cm W SPECI- NEUTRON-DEF: Y ' ‘21 an ezfort to obta =r=I=atics in the neutron "‘31? Spectroscopic stud‘e 33'. 2' ““h- .Pb 3'7, vaw, P": ..or an isomeric StaU mese studies were Sigles . in coincidence a mum ban in two-di~. 35:53 :9 "“II .7 ABSTRACT GAMMA RAY SPECTROSCOPIC STUDIES OF STATES IN NEUTRON-DEFICIENT Pb and T1 ISOTOPES By Raymond Edwin Doebler In an effort to obtain more information on the energy level systematics in the neutron-deficient lead and thallium isotopes, y-ray spectroscopic studies have been made on the following isotopes: szown, szoan, szozn, Pb201, Pbloo, and Pb199. A search was also made for an isomeric state in szoo. These studies were conducted largely with Ge(Li) detectors in singles, in coincidence and anticoincidence arrangements with NaI(Tl) detectors, and in two-dimensional, Ge(Li)-Ge(Li), multiparameter systems. Using a single-crystal Ge(Li) conversion coefficient spectrom- eter, K-conversion coefficients for the 899.2-keV E2 and the 911.7- keV‘ES transitions in Pb2°“m were measured. In the study of the 6.1-s isomer, szoyfl, a 5.1-keV M2 transition was found to compete with the 825.2-keVflM4 isomeric transition. After correcting for i this branching decay, thelMfi transition probability was found to be consistent with those of the other‘Mfi transitions in the lead region. Six new y transitions have been assigned to the decay of Pb202m and a new level has been proposed at 1550-keV in T1202, populated by. dam-capture from tne I ! upper limit of 31 s was Lav-Spin states in I 55:. Nineteen y trafls‘li timbeen placed in a Inlazed in the decay of 33.91.160.56, 525.54, a: Atotal of 72 w, tra 4 " Q“ 3.3“ “A“ w . Excited Sta L‘miticas have been pie 137.41, 1238.82, 1277 .C‘ 139.37, 1550.5. 1617 .~’+5 Eighty-nine of the .199, 124W, 1&82.2 electron-capture from the isomeric state. After an intensive search, an upper limit of =1 3 was placed on any possible isomeric state in PbZOO. Low-spin states in T1200 have been studied via the decay of 21.5—h szoo. Nineteen 7 transitions have been assigned to this decay and all have been placed in a consistent level scheme. States in T1200 populated in the decay of Pb200 lie at 0, 147.63, 257.19, 289.24, 289.92, 450.56, 525.54, and 605.44 keV. ' A total of 72 7 transitions have been observed in the decay of 9.4-h Pb201. Excited states in T1201 accommodating 65 of these transitions have been placed at 331.15, 692.41, 1098.46, 1134.81, 1157.41, 1238.82, 1277.09, 1290.05, 1330.38, 1401.21, 1420.00, 1445.85, 1479.87, 1550.5, 1617.45, 1639.47, 1672.00, 1712.5, and 1755.31 keV. Eighty-nine of the 117 7 transitions assigned to the decay of 90-min IPb199 have been placed in a decay scheme with levels at 0, 366.90, 720.26, 1120.90, 1241.67, 1482.25, 1502.00, 1528.2, 1554.10, 1632.00, 1658.47, 1695.25, 1725.4, 1749.6, 1768.5, 1891.0, 1898.1, 1930.4, 1959.45, 1977.8, 2031.5, 2159.0, 2206.7, 2226.5, 2237.3, 2367.3, 2433.5, 2547.4, and 2643.2 keV. unique spin and parity assignments have been made for many of the states observed in these studies, with limits placed on most of the remaining ones. Spin and parity assignments were based on measured conversion coefficients, log ft values, and relative photon intensities. The structures of the states in odd-odd T1200 are discussed in was cf the coupling of xix-{nuclei The syster and 7139, are dis- 32:25 coupled to core st terms of the coupling of possible single-particle states in adjacent oddeA nuclei. The systematics of the oddeA Tl nuclei, including '1‘1201 and T1199, are discussed in terms of single-particle shell model states coupled to core states of the corresponding Pb nuclei. .AMHA RAY SE IN NEUTRCN- R3 De GAMMA RAY SPECTROSCOPIC STUDIES OF STATES IN NEUTRON-DEFICIENT Pb AND T1 ISOTOPES By Raymond Edwin Doebler A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY _Department of Chemistry 1970 Iuish to the“ 2th of study. His Pat; '|'- ‘1'"! g the experiment getely appre iated. Dr. H. H. Kelli: Elsie suggestions and Estatefully acknov l e c' g e Dr. B. G. 81055 We! ’ ' .. with the operat: Er. ,l ‘ ~ at. Cyuotron, vhicn 37431: for t hese invest 517? 0f the en 67/ f .24. ACKNOWLEDGMENTS I wish to thank Dr. Wm. C. McHarris for suggesting this region of study. His patience, encouragement, and readily available help during the experimental work and preparation of this thesis are greately appreciated. Dr. V. K. Kelly of the Physics Department has provided many valuable suggestions and discussions during this project and his help is gratefully acknowledged. Dr. H. G. Blosser, Mr. H. Hilbert, and Dr. W. P. Johnson assisted with the operation of the Michigan State University Sector- Focused Cyclotron, which was used to prepare most of the radioactive sources for these investigations. Many of the energy and intensity standards used in this Study were produced by neutron irradiation in the 11.8.0. Triga reactor under the supervision of Dr. B. Wilkinson, Department of Chemical EnSines ring . Dr. D. B. Beery, Mr. J. Black, Mr. W. B. Chaffee, ”r- J- B. Cross, Dr. R. s. Eppley, Mr. G. c. Geisler, Mr. R. Sales, “1" K- Kosanke, and Mr. R. Todd all deserve special mention, for with“: their advice and "extra hands," most of the experiments deacribed in this thesis could not have been done. Miss T. Arnette, Mr. and Mrs. W. Merrit, and the cyclotron c ”Pater staff aided greatly in the data acquisition and evaluation ii 2:;gtzheir programing ~ {wish to thank t agitatian and financial 5 Mrs. P. liarstler here of many details 1“ The National Scie LIL-551m, and Hichigan St Sistacew'nich made this Ian grateful to I “than throughout the ; Finally, I wish I( 9:» 1er011 of out engages. through their programming of the XDS Sigma-7 computer. I wish to thank the Department of Physics for their friendly cooperation and financial support during this work. Mrs. P. Warstler helped in the typing of this thesis and took care of many details connected with its final preparation. The National Science Foundation, U. 8. Atomic Energy Omission, and Michigan State University have provided the financial assistance which made this study possible. I am grateful to my parents for their continued interest and support throughout the many years of this study. Finally, I wish to thank my wife, Kathleen, who spent the short period of our engagement typing this thesis. iii MIME ............. ‘AI . Cl HUI-”.5 ............. l I. MEUCUON. . .. ...... 2. 2231153211. METHOIE Al N ‘. J: 1625 ssssssssssssss .212: 2.1. 2.2. - Determination “.512 y-Ray Singles Ge(Li)-Nal("l'l ) 2.2.1. Antics: Spectr 2.2.2. lntegr Spectr “O‘Dimnsiona Calibration of D!“ Mnysis.‘ 3.1. 3'2' P1320311 The Decay Sche- ll'l' Introd. 3'1'2' The Ge SPECtrc 3.1.3. SOurQe 3.1.6. “peril Decay at 3M. IntrOd: TABLE OF CONTENTS Page aotnmmmw. ...... u IJST'OF TABLES.................................................. x LIST OF FIGURES................................................. xii Qumter L. INTRODUCTION.............................................. 1 LL. EXPERIMENTAL METHODS AND DATA ANALYSIS.................... 7 2.1. y-Ray Singles Spectrometer........................ 8 2.2. Ge(Li)-NaI(Tl) Coincidence Spectrometer........... 10 2.2.1. Anticoincidence and Anti-Compton Spectrometers............................. 10 2.2.2. Integral, Gated, and Triple Coincidence Spectrometers............................. 13 2.3. Tho-Dimensional y—Y Coincidence Spectrometer. .. . . . 17 2.4. Determination of Photopeak Efficiency Curves...... 21 2.5. Calibration of Y-Ray Standards.................... 26 2.6. Data Analysis..................................... 32 III. Tan IBCAY scams 0? SOME NEUTRON-DEFICIENT Pb ISOMERS.... 34 3.1. The Decay Scheme of Pb2°“’"........................ 35 3.1.1. Introduction.............................. 35 3.1.2. The Ge(Li) Conversion-Coefficient Spectrometer.............................. 36 3.1.3. Source Preparation........................ 39 3.1.4. Experimental Results...................... 41 3.2. 91:20” Decay and M4 Transition Probabilities...... 44 3.2.1. IntrOductionOOOOO000.......OIOOOOOOOOOOOOO 44 iv 1:111 3.2.2. Exam 3.3. lhe Data? of P 3.3.1. intro: 3.3.2. Souro 3.3.3. Erper 3.1. Search for a' .7. EELECIMN CAPTlT-l’i 1.1. Introduction 1.2. Source Prep: 4.2.1. lnt 4.2.2. 111 1.2.3. 112 1.3. —.—::l‘ n Chapter Page 3.2.2. Experimental Method and Results........... 45 3.3. The Decay of PbZOZ"...... ....... . ...... ........... 51 3.3.1. Introduction.............................. 51 3.3.2. Source Preparation........................ 51- 3.3.3. Experimental Results and Discussion....... 54 3.4. Search for an Isomeric State in szoo... ..... ..... 58 IV. mnmcrmu CAPTURE new orprOO..... ...... ...... . 61 4.1. Introduction..................... ............. .... 61 4.2. Source Preparation................................ 64 4.2.1. Introduction.............................. 64 4.2.2. T12°3(p,4n)p62°° .............. . 64 4.2.3. T12°3(ae3,6n)312°°326200................ 66 4.2.4. Hg2°2(ue3,5n)Pb2°°........................ 67 . 4.3. Experimental Results. 70 4.3.1. y-Rsy Singles Spectra...................... 70 4.3.2. Coincidence Spectra...... ....... .......... 76 4.3.3. Conversion Coefficients................... 91 4.4. Decay Scheme...................................... 96 4.5. Spin and Parity Assignments....................... 99 4.6. Shell-Model Assignments and Discussion............ 102 .V- furnace or PM“ 116 5.1. Introduction...................... ............... . 116 5.2. Source Preparation................................ 121 5.2.1. T12°3(p,3n)Pb2°1.......................... 121 5.2.2. T12°3(Be3,5n)812°1 5 Pb29}................ 122 5.3. lrperirental ‘. 5.3.6. - ‘1 4131: Anti-. ~1th: lnteg' Z-dy. 5.3.5 5.3.5 5.3.5 5.3.5 Com'e Decay Scheme 54.1. Leve 1 5.4.] Chapter 5.3. 5.4. Page Experimental Results............. ................. 124 5.3.1. 'Y-Ray Singles Spectra..... ................ 124 5.3.2. Anti-Compton Spectrum....... .............. 134 5.3.3. Anticoincidence Spectra ................ ... 136 5.3.4. Integral Coincidence Spectra ............ .. 138 5.3.5. 2-d'YdY Coincidence Experiment............ 140 5.3.5.a. Integral Coincidence Spectra.... 140 5.3.5.b. Gated Coincidence Spectra....... 141 5.3.5.c. Compton Pair Peaks.............. 163 5.3.5.d. The 946-keV Doublet............. 165 5.3.6. Conversion Coefficients................... 172 Decay Scheme of Pb201... ..... . ....... . ............ 177 5.4.1. Level Placements.......... ...... 177 5.4.1.a. 331.15-keV Level.. .............. 177 5.4.1.b. 692.41-keV Level................ 180 5.4.1.c. 1098.46-kev Leve1.. ...... ....... 180 5.4.l.d. 1157.41-keV Level............... 180 5.4.1.a. 1238.82- and 1277.09-keV Levels. 181 5.4.l.f. 1330.38-keVLeve1.... 181 5.4.1.g. 1401.21-keV Level........ ..... .. 182 5.4.1.h. 1445.85-keV Level............... 182 5.4.1.1. 1479.87- and 1639.47-keV Levels. 183 5.4.1.1. 1672.00-keV and 1755.31-kev Levels.......................... 184 5.4.1.k. 1134.81-, 1290.05-, and 1420.00- kev Level.OOOOOOOOOOOIOOOOOOO... vi >184 1;:2: 5.4.2. 3*-“ 5.4.3, Log .3 5‘5' 8Mn and Perl 5'5'1- Grow 5.5.2. 692-' 505-10. 161 Chapter Page 5.4.1.1. 1550.5-, 1617.45-, and 1712.5- keV Levels...................... 186 5.4.2. B+-feeding ...... . ........................ . 188 5.4.3. Log fi's......... ...... .......... ...... ... 191 5.5. Spin and Parity Assignments....................... 193 5.5.1. Ground and 331-keV States ....... . ........ . 193 5.5.2. 692- and 1277-keV States. ................. 193 5.5.3. 1098.46-keV State......................... 194 5.5.4. 1134.81- and 1290.05-keV States.. ......... 195 5.5.5. 1157.41- and 1479.87-keV States........... 197‘ 5.5.6. 1238.82-kev State............... ......... . 198 5.5.7. 1401.21- and 1445.85-keV States........... 198 5.5.8. 1420.00-keV State......................... 199- 5.5.9. 1550.5-keV State... ........ ..... .......... 199 5.5.10. 1617.45- and 1712.5-keV States........... 200 5.5.11. 1639.47- and 1672.00-keV States.......... 200 5.5.12. 1330.38-keV State........................ 201 5.5.13. Summary of Spin and Parity Assignments... 201 V“ THE DECAY or Pb199 AND STATES IN ODD-MASS T1 ISOTOPES ...... 203 6.1. Introduction...................................... 203 6.2. Source Preparation................................ 206 6.2.1. Hg2°°(He3,4n)Pb199........................ 206 6.2.2. T12°3(p,5n)rbl99.......................... 207 6.3. Experimental Results.............................. 209 6.3.1. y-ray Singles Spectra..................... 209 vii 12:2? 6 l 8- Decay Scheme c 6.4.1. . Conver Antic: Integr Z-d y- Assig: Level 6.4.1. 6.4.1. 6.4.1. 6.4.1. 6.4.1. 6.4.1. 6.4.1 6.4.1 6.4.1 6.4.1 6.4.1 6.4.» 5.4.1 6.4.4 6.4.1 Chapter 6.4. Page 6.3.2. Anticoincidence Spectra... .......... ..... 217 6.3.3. Integral Coincidence Spectra... .......... 220 6.3.4. 2-d y-y Coincidence Experiment ....... .... 220 6.3.5. Conversion Coefficients and Multipolarity Assignments.............................. 259 Decay Scheme of Pb199........... ................. 261 - 6.4.1. Level P1acements......................... 261 6.4.1.a. 366.90-keV Level ......... ...... 261 6.4.1.b. 720.26-keV Leve1.. ............ . 261 6.4.1.c. 1120.90-keV Level.............. 264 6.4.1.d. 1241.67-keV Level.............. 264 6.4.1.e. 1482.25-keV Level.............. 265 6.4.1.f. 1502.00-, 1632.00- and 1658.47- keV Levels..................... 265 6.4.l.g. 1725.4-, 1749.6-, 1768.5-, and 1891.1-keV Levels.............. 266 6.4.1.b. 1898.1-, 1959.45-, and 1977.8- keV Levels..................... 267 6.4.1.1. 2226.5- and 2367.3-keV Levels.. 267' 6.4.1.1. 1554.10-keV Level.............. 268 6.4.1.k. 1930.4- and 2031.5-keV Levels.. 270 6.4.1.1. 2237.4— and 2433.7-keV Levels.. 270 6.4.1.m. 1528.2 and 1695.2-keV Levels... 272 6.4.1.n. 2159.3-keV Level............... 273 6.4.1.0. 2206.7-, 2547.4-, and 2643.2- keV Levels..................... 274 viii ‘2“:3 ~I-O '4‘. La. h‘. ... ‘ .J. “35:44:; _ xx 661 A‘ y 0 I . 371‘ Chapter 6.5. 6.6. Page 6.4.1.p. Possible Additional Levels ...... 275 6.4.2. Log ft's. ........... - ...................... 276 6.4.3. B+efeeding.... ............................ 277 Spin and Parity Assignments ....................... 281 6.5.1. Ground State......... ..................... 282 6.5.2. 366.90- and 720.26-keV States ............. 284 6.5.3. Remaining States with Log ft's 57.4....... 285 6.5.4. States with Log ft's 27.5 ................. 287 Theoretical Description of Odd-mass Tl Isotopes... 289 6.6.1. Shell Model Description of Odd—mass T1 Isotopes........... ....................... 289 6.6.2. Core-coupling Model ....................... 295 BIBLIOGRAPHYOOOOOOO......OOOOOOOOOO00............OOOOOOOOOOOOOO 303 APPENDICES... ......OOOOOOOOOO0.0.0.000... ..... O OOOOOOOOOOOOOOOO 310 '11: I 1.15' 2-1 ; Values for Reacti y-lay Relative Int 2'2. v-Rjy Calibration S 81 s L.- '74:“? '1" w .. Stable . insults of y-Ray E: -.. Ccnversion Coeffici . 1123:“ Conversion 1 » Ritiial Hattie Eleni Pb IsotOpes ....... ° Enersieez and Relat: Decay of 2425?"... Isotopes of ' 7‘43?! Used as Ene Table I-l. II-l. II-2. II-3. III-1. III-2 . 111-30 III“. Iv.1s IV-Zs Iv.3s IV“. Iv-s s IVb6. v.10 v-Zs V~3. V~4 LIST OF TABLES Page Q Values for Reactions Used to Produce Pb Isomers....... 6 y-Ray Relative Intensity Standards................ ..... 22 y-Ray Calibration Standards.................. ..... 27 Results of y-Ray Energy Calibrations.................... 30 Conversion Coefficient Calibration Standards....... ..... 43. P1324“m Conversion Data 43 ' Radial Metric Elements for M4 y-transitions in Odd-mass Pb Isotwe‘OOOO....OIOOOOOOOOOOO......OOOOOOOOOOOOOOO... so Energies and Relative Intensities of y-Rays from the men, Of PbZOMOOOOOOOOOOOOOO..0OOOOOOOOIOOOOOCOOOOIOOOI 57 Stable Isotopes of Hg and Z Abundances.................. 68 Y-m‘ UseduEnery stmdardBOOOOIOO......OOOIIOOO.... 72 Energies and Relative Intensities of y-Rays from the Decay 0f szooCOOCOOOOOOOC0.00.00.00.00.....OOOOOOCOOOOOOOOOOC 73 Results of y-y Coincidence Study Using 2-Dimensional halyaiaOO......OOOOOOO.........OOOOOOOC00.000.000.00... 87 Trmsition Data for PbZOOOOOOOO............OOOOOOOOOOOOO 92 Possible Configurations for Producing Some Low-Lying Odd-Odd States in leooOOOOOOOOOOOOOOOOOOOO0.00.00.00.00 109 y-Rsys Used as Energy Standards......................... 125 Energies and Relative Intensities of v-Rays from the mcay OfPbZOIOOOOOOI..........OOOOOOO......OOOOOOOOOOOO 130 Results of y-y Coincidence Study Using 2-Dimensional may.1800000000000000...000............OOOOOOOOOOOOOOOO 160 Trmition Data for PbZOIOOOOOOOOOOOOOOOOO.......OOOOOOO 176 '11 r-Rays L'sed as Ener -. Energies and Relat; Decay of 16199.. . .. . Possible Levels in and Integral Coinci 4. Results of y-y Coir I 2~Dinensional Anal; .1 . "‘ . s-feeding in Pb"? ' 31111 Assignments f. Decayuuuumm }Table VI']. 0 v1.20 VI-3o VI-4. v1-5 o VI-6. Page y-Rays Used as Energy Standards ........................ 210 Energies and Relative Intensities of y-Rays from the Decay Of PbIQSOOOOO...00.000.000.000........OOOOOOOOOOO. 212 Possible Levels in T1199 Indicated by Anticoincidence and Integral Coincidence Experiments.................... 219 Results of y-y Coincidence Study of Pb199 Using 2-Dimensiona1 Analysis.................................. 254 8+-feedin8 in Pblgg DeCBYOOOOOOOI......COOOOOOCIOCOCCOOO. 279 Spin Assignments for Levels in T1199 Populated in Pb199 mcayOOOOOOOOOOOOOO...0.0.00.0...0.0.0.0...0.00.00.00.00 283 xi H me Ge (i1) ~32, -. Schenstic Illus tra is sae apparatu Canton, and trip‘. elimination of the -. Schaatic illustra not: used in Anti. integral coinciden ‘3' “Cd diagra of t dimensional "ne 3 ac the 518“” Comp ut . Relative photopea‘x determined using t It 2 inches frOm t COME rs mentation of Tap: 53137 the 8PECtx ML conversior taken Vlth an ‘11 E detector to block LIST OF FIGURES Figure Page II-l. Schematic Illustration of the anticoincidence apparatus. This sale apparatus is used for the integral, anti- Conpton, and triple coincidence experiments after the elimination of the 3t3-in. NaI(T1) detector [Ep70]...... ll II-2. Schematic illustrations of source and detector arrange- ments used in Anti-Compton, anticoincidence, and integral coincidence experiments........................ 14 II-3. Block diagram of the apparatus used to collect two- dimensional "megachannel" y-q coincidence spectra, using the81m.7 ”uterus...soooosoooooooooosooooooosoooooo 18 II-ls. Relative photopeak efficiency curve for the 2.51 detector determined using the sources listed in Table II-l placed at 2 inches from the front face of the detector......... 25 111-1. The Ge(Li) conversion-coefficient spectrometer showing the orientation of the crystal and the source mount..... 37 111-2. Top: Cs137 spectrum showing the 661.6-keV y-ray and its K andL conversion electrons. Bottom: C8137 spectrum taken with an A1 absorber between the source and the detector to block the electrons. ‘By subtracting the bottom spectrum from the top one a clean electron spectrum can be obtained...........,.................... 38 III-3. Efficiency curves. The solid curve represents the composite y-ray-electron efficiencies. Triangles are calculated from theoretical conversion coefficients, 3's from experimental ones. The dashed curve shows a (displaud) Y-ray effidency cum._.. 0.0.0.0... O. 0.... O. 40 111-4. szo'm spectra obtained with the Ge(l.i) conversion- coefficient spectrometer. Only the szow transitions are labelled. Top: Spectrum showing both Y-rays and electrons. Bottom: Spectrum taken through an Al absorber to remove the electrons........................ 42 xii 1' , ' . hey'fly spectru: 3?. ' “138168 Y-ray a; ' Sistmatics and de so! 3125“. which after chmical se; . lire decay scheme 0 present study ..... -. Deeay scheme of P': crossed-dotted lir. The transition in: intensities only. transition may see an E4 transition, efficient Ge(l.i) c’ the spectrum was p enriched r1203 (7 C. Pb “merit states H17 singles spec 2'51 efficient cg mailing to the d- L01 energy Y‘l‘ay 5‘ K bray inteflsity Anticoincidencg a Figure Page III-5. They-ray spectrum of: a) 31203 (unavoidably containing some 3120“ , which has the same half-life); b) 9903’" after chemical separation from its 31203 parent......... 47- 111-6. The decay scheme of Pb2°3m as determined from the pm’ent studYOOOOOOO......OOIOOOIOOO.....IOOOOOOOOOOOOOOO‘ 48 111-7. Decay scheme of P1320?" . The transitions marked with the crossed-dotted lines were added in the present study. The transition intensities given here are y-ray intensities only. The intensity of the 129.09-keV transition may seem improbable, however, as this is an 34 transition, it is very highly converted.... 52 111-8. A singles y-ray spectrum of szozm recorded by a 2.52 efficient Ge(Li) detector. The source used to obtain the spectrum was prepared by the 09,271) reaction on enriched T1203 (70:).................................... 56 III-9. Systematics and decay schemes of the known even-even Pb1’mdcstate.0000000000......OOOOOOOOOOOOOOO0...... 59 Ivel. y-ray singles spectrum of szoo obtained in 7 h with a 2.52 efficient Ge(Li) detector. Only those peaks belonging to the decay of szoo are labelled............ 71 IV-2. Low energy Y-ray spectra of szoo, and T1200 taken with 0.421 efficient Ge(-Li) detector and used to obtain the Kx-ray1nten81ty foerzoo decay-......0.00.00.00.00... 75 IV-3. Anticoincidence spectrun of szoo was obtained with a 0.422 efficient Ge(Li) detector placed inside an 8X8-in. Na1(Tl) split annulus with a 3X3-in. NaI(Tl) detector blocking the other end. Only peaks belonging to Pb20° decay are labelled............. 77 Y-rays . This spectrum IV-b. Integral coincidence spectrum of Pb200 Y-rays. This spectrum was obtained by using a 0.421 efficient Ge(Li) xiii in!!! H. Lei 53—1 . integral coincide: . “election of the detector in coincl mulls. All y-r; in the gate ...... r detector (Y-integ: detector (X-integ: dinensional y-y c: £10! the two-dime: me230. All 3a: subtracted except Spectra were obta; detector) and dis; ”Capt Hhere speci identified as big-... Kiting on the back new“ of the two mad to mafia th irroufi f0 CU Vet! drawn Figure IV-lb e Cont ' d IV'S e IV-6 e Iv-7e Iv.8e Page detector in coincidence with an 9‘8—1n. NaI (Tl) split annulus. All y-rays above theK x-rays were included in th‘ satQOOOOOOOOOOO......OOOOCOOOOOO00.000.000.000... 78 Integral coincidence spectra obtained with the 2.02 detector (Y-integral coincidence gate) and the 2.52 detector (X-integral coincidence gate) during the two- dimensional y-y coincidence experiment on szoo......... 81 A selection of the gated coincidence spectra obtained from the two-dimensional y-Y coincidence experiment on szoo. All gated spectra have had the background subtracted except where otherwise specified. All spectra were obtained by gating on the .Y-side (2.01 detector) and displaying the X-side (2.52 detector) except where specified as .Y-display. Spectra identified as high or low background resulted from gating on the background adj scent to the specified peak. 82 Results of the two-dimensional coincidence experiment used to confirm the doublet nature of the 289.6-kev peak.- A. y-side integral coincidence spectrum used for the gates. B. x-side integral coincidence spectrum showing the region near the 289.6-kev peak on an expanded scale. C. z—side spectrum in coincidence with the 161.3-keV peak (background subtracted). D. cur-side spectrum in coincidence with the 235.6-keV peak (background .wtr‘Ct.deeeseeeeeseeeeeeeeseeeeeeseeeseseseeesseseseee 89 Experimental and theroretical K¥shell conversion coefficients for transitions following the decay of szoo. The smooth curves‘were drawn to fit the theoretical values of Eager and Seltzer [Ha68]....................................... 94 xiv .. hfxe H. ‘PWI TJ :u—D Decay schm of P': transitions are g: tions. The per c Hints for that s it the extreme ti decay of 11237-5; Populated by the Suzanne; of the in Odd-1333 n 150 'U2‘d3/29 ”id is] . Systeutics of 10‘. min; 119 neutrm purepl/l'fS/z. a ”Stain“ Of the in the “finest ne Figure IV-9 s IV’IOO IV-ll. Iv-12e IV-13. v-1 0 . v-Zs V-3. Page Decay scheme of P172” . The intensities of all (total) transitions are given in per cent of the P132” disintegra- tions. The per cent 2 decay to each state and the log ft values for that state are listed to the right of the state. At the extreme right we show the states populated by the decay of T1209"; these higher-spin states were not populated by the decay of prOO 97 Systematics of the low-lying 1/2+, 3/2+, and 5/ 2+ states in odddmass Tl isotopes. These should be relatively pure 81/2, d3/2’ “d d5/2 .hell-NOdel Statussesssesesssssess 105 Systematics of low-lying states in odd-mass iso tones having 119 neutrons. These correspond to relatively pure P1/2 , f5/2u and i13/2shell-model states. . . . . . . . . . . . 106 Systusatics of the f5] 2, 191/2 , P3] 2 , and ‘51 3]; states in the odd-mass neutron-deficient Pb isotopes. . . . . . . . . . . 107 Systematics of some selected states in odd-odd Tl isotopes. The states connected by lines are assumed to be primarily the same configurations. The 289.24-kev state in T1200 is marked by a 7, for we have been unable to decide between 1- and 2- for its assignment.......... 111 The decay scheme of Pb”1 as known before the present study. The energies of the transitions and states are given in mVOOOOOO.........OOOOOOOOOO......OOOOOOOOOI... 120 Pb”1 singles y-ray spectrum taken with a 0.421 efficient Ge(Li) detector. The source used to obtain this spectrum was prepared by the (P.3n) reaction on natural Tl....... 127 A singles y-ray spectrum recorded by a 3.62 efficient Ge(Li) detector during a 24-h period. The source used to obtain this spectrum was prepared by the (19.3?!) reaction on enriched T1203 (701)................................. 129 I II .- ..és. H. " a 7.5. . hti-Cmton spect . hticoincidence sp 3,: low energy y-ra'y s 2.51 efficient Ge{' i may intensity efficient Ge(l.i) 6' dud-in. 881(11) an: the other end of t'. decay of PW1 are indicated ..... . . . . efficient Ge(l.i) de hifll) annulm vii binding the other decay of new} are indicated... ...... Integral Coinciden: “Puma was obtair dHector in Coin ci< detector. All Y‘r‘ V-S s V-6 s V-7. Page Low energy y-ray spectra of szo1 and Pb“3 taken with a 2.51 efficient Ge(Li) detector and used to obtain the r K x-ray intensity for Pb201 decay...OOOOOOOOOOOOOOOOOOOO 133 mti-Comton spectrum of Pb201 y-rays using the 2.52 efficient Ge(Li) detector placed inside one end of an 3x3-in. NaI(Tl) annulus with a cOllimated source at the other end of the tunnel. Only peaks belonging to the decay of szo1 are labelled except where otherwise indicatedOOOOCOOOOO0.00.0.........OOOOOOOOIIOOOOOO...... 13.5 Anticoincidence spectrum of Pb”1 y-rays using a 2.52 efficient Ge(Li) detector placed inside an 8x8-in. NaI(Tl) annulus with a 3XB-in. NaI(Tl) detector blocking the other end. Only peaks belonging to the decay of szo1 are labelled except where otherwise inuC.md.CCIOOOO0.00.0000............OOOOOOOOOOOOOOOOO. 137 Integral coincidence spectrum of szo1 y-rays. This spectrum was obtained by using a 2.52 efficient Ge(Li) detector in coincidence with a 3. 61 efficient Ge(Li) detector. All y-rays above the K x-rays were included in the gate.................................... 139. Integral coincidence spectra taken during two- dimensional y-y coincidence experiment on szm. A. I-side spectrum taken during first day with a 3.62 Ge(Li) detector. B. Y-side spectrum taken during second day with 3.62 Ge(Li) detector. 0. X-side spectrum taken during two-day run with 2.51 detector.... 142 A selection of the gated coincidence spectra obtained from the two-dimensional y-y coincidence experiment on Pb201. All gated spectra have had the background subtracted except where otherwise specified. All spectra were obtained by gating on the Y-side (3.62 xvi Tun H. M except where upecii as. ' I“ “"0 Portion detector) and disp Lou energy portion grand subtraction ”"0 background 3 M100“ Y‘Y coi “hm “Wrfl "fair scattering 0 Hell Pei ”..°"OOssssss Found 'tbtraction at” b‘Ckgro‘md i filmiond 7"! co‘ q .hW' .enrd "fa scattm118 II “11 Peak Faults of 2_d Y"! thlt t1)! “(i-Rev P! 30d“ m“ the 36] Mia peak fro. l but. Of the 2-d ““an the One r] Figure V-9. Cont'd v.10 e v.11 e V-12 . V-13. Page detector) and. displaying the X-side (2.52 detector) “apt mt. Ipfidind ll Y-displ‘yeeeeeeeeeeeeeeeeeeeee 143 Low energy portion of the 130-keV gate without back- ground subtraction along with the adjacent. low and high energy background gates obtained fro. the two- dinensional y-ey coincidence experiment on Pb201 . his shows several "false" peaks generated by Canton scattering as well as the lSS-kev "true" coincidence p.*..................OOOOOOOOOOOOI......OOOOOOOOOOOOOO. 164 Low energy portion of the lSS-kev gate without back- ground attraction along with the adjacent low and high energy background gates obtained fro- the two- dinensional y-ey coincidence experiment on sz‘”. This shows several "false" peaks generated by Canton scattering as well as the 130-keV "true" coincidence ”“00000000000000000000000.00.00.00.00.....IOOOOOOOOOO. 166 Results of 2-d y-y coincidence experiment used to show that the 946-kev peak is composed of two y-transitions. Notice that the 361-keV peak gets larger as we scan the 946-ltev peak from lower to higher energy....... ...... ... 168 beults of the Z-d coincidence experiment used to determine the energies of the 945.96- and 946.78-kev doublet. A. .Y-side integral coincidence spectrum used for the gates. The y-rays which have a 966 in parentheses after the energies were in coincidence with only the lower energy member of the doeblet and those y-rays noted by (967) were in coincidence with only the higher energy. B. I-side integral coincidence spectrum showing the region near the doeblet on an expanded scale. C. Sun of gated spectra in coincidence with the 965.96-kev transition. D. See-of gated spectra in coincidence with 946.78-kav amnion...”............. 17o xvii ‘1 F13. Experimental and t for ttmitione to curves were drawn end Seltzer [8368} ’5- Decay lchene of Pb (“‘11) transition Pbm disintegrati and the 108 ft val right 0f the state Mingle: y-ray s? efficient “(14) d Pb199 “mm was F eeparated 130t0pe my: obaemd am this 'Pectm not decay of phleg are mticoinqdence 3; 2.51 efficient Ge ( Balm) maul“ '1 81! Open and. Onl Figure v.14. v.15 e 'VI-3. vx-4, Page Experimental and theoretical K-shell conversion coefficients for transitions following the decay of Pb201. The smooth curves were drawn to fit the theoretical values of Eager and Seltzer [Ha68]...................................... 176 Decay scheme of Pb201. All energies are given in keV and (total) transition intensities are given in percent of the Pb2°° disintegrations. The percent e decay to each state and the log'ft values for that state are listed to the right of the state...................................... 178 A singles y-rsy spectrum of Pb”9 recorded by a 2.52 efficient Ge(Li) detector during a 45 min period., The Pb199 source was prepared by the (Be3,4n) reaction on separated isotope HgZOO. Because of the large number of y-rays observed and because of the poor statistics in this spectrum not all of the peaks belonging to the decay of Pb199 are 1abe11ed............................. 211 Anticoincidence spectrum of Pb199 y-rays obtained with a 2.51 efficient Ge(Li) detector placed inside an Bxs-in. NaI(Tl) annulus with a 3X3-in. NaI(Tl) detector blocking the open end. Only peaks belonging to Pb199 decay are labelled except where noted othenvise................... 213 Integral coincidence spectrum of Pb199 y-rays obtained by using a 2.51 efficient Ge(Li) detector in coincidence with a 3.61 efficient Ge(Li) detector at 99' to the source. In this case no chemical separation was performed on the Bg2°° target after the bombardment..... 221 Integral coincidence spectra recorded during the two- dimensional y-y coincidence experiment on Pb199. The Y-side spectrum.wss recorded by the 3.62 Ge(Li) detector and the Xiside spectrum by the 2.52 Ge(Li) detector.............OOOOOOOOOOOOO...0.00.00...0........ 222 xviii A 5(‘Jl‘t‘li‘m “f t from [NC W041" Pb All can“ subtracted 9x50? spectra were obt detector) and d: Decay scheme of and (total) trar‘. percent of the F decay to each 5: state are listed Shell-model o rbi the 3-82 closed change somewhat neutrons change. A ouserved near t?’ S:‘v’Stematics of s Tl isotopes belt radioactive deg. included exceat observed by set. the reaction of Acmarlson of Etay SChene pi le' 1 gee res Vl-'). V1-6 e VI-7. VI-Be VI-9 . A selection of the gated coincidence spectra obtained from the two-dimensional y-y coincidence experiment on Pblq”. All gated spectra have had the background subtracted except where otherwise specified. All spectra were obtained by gating on the Y-side (3.6% detector) and displaying the X-side (2.5% detector).... Decay scheme of Pb199. All energies are given in keV and (total) transition intensities are given in percent of the szoo disintegrations. The percent 5 decay to each state and the log ft values for that state are listed to the right of the state............. Shell-model orbitals near the N=126 closed shell and the z-82 closed shell. The order of these states may change somewhat as the total number of protons or neutrons change. The order shown here is that observed near the double shell closure at 82Pbigg...... Systematics of states in neutron-deficient odd-mass T1 isotopes below 2.0 MeV. All states populated by radioactive decay and nuclear reactions have been included except for the high spin states in T1199 observed by Newton et al. [Ne70] in their study of the reaction of Au197 (a,2ny)T1199..................... A comparison of the levels in T1201 and T1199 from our decay scheme with those calculated by [CV67] and [A167]. The parities for all the levels are positive except Where indicatedseseeeeeeeeeeeeeeeseeeeeeseeeeeeeeeeeeee Page 224 262 290 291 298 lmediately after slayer [31349] and Jen tile shell closures b .7 n a : Eeuutleus “Pow is 5" 1L6 closed shells of 126 ne- particularly stable in ‘ {:36 . and .eellshed levels a d Sititiu . The states 1 1x6: CHAPTER I INTRODUCTION Immediately after the formulation of the nuclear shell theory by Mayer [H349] and Jensen [Je49] in 1949, nuclei in the region of double shell closures became of tremendous theoretical interest. The nucleus 82Pb§gg is one of these "doublydmagic" nuclei, with closed shells of 126 neutrons and 82 protons. This nucleus is particularly stable in its ground state configuration, its first excited level lying at 2.62 MeV. The great stability of Pb208 suggests that one could treat those nuclei differing by only a few nucleons from this doable-closed shell by the independent particle model. The nuclei azpbfigg, ezpbfigg, alrifigg, and 83Bi§gg, all of which have the "hard" core of szo8 with one hole or one additional nucleon, should therefore exhibit a sequence of low-lying energy states in particularly good agreement with single-particle levels of the independent-particle shell model. Of these four nuclei, Pb207 is the best known, and the correspondence between the experimentally established levels and those predicted by the shell model is very striking. The states in Pb207 at low to moderate energies should ‘consist of single neutron holes, and from.simp1e shell—model Predictions, one would expect these states to be, in increasing energy, p1/2’ fS/Z’ p3/2’ 1113/2, f7/2, and 719/2. This is exactly the order found experimentally. The next step was to use the experimentally determined single- :eutron levels of Pbi7 rte neutron-deficient successful in the dest- Eros the #126 closed into a discussion of t excellent review of th results up to 1964 by E 5!: for this seeningl; i335 from the t decay shell model calculatior .unature. As we get t he effects play an in Names, and in th etsetsect to find suc .4: Mrted nuclei by Viv its" H011 States. neutron levels of Pb207 as a basis for calculating levels in other, more neutron-deficient Pb isotopes. These attempts have been most successful in the description of Pb206 which is two neutrons removed from the N-126 closed neutron shell. However, instead of getting into a discussion Of these calculations here, we would recomend an excellent review of these calculations, as well as the experimental results up to 1964 by Hyde [Hy64]. The important point here is that, even for this seemingly ideal case, two of the levels observed in Pb206 from the e: decay of 31206 could not be accounted for by the shell model calculations and had to be characterized as collective in nature. As we get to the more neutron-deficient isotopes, collec- tive effects play an increasingly more important role in the low- lying states, and in the very neutron deficient Pb isotopes we might even expect to find such species as "closed-shell" (82 protons) deformed nuclei by virtue of the distorting powers of the 1313/2 neutron states. The shell model calculations become more complicated as we get to even more neutron-deficient isotopes of Pb; however, the situa- tion becomes even more conplex for the theoretician because of the lack of good experimental data for these isotopes. In the 1950's much work was done on these decay schemes. This work was done using conversion electron spectroscopy and Hal (T1) y-ray spectroscopy. Hairever, because of the extreme complexity of these decay schemes and the limitations of the available tools, this work was only Partially successful and much work had to be left unfinished. With the advent of Ge(Li) detectors, it has become possible to examine get-ray Spectra of If additional information Ta: techniques of y-ra; deszribed in Qtapter 1‘. After selecting t? ttstudy, we felt that tiny schemes of the 11' ieizrj of the isomeric : 557917 useful, as the aliaited number of st basic framework on uh: Smut states in the 1 .' . ““3 5male particle the y-ray spectra of these isotopes in more detail and from this additional information to construct more complete decay schemes. The techniques of y-ray spectroscopy used in the present study are described in Chapter II. After selecting this general region of the nuclidic chart for our study, we felt that before anyone attempted to determine the full decay schemes of the lighter Bi isotopes, preliminary studies on the decay of the isomeric states of the correSponding Pb isotopes would be very useful, as the decay of the isomeric states populates only a limited number of states. These states could then serve as the basic framework on which to build the full Bi decay schemes. The isomeric states in the Pb isotopes are due to the presence of low- lying single particle 1113/2 states. This is true of the even mass as well as the odd-mass isomers. Originally, we proposed to study only the isomeric states of Pb from szo‘” to 913195“. Of these, odd— IIass isomers from Pb203" to Pblgm and the even mass isomers, PbZOL'm and szoz’" had been observed in the earlier studies. We were there- fore primarily interested in searching for the even-mass isomers szoom, Pblgem, and P1319601. Early in our search for szoom we were Bidetracked into a study of the decays of the neutron-deficient Pb isotopes to levels in the T1 daughters. However, we did complete 8tudies on several of the Pb isomeric states and these results of our studies are given in Chapter III. Our interest in the decay of szoo was stimulated by several factors. First, we were making a large amount of this isotopes in our attempts to find szoom and as no Y—ray studies of this decay sing Ge(Li) detectors right aid in tying “P 2:3 and the other oc‘ ans: favorable regions add-odd system, for t'. neighboring odd-mass m he results of our stun lapter IV along with a 1 "fi; 11171" in term of t? adjacent odd-A nuclei. In Chapter v and l using Ge(Li) detectors had been published, we felt that such a study might aid in tying up the loose ends in this decay scheme. Second, T1200 and the other odd-odd thallium isotopes are in one of the most favorable regions for explaining properties of non-deformed odd-odd systems, for the single-particle states in many of the neighboring odd-mass nuclei are reasonably well characterized. b200 are given in The results of our study of the a decay of P Chapter IV along with a discussionsof the structure of the states in T1200 in terms of the coupling of single-particle states in the adjacent oddeA nuclei. In Chapter V and VI we present the results of our studies of the e/B+ decays of Pb201 and Pb199 to states in their odd~mass Tl daughters. Because 81T1§gg and 81T1133 are only one proton removed from the Z-82 closed shell and have an even number of neutrons not too far removed from the ”-126 neutron closed shell, most of their low-lying states should be successfully described by the single- particle shell model. The proton single-particle states available in this region are 81/2, dg/z, d5/2, h11/2, and 97/1. Of these, only the 81/2, d3/2, and 615/2 states lie low enough in energy to be fairly "pure" single particle states. The systematics of this series are eSpecially interesting, as the neutron-deficient odd-A Tl isotopes I>lnavide one of the few series where we can observe the effects of successively plucking out pairs of neutrons on fairly "pure" single Particle proton states. Most of the isotopes used in the studies described in this theSis were produced by the bombardment of the appropriate targets rlt': proton or He3 bea. Energy Sector-Focused available were 50 NW ' :rtains a list of the My. 1‘ x vetoes, which were isotopes were produced his-mom and ground It: ' ’ .. the mchigan State 1 Y' ed | with proton or He3 beams from the Michigan State university Variable- Energy Sector-Focused Cyclotron. The maximum beam energies readily available were 50 MeV for protons and 70 MeV for He3. Table I-l ' contains a list of the principal reactions and their associated "Q"-values, which were of interest to us in these studies. The Bi isotopes were produced principally for use as "cows" from which the Pb isomeric and ground states could be "milked". We also made use of the Michigan State University "Triga" Reactor facility to produce many of the y-ray energy and intensity standards listed in Chapter II. Ql'alues for Re R \X‘ \ f‘ s i Valu: Reaction (he: i \ Eifékvhmm“ '20.l 5‘?6(;,4g)31233 mason”? sigma,“ "":l.=.2r.)tb20~n ":fl?’3't)Pb2:3m “H'Qritll’bzchr ”2.: ‘125 , “.“(Qts'ilpbzclm ~32 “"2(§.6r.)pb230 1 “Jih- . VMPb 2C3»; 1 l N .3Jh ‘ ..‘t'lhnbéUZm “:"(F:3‘.)Pb201,, ‘:.b’l'-)Pb230 1 I H C) "JPblssn K lemme /; s ! Valu 83 b 38ed 0n Table I-l Q Values for Reactions Used to Produce Pb Isomers Q values Q values Reaction (HeV) Reaction (MeV) Pb2°5(p,3n)312°‘* -2o.o T12°3(He3,2n)3120‘* - 6.3 sz°5(p,tm)312°3 -27.o . T1203(He3,3n)B1203 -13.3 ”206(p’5n)31202 -36.2 T1203(He3,4n)Bi7-02 -22,5 Pb2°5(p,6n)312°1 -43.8 T1203(He3,5n)B1201 -30.1 T1203(He3,6n)81200 -39.6 r12°5(p,2n)pb2°“m - 9.7 T1203(He3,7n)Bil99 4.7.2 T1205(p,3n)Pb203m -16.7 T1203(Hea,3n)31198 -57,1 112°5(p,4n)rb2°2’" -253 T12°5(p,5n)Pb201m -32.:3 H3200(He3,2n)Pb201m — 6,0 T12°5(p,6n)Pb2°° 4,0,0 HgZOO(He3,3n)Pb200 -12.7 HgZOO(He3,lm)Pbl99m -22.0 T12°3(p,n)Pb 203m - 2,4 HgZOO(He3,5n)Pb198 -29.2 r1203cp,2n)m,202m -1o.9 H3200(He3,6n)Pb197m -39.1 T1203(p,31)Pb201m -18.1 HgZOO(He3,7n)Pb196 —46,7 T12.03(p,ln)Pb200 -24.7 T1203(p.5n)Pb199m -34.1 T12°3(p,5n)eb198 4.1.2 Q values based on "experimental" masses listed in reference [My65]. EXP ERIE The construct: iesis required a great . . I P!" I ..L..ues. In this ch expose of each type of 1‘5 aPearattxs used. we 33 the electronics use: :-:"~ 4 4 .nptoved, making CHAPTER II EXPERIMENTAL METHODS AND DATA ANALYSIS The construction of the decay schemes described in this thesis required a great variety of experimental apparatus and techniques. In this chapter we have attempted to explain the purpose of each type of experiment and to describe, in a general way, the apparatus used. We have deliberately avoided a detailed listing of the electronics used in these experiments, as these are constantly being improved, making any such listing obsolete within a few years. The first expet 2.1. iaéetay scheme by y-r. extent is used to de: fine w-rays given off i :fz'ne singles experiment sizg 32(li) detectors , a ‘fiade use of a Si(Li) d A complete Ge( 3: 5 39“) detector b 15,: ..nected to a charge set "ailitier whose outpu: “I“ I a “793‘ ..er WtPUt goes on 2.1. ;y—R§y Singles Spectrometer The first experiment generally performed in the elucidation of a decay scheme by y-ray spectrosc0py is a singles experiment. This experiment is used to determine the energies and relative intensities of the y-rays given off in the decay of a radioactive isotOpe. Most of the singles experiments described in this thesis were performed using Ge(Li) detectors, although for very low energy (5-60 keV) y-rays we made use of a Si(Li) detector. A complete Ge(Li) or Si(Li) Spectrometer system consists of a Ge(Li) detector biased with a regulated high-voltage supply and connected to a charge sensitive field effect transistor (FET) preamplifier whose output goes to a linear amplifier. The linear amplifier output goes on to an analog to digital converter (ADC) which is connected‘with some type of memory or storage unit:that has an associated readout system. During the 4—year course of this work, there have ibeen many refinements in all of these components. Our "best" system 4 years ago contained a Ge(Li) detector with a photopeak efficiency of 0.422 (relative to a 3X3-in. NaI(T1) detector) and a resolution of >4.0 keV-FWHM for the l332-keV y from 0050. This was connected with a 1024 channel analyzer. Our present "best" system possesses a 3.61 efficient Ge(Li) detector with a resolution of 2.0-keV FWHM connected with an 8192 channel ADC interfaced with an XDS Sigma-7 computer. A definition of "best" system is hard to define as it depends on the purpose of a particular experiment, but generally we would like a detector with the best resolution and the highest e‘iiziancy connected to a etirespectrun at a rea~ efficiency connected to an ADC with enough channels to diaplay the entire spectrum at a reasonable gain. 9..) OJ having he .-:ays belong marineats, one 21:21: me relat We". ' . o ’ f‘ flip! A “..‘..2~‘uenhe "‘5? (iztegral c :35 :3 be invol “.533 they are i u". 410m mes. :' oh .3 ..e anti-Co; mi“! . '1‘ "h W For th ‘e an 0.427. or an “.1" "ln' "th ..cre 11.1 Show A, «a, V‘L “8 and z: ., ,. u 5‘?” N ‘38 Si: 1:? t'r 10 2.2. Ge(Li)-NaI(T1) Coincidence Spectrometer Having determined the energies and relative intensities of the y-rays belonging to a particular isotope from the singles experiments, one would then like to know something about the coincidence relationships between these y-rays. This includes determining which y-rays are 222 in coincidence with any others (anticoincidence experiment), which are in coincidence with any other y-ray (integral or any coincidence experiment), and for those y-rays found to be involved in a coincidence we would like to know with which y-rays they are in coincidence (gated coincidence experiment). In addition to these standard coincidence experiments with NaI(T1) detectors, we have also used other more specialized experiments such as the anti-Compton and triple coincidence [511 keV-Sll keV-any] experiments described below. 2.2.1. Anticoincidence and Anti-Compton Spectrometers For the anticoincidence experiments performed in these studies, we used 0.422 or 2.52 efficient Ge(Li) detectors in conjunction with. an 8x8-in. NaI(Tl) split annulus detector and a 3X3-in. NaI(T1) detector. Figure II-l shows the general setup for this experiment. The source is placed inside the annulus with a Ge(Li) detector in one end and the 3m3-in. NaI(T1) detector blocking the other end. The signals from the 'NaI(Tl) detectors go through cathode followers to correct for impedance adamatching and then to linear amplifiers. The amplified signals then 30 to timing single-channel analyzers (TSCA) to obtain a logic signal for the coincidence unit or the AND/OR gate. The TSCA's are also used ll ----.“____-_._____1 r uuuuuuuuuuuuuuuuuuuuuuu I nacasxe .ozssa..pp:¢z com sorrow» 02.2.... .msuouomem monogoowoofiuss o5 mo consumes: owumfinom ..oaam. uouuuuoe .Havaz .ahunxm was no caduceus“? on“. nouns muaoawuoexo messages can» use .aoueaoouwuam Johnson.“ on.» you one: o." nauseous? damn 3:5. one osnmu up<4ua m4<2o_m «on» cu.u_4m3< ¢21) will also be enhanced as well as transitions from such states going directly to the ground state. In addition to reducing the intensity of cascade transitions, the Compton background is also greatly reduced in the anticoincidence eXperiment, for any y-ray scattered in the Ge(Li) detector has a good chance of being picked up by one of the NaI(Tl) detectors. With a slight modification of the anticoincidence apparatus, we can obtain a'sizrles" spect statement of t'r. i:?ir.'re ll-Z an iiictegral coir. Fireplace the 3‘ the the son o” V‘s .' .-E .I:.( .1) annu‘. tithe Ge(Li) det "15iappens the s: the Ge(Li) de: :15 ‘a-w ....er of the '0:th aRector a "Esta; than thos 5 '. ~. mu ‘ Mud coup‘on e :2 Stud? of p55. ‘. :33: .. mad “as re t" a 53:2. “S are Set .... ‘ ml: :b‘." Q‘ns 1‘ SIIOI", s H‘G ‘. ’J v fit; 13 a "singles" spectrum with a greatly reduced Compton background. The arrangement of the detectors in this anti-Compton experiment is given in Figure II-2 and compared with that used in the anticoincidence and integral coincidenmeexperiments. In the anti-Compton experiment, we replace the 3x3—in. NaI(Tl) detector with a graded Pb collimator, and place the source inside the collimator so that it is shielded from the NaI(T1) annulus. In this configuration only y—rays Compton scattered by the Ge(Li) detector will be picked up by the annulus. But whenever this happens the gate will be closed and the Compton event picked up by the Ge(Li) detector will be rejected. It is also helpful in reducing the Compton background if the source is collimated toward the center of the Ge(Li) detector, as y-rays scattered in the center of the detector are more likely to be picked up by the rest of the crystal than those scattered near the edges of the detector. Such an anti-Compton experiment was performed with excellent results in our study of Pb201 (Figure V-S). In this experiment the Compton background was reduced enough to reveal several new transitions. 2.2.2. Integ§a1,,Gated, and Triple Coincidence Spectrometers The apparatus shown in Figure II-l can also be modified for obtaining integral, gated,and triple coincidence spectra. In the integral coincidence experiment the TSCA's associated with the NaI(T1) datectors are set to accept all y-ray energies above the x-ray, and the linear gate is used in the normal mode, that is, closed until Opened by a signal from the fast coincidence unit. Therefore, the spectrum one obtains should ideally contain only those y—rays involved in a cascade within the resolving time of the fast coincidence unit, usually .. Z? ... 1...... mozmo-oz-oo-I-Izq -m 1h .mucoE«uoaxo mononwocwoo Hmuwouna was .ooaovaoawooaucm .oouasoouwuc< :a new: mucoammamuum neuoouov new season mo mcoaumuumsaaw uwumaosom mozmo_oz_oo 4.+omo.omm . tetoocou ..ev_mz .c_-nxm . co+ne___oo on noumco . m3.3ccm to mo>.mI . mco;a_+_=so+oga . co+oo+ou ..mvoo . MDUUOM— .~-HH assume 20.55.00 .. E26. .6. E}30ns. The or: in Figure 7 22:21:15 and “30‘ its Csnpton I335"L .:i:g the 813-1?- ?ig‘xe I'M. The se is same as that 2:515 case we fixer-rays fal E;tseto the f 13min only tho Eternally cho I f V g ‘ s -.\‘al(ll) ~33. for the 133 “1‘ o <4 the interpr its k, 15 50-100 us. The orientation of the detectors used in this experiment is shown in Figure II-2. We have moved the Ge(Li) detector outside the annulus and removed the 3x3-in. NaI(T1) detector in order to reduce the Compton background. A typical integral coincidence experiment using the 8x8-in. NaI(Tl) annulus and the 0.422 detector is shown in Figure IV-4. The setup for the gated coincidence experiment is exactly the same as that for the integral coincidence experiment; however, in this case we set an energy window with the TSCA's so that only those y-rays falling within this window will cause the TSCA's to output a pulse to the fast coincidence unit. The resulting spectrum will then contain only those-y—rays in coincidence with this energy window, in which one normally chooses to include a-y-ray peak of interest. We used such a Ge(Li)—NaI(Tl) gated coincidence spectrometer in our integral studies of Pb200. However, the resolution of the NaI(T1) detectors were about 102, for the 1332-keV y-ray of C050, and this poor resolution made the interpretation of gated coincidence spectra very difficult, as each gate contained several y-rays. Therefore, after the 2-d Ge(Li)-Ge(Li) megachannel coincidence spectrometer was developed we abandoned the N81(1'1) detectors for obtaining gated coincidence spectra. As we mentioned in Section 2.1.1., the 8x8-in. NaI(T1) detector is a split annulus, having two optically-isolated halves. This allows us to perform triple-coincidence experiments. With the two sides of the annulus being used as the gates and a Ge(Li) :e:e:::r again it :ri;le-ccinciden trap: to obtai trifle-coincides. 573.: in Figure IaI’Tl) detector '54 3?. gate is r T” 33's assoc "3‘5 in the 51 at for detemi o'. , “Lt integral coi l6 detector again used to obtain the coincidence spectrum. Such a triple-coincidence setup was used in our study of Pb201 in an attempt to obtain the relative B+Ffeedings to the variouslevels. The triple-coincidence [511 keV-Sll keV-any] experiment used the apparatus shown in Figure II-l with the following modifications: The 3X3-in. NaI(T1) detector and its associated electronics is removed, the AND/0R gate is replaced with another fast-coincidence unit, and the two TSCA's associated with the split annulus are set to accept only Y-rays in the 511-keV region. The orientation of source and detectors used for determining relative B+Ffeedings is the same as that used in the integral coincidence experiment, Figure II-2. VJ . - 3 In the warrants (incl listing. At t‘ L21 coincide ::i-:ide:ce with 35: section, an ascziated elect {tartar-lived is 2E t} J! sources iticcincic'ence 9&3: to th ‘ H 331. 39in 17 2.3. Two-Dimensional 131 Coincidence Spectrometer In the elucidation of a nuclear decay scheme, coincidence experiments (integral, anti, and gated) are generally the most time consuming. At the beginning of the studies described in this thesis all y-y coincidence experiments employed one Ge(Li) detector in coincidence with one or more NaI(T1) detectors as described in the last section, and each experiment required the use of the detectors, associated electronics, and a multichannel analyzer. In addition, for shorter-lived isotOpes we required the repeated use of the cyclotron to make the sources. This did not present a real problem as far as the anticoincidence and integral coincidence experiments were concerned, as these were generally performed only once (successfully) on a given isotope; however, for a moderately complicated decay scheme one is required to set many gates,and these consumed the largest amount of time and effort in elucidating a decay scheme. we were rescued from this drudgery by the introduction of a program called EVENT, written by D. Bayer to run under the JANUS time sharing monitor of the Sigma-7 computer. Figure II-3 shows:UIflowsheet form the apparatus used in a typical two-dimensional coincidence experiment. When two photons are detected in coincidence, the linear gates are opened, allowing the output from detector 1 to go to the x-ADC while the output of detector 2 goes to the y-ADC. With the x- and y-ADC's operating in the synchronous mode, the signals going into the two ADC's must arrive within 1 usec of each other in order to be recorded as a coincidence event. The 8192- channel ADC's are interfaced to the Cyclotron Laboratory's Sigma-7 "“ -- ” T'I LINEAR AMPLIFIER AM’LIFIER .L___“___J LINEAR FAST SCA SCALARb—j * TMMNG AMPLIFIER LINEAR AMPLIFIER I I 1 COINC. SCA LINEAR GATE ‘ Figure II-3. X-SIDE 8|92 CHANNEL 0- 8V ADC I882 CHANNEL Y-SIDE ADC GATE O-8V SYNCHROATQWE .NTERFMCEOTO SIGMA 7 CHANNEL NO. OF EVENT FROM SIDE X CHANNEL NO. OF EVENT FROM SIDE Y BUFFER IN SIGMA 7 UNQEj "JANUS' CHANNEL NO, I I CHANNEL No Lg ONE WORD j MAGNETIC TAPE CHANNEL '0. OF EVENT I/Z WORD CHANNEL m. OF EVENT l/Z WORD Block diagram of the apparatus used to collect two-dimensional "megachannel" y-y coincidence spectra, using the Sigma-7 computer. mater, and the 21:11: a multipu: ‘ ”\- .:;i:a:ed buf fer : i‘mntents of t! :si::i:'e::-t ‘r-rays 2:23: in the cor: restitten to ta wan. :fi ‘R me: than t‘” I!.l h'uv The dat.‘ ME EDOII‘IEI‘ pm 21': event off th' itzuse up to 13 Iiztiience spect ’92: is, an Open Wang a peak :y- a: .gccuress an ’3- "M‘mS one-dim w: . 1‘4 , ' .or later p 33: «Hg “'0 G ( “uteri 19 computer, and the program EVENT reads the data from both ADC's through a multipurpose interface. The events are then stored in a dedicated buffer in the computer until the buffer fills, whereupon the contents of the buffer are written on magnetic tape. The two coincident y-rays are recorded by channel number in one word of memory in the computer and on the magnetic tape. Since the events are written to tape, this system cannot be used for count rates greater than 32000 coincidence events per second. The data on the magnetic tape are later recovered off-line using another program, event recovery [EVENT]. This program reads each event off the tape and separates the x and y addresses. It will then use up to 10 digital gates per pass of the tape to define coincidence spectra. Each gate can be either an "integral coincidence", that is, an open gate, or a selected gate of a few channels' width containing a peak of interest. The gates can be set on either the x or y address and background subtraction can be performed. The resulting one-dimensional coincidence spectra are then punched on cards for later plotting and data analysis. The first experiments with this 2-d megachannel coincidence system made use of the 0.422 Ge(Li) detector and a 3XB-in. NaI(Tl) detector. However, we soon abandoned this arrangement in favor of one using two Ge(Li) detectors. During our study of Pb200 decay we performed the first successful Ge(Li)—Ge(Li) 2-d coincidence experiment at M.S.U. For this first experiment we used the 0.422 and 0.672 detectors. As the work described in this thesis progressed, 5 detectors used 3512! used in the 3132 detectors V Besides wheat to obta as the additional jaizfor easy in”- ‘aterpretated back I: integral coinc awake an integ ”abate also used slightly in enetgj itchy gating on flight and loo w. ..acussion of h g: abZCI is give 20 the detectors used in the 2-d experiments became larger. with the final system used in the elucidation of the Pb”1 and Pb199 decay schemes having detectors with efficiencies of 2.51 and 3.62. Besides the obvious advantage of having to do only one experiment to obtain all the gated coincidence apeCtra, this system has the additional advantage of presenting all the results at the same gain for easy interpretation and allowing one to perform a linearly interpretated background subtraction of the adjacent background. The two integral coincidence spectra one obtains from this experiment may also make an integral coincidence run using a NaI(T1) detector unnecessary. We have also used this system for resolving doublets that differ only slightly in energy and cannot be detected by peak broadening. This is done by gating on peaks that food only different members of the multiplet and looking for a shift in the centroid of the multiplet. A discussion of how this technique was used in our study of Pb2°° and Pb”1 is given in Sections 6.3.2 and 5.3.5.d.,respectively. 2d. DI The pht ratant with en ergy. The en fa:rs,such as :5 depth to vidt iis-taace. Ther list-rays emitt éfiziency curve Effie: “15195 were de fitted tvo or . Ease sources I: EaSet with ea lather. This 3d 1“” finer; 3* t° 0min Prev 21 2.4. Determination of Photopeak Efficiency Curves The photopeak efficiencies of Ge(Li) detectors are not constant with energy but generally decrease with increasing y-ray energy. The exact behavior of this decrease is dependent on many factors, such as: The total active volume of the crystal, the ratio of depth to width of the active region, and the source to detector distance. Therefore, in order to obtain the relative intensities of the y-rays emitted from a source, we must have available a photopeak efficiency curve for the particular detector and geometry used. Efficiency curves for the detectors used in the present studies were determined by using a set of y-ray sources, each of which emitted two or more y-rays whose relative intensities were well-known. These sources were chosen to cover the widest energy range possible, and yet with each source overlapping, at least at one point, with another. This allowed us to bootstrap our efficiency curves to higher and lower energies. A list of the y-ray relative intensity standards used to obtain the efficiency curves is given in Table II-l. Previous to our work on the efficiency curves, the data were fit to a straight line on a log-log scale. However, we noticed a systematic deviation of our data from a straight line which suggested a 3rd order £1: of the form. insufficiency) - A + 31053 + cuogE)? + D(logED3, where A, B, C, and D are empirical constants and E is the energy in keV. A computer program for fitting the efficiency date to this equation was written by G. Giesler. Actually, this program divides the curves into two sections, calculating one curve for heme Photon Energy (keV) 32.1 36.5 661.6 66 52 86.8 (197) 22 Table II-l Y-Ray Relative Intensity Standards Isotope Photon Relative Ref. Energy Intensity Isotope Photon Relative Ref. Energy Intensity (keY) gkeV)_ c3137 32.1 6.85 a A3110“ 446.8 3.5 a 36.5 1.54 a 657.7 100 a 661.6 100 a 706.7 17.2 a 763.9 24.0 d 28160 46 116 a 818.0 7.8 a 52 28.8 a 884.7 79.6 a 86.8 100 a 937.5 36.5 d (197) (46.7) a 1384.2 27.7 d 1475.7 4.5 d Hg203 72 11.9 a 1504.9 14.8 82 3.44 a 1562.2 1.33 a 279.2 100 a 0056 846.8 100 e n£18°m 93.3 16.8 a 1037.9 14.3 e 215.3 80.6 a 1175.1 2.30 e 332.5 94.8 a 1238.3 67.6 e 444 83.0 a 1360.2 4.34 e 501 14.2 a 1771.4 15.8 e 2015.4 3.10 e 18177” 105.3 100 b 2034.9 7.95 e 113.0 184 b 2598.6 16.8 e 128.5 131 b 3010.2 1.01 e 153.3 144 b 3202.3 3.03 e 204.1 117 b 3253.6 7.39 e 208.3 512 b 3273.3 1.76 e 228.4 310 b 3451.6 0.875 e (233.8) (:48) c 3547.9 0.178 e 281.8 118 b (283.4) (=5) c 288 898.0 93 a 327.7 152 b 1836.1 99 a 378.5 240 b (385.0) (=25) c C060 1173.2 100 a 413.6 135 b 1332.5 100 a 418.5 172 b 466.0 20 , b Naz“ 1368.5 100 a 2753.9 100 a a 0.11. Lederer, J.M. Hollander and I. Perlman, Table of Isotopes, J. Wiley and Sons, New York 1967, 6th Ed. Table II-I (cont '3..".. Bernthal, .159; [CM-186 ‘LI. Eaverfiel 337 (1967). Edi. Brahmavar E--'-- 56:15, Jr a sa. hever, 1.- Cmraication 23 Table II—l (cont'd) b F.M. Bernthal, Ph.D. Thesis, University of California, Berkeley, 1969; UCRL-18651 (1969). c A.J. Haverfield, F.M. Bernthal.and J.M. Hollander, Nucl. Phys. A94, 337 (1967). d S.M. Brahmavar, J.H. Hamilton, A.V. Ramayys, E.E.F. Zganja13rand C.E. Bemis, Jr., Nucl. Phys. A125, 217 (1969). e R.A. Meyer, Lawrence Radiation Laboratory, Livermore, private communication. carries above 4 tea aomiizes The at 533.: in Figure efficiency gene Ls: falls off ' the aluminum ca '91 32 Crystal. 3. (2‘1 to the ELIE in {IN Wife [3867] 32:33 were C3 1: e ” ‘he 0119 sho 2h energies above 400 keV and one for energies from 100 to 400 keV. It then normalizes the two curves so they are continuous. The efficiency curve for the 2.52 detector at 2 in. is shown in Figure II-“u As we mentioned earlier, the photOpeak efficiency generally goes down with increasing energy however, it also falls off very rapidly below 100 keV because of absorption in both the aluminum can surrounding the detector and in the "dead" layer of the Ge crystal. Because of the difficulty in fitting the data below 100 keV to the rest of the efficiency curve, these points were not included in the calculated efficiency curve used in the MDIRAE E(I) program [Be67]. The intensities of y-rays or xrrays in this energy region were calculated by hand, directly from efficiency curves such as the one shown in Figure II-U. ' i V ' ' , a?" v 70.0 i 60.0 L S o RELATIVE EFFICIENCY 25 LO 'IUIIl 0.5 :FH'I'HI l 1117!” 1 [I 2.5% Ge(Li) DETECTOR AT 2.0 inches 1 LILJ 1141L111I I lllllll 1 ll 50 Figure Ilsa. 100 200 500 1000 2000 X“ RAY ENERGY Relative photopeak efficiency curve for the 2.52 detector determined using the sources listed in Table II—l placed at 2 inches from the front fact of the detector. i: :as .eco: :;"I .3. "lens 0 5:. h. g ...: yr‘z‘fin 'ngu" a: . "1‘s s “end. V :5;:u’“ ‘ ‘. "J: ‘ 1 I Aith‘ :IStgzs 1 the i5 26 2.5. Calibration of Y-Ray Standards With the development of high resolution Ge(Li) detectors, it has become possible to measure y—ray energies with a precision of better than 100 eV. This does not present any significant problem for energy determinations in the range from 100 to 1000 keV, for a great number of energy standards are available in this range, which are known precisely to tens of eV. However, the y-ray energies greater than 1 MeV, there are significantly fewer y—ray standards with this precision. We decided, therefore, to remeasure the energies of some common standards listed in the 1967 edition of the Table of Isotopes [Le67] in the region 1000 to 2000 keV. The Ge(Li) detectors used in these measurements had resolutions of 2.1 and 2.3 keV for the l332—keV peak of Co60 and efficiencies of 3.6% and 2.52,respectively. The spectra were taken with both a Northern Scientific 4096-channel ADC interfaced with the PDP-9 computer and a Northern Scientific 8192-channel ADC interfaced to the XDS Sigma-7 computer. Energy determinations were made by counting the "unknown" simultaneously with several of the standards listed in Table II-2. The centroid channels were determined using the "MOIRAE" spectrum analysis program. From the centroid channels of the standards a least-squares quadratic calibration curve was calculated and the energies of the "unknown" y—rays computed from this curve. Although we did not determine the linearity of our detector systems, the quadratic calibration curve was found to compensate for any non-linearity over the range of channels used. Because the Ilk‘.E(-rhcu. Kai; e \ A.V. Ra P523. ' L 27 Table II-2 1:3ay Calibration Standards Isot0pe Energy (keV) Reference 11 203mm") 583.14:0.02 a C3137 66l.61i0.04 Average of a and b Mus” 834.83:0.04 Average of b, c, d, and 9 Y88 898.0310.04 Average of b, c, e, and f C060 1173.23:o.oa a Na22 1274.55i0.04 Average of c, e, f, and 9 C060 1332.50:o.03 29 N32“ 1368.53t0.04 a Y88 1836.1310.04 Average of a, c, e,‘f, and g nzoeuhc") 2614.47:0.10 a Nazu 2754.08i0.08 Average of a and b Co56 846.7810.06 Average of b, h, and i 1037.89i0.07 Average of b, h, and i 1238.30i0.05 Average of b, h, and i 1360.25i0.05 Average of b, h, and i 1771.43i0.05 Average of b, h, and i 2015.37:0.06 Average of b, h, and i 2034.93i0.06 Average of b, h, and i 2598.5810.06 Average of b, h, and i 3253.63:0.06 Average of b, h, and i W a G. Murray, R.L. Graham, and 6.8. Geiger, Nucl. Phys. fig, 353 (1965) b R. Gunnink, R.A.'Meyer, J.B. Niday, Methods 22. 26 (1968). ° w.w. Black and R.L. Heath, Nucl. Phys. A90, 650 (1967). d H.H. Williams, E.K. warburton, K.W. Jones, Rev. £95, 801 (1966). Phys. Lett. gig, 49 (1967). and R.P. Anderson, Nucl. Instr. and J.W. Olness, Phys. A.V. Ramayya, J.H. Hamilton, S.M. Brahmavar, and J.J. Pinajian, Table 11-2 (con! O J. Leerand, J 11963). 5M. Write an L ‘!.E. Phelps, ' Pfilished. 3 ”LA. Meyer an Lab-emery, L 28 Table II-2 (cont'd) f J. Legrand, J.P. Boulanger and J.P. Brethon, Nucl. Phys. A107, 177 8 D.H. White and D.J. Groves, Nucl. Phys. A91, 453 (1967). h M.E. Phelps, D.G. Sarantites, and W.G. Winn, Nucl. Phys. to be published. 1 R.A. Meyer and D. Camp, Private communication, Lawrence Radiation Laboratory, Livermore, Calif. (1970). It zt-linearity iaznels. we a iezeziaations Tabl 1:55, SHE, 51 Have compaz the table. A] :8”: the fa? ‘8 ran u ;:e: ' 3195 for ( {he 29 non-linearity for most ADC's is most severe for the highest and lowest channels, we avoided the first and last 500 channels in these energy determinations. Table II-3 contains the results of our measurements on Zn65, Scke, 31207, and selected y—rays from Aglloni Ta182,and Gees. We have compared these with other recent measurements as noted in the table. Although we have not completed the energy calibration of Cass, the few y—ray energies we have measured are in better agreement with those of Phelps, Sarantities, and Winn [Ph70] than with the V unpublished results of Meyer and Camp [Me70]. This is worth mentioning here as the unpublished energies reported by Meyer and Camp for 0056 were found to be in excellent agreement with the other standards listed in Table II-2, when these were run as internal standards. However, when we ran the 0056 as an internal standard with the Gees, the resulting energies for Ga66 do not agree within experimental errors with those reported by Meyer and Camp. The energies of the 2751.99 and 3229.08- keV y-rays from Ga66 were determined using the pair-peak method, that is, we determined the energy of the double—escape peaks using internal standards as described above and added 1022.01 keV to obtain the full- energy y-rays. '— I 30 Table II—3 Results of y-Ray Energy Calibrations Energies (keV) Isotope Previous Standard Energies Present Work a b Tgble of Isotopes J.B. Marion g Sc“5 889.18i0.10 889.14t0.15 889.28:0.06 1120.41:o.1o 1120.29:o.25 1120.58:0.06 2n55 1115.44io.1o 1115.40:o.12 1115.57:0.06 31207 1063.58i0.06 1063.44:o.09 1063.64i0.06 1769.7110.13 1769.71i0.13 1770.22:o.os Ta182 1121.28i0.12 1121.31:o.os 1189.0310.20 1189.06i0.05 1221.42:o.1o 1221.42:o.05 c d Brahmavar et al. Legrand et a1. A8110“ 884.67to.oa 884.66¢0.04 884.68t0.04 937.48:0.04 937.46:0.06 937.48i0.04 1384.22:0.04 1384.35t0.06 1384.26:0.05 1475.7310.04 1475.8110.08 1475.7610.07 1504.90:0.08 1505.11:o.14 1505.01:o.o7 1562.22:0.06 1562.37tO.12 1562.35:0.08 e f Phelps et a1. Meyer and Camp 6866 833.46:0.04 833.65:0.08 833.52:0.06 1o39.2a:o.05 1o39.35:o.08 1039.20:0.06 2189.85:0.06 2190.24:o.15 2189.73:0.08 2422.75:0.06 2422.51:o.15 2422.60:0.08 2751.9210.06 2752.27:o.1o 2751.99:o.os(n) 3229.16i0.06 3229.37:o.2o 3229.08:0.15(D) a C.M. Lederer, J.M. Hollander and I. Perlman, Table of IsotOpes, John Wiley and Sons, New York 1967, 6th Ed. J.B. Marion, Nuc1.'naca,'ég, 301 (1968). S C. . Bemis, Jr., Nucl. Phys. A125, 217 (1969). .M. Brahmavar, J.H. Hamilton, A.V. Ramayya, E.E.F. Zganjar, E fable II-3 (con! ‘1. Legrand an: a”. Phelps, 2 ;ublished. M. Meyer an Laboratory, L 31 Table II—3 (cont'd) d J. Legrand and J.P. Boulanger, Compt. Rend. 265, 697 (1967). e M.E. Phelps, D.G. Sarantites, and W.G. Winn, Nucl. Phys. to be published. f R.A. Meyer and D. Camp, private communication, Lawrence Radiation Laboratory, Livermore, Calif. Alon e: lave been f feta analysis mining data 733331: Mm maids and 33C" into the 3:331 Card 9 35135 backg 756 Essential 26 531.0,): prc ifiétfaCed V11 26 sPectrum r aim“, a ata' Mark witch is use fer fitting t n ‘36 1 bamgmuz 32 2.6. Data Analysis Along with the rapid increase in data acquisition capabilities, we have been fortunate to have seen a similar rapid development in the data analysis routines available to us. The standard program for analyzing data at the beginning of these studies was the MIKIMAUS program written by G. Berzins [Be67]. This program calculated the centroids and intensities of photon peaks specified by control cards read into the computer along with the data deck. For each peak a control card was required, specifying the channel numbers of the low and high background intervals, as well as specifying the peak interval. The essential features of this program were later incorporated into the MOIRAE program using a live display scope with sense switches interfaced with the Sigma-7 computer. The scape is used to display the spectrum or a portion of the spectrum to be analyzed, the fitted background, and the difference between the background and the raw data. A marker displayed on the scOpe and controlled by a sense switch is used to specify the high and low background intervals used for fitting the background as well as specifying the peak interval. The background can be fitted to an nth-degree (up to n - 9) polynomial at the discretion of the operator. The great value of this program is that one can try different background intervals and different order fits until he is satisfied with the shape of the background under the peak. After subtraction of the background from the specified peak interval, the centroid is calculated using either the full width of the peak or only the upper two-thirds of the peak, again, at the discretion iv. :f the operatc an then be ox cards 0 taine: 1'1) [3867], t: :eztteids of ' fine from 9h iatensities 3 536ml eff 1 The 33130, a com; after subtral :0 a Caussiay Farmers 0' in: the Spa 532::01 d8 of his “er. Al . L 1115 not u 33 of the Operator. The centroid and the net area within the peak interval can then be outputted to a line printer and card punch. The punched cards obtained from MDIRAE are then used with another program, MDIRAE E(I) [Be67],to obtain the energies and intensities of the peaks. The centroids of known peaks are used to define a quadratic calibration curve from which the energies of the other peaks are calculated. The intensities are calculated using the areas from the MOIRAE cards and internal efficiency curves for the detector. The most recent addition to our data analysis routines is SAMPO, a computer program developed by J. Routti [R069]. This program, after subtracting the quadratic background, fits the experimental peaks, to a Gaussian function having exponential tails using internal shape parameters or shape parameters calculated from singlet peeks chosen from the spectrum being analyzed. This program outputs the area and centroids of all the statistically significant peaks via the line printer. Although this routine is very useful for stripping multiplets, it was not used for general spectrum analysis in these studies because we had no control over the background which was fitted by a quadratic curve,snd in many cases made for a poor fit. A more detailed discussion of the SAMPO and MOIRAE programs is given in theses by J. Cross [Cr70] and R. Eppley [Ep70]. THE DEC:l ll ' lived eetastab see-even isor states, here 7 tale to other 35 saeral of State in F152? CHAPTER III THE DECAY SCHEMES OF SOME NEUTRON—DEFICIENT Pb ISOMERS All the odd-mass Pb isotopes known below N - 126 have long- lived metastable states based on the i13/2 neutron state. The three eveneeven isotopes Pb205, szo“, and Pb202, also have reported isomeric states, here 7- or 9- states resulting from coupling the i13/2 neutron hole to other holes. In this chapter we report on our investigations of several of these isomers as well as our search for an isomeric state in Pb2°°. 3h m? previou atellent su it will not Sine relie 521: that b) ‘5‘ trarsitl' 35 3.1. The DecaygScheme of szoum 3.1.1. Introduction The decay scheme of 66.9 m PbZOWW has been the object of many previous studies starting with Sunyar in 1950 [Su50]. An excellent summary of this previous work is given in reference [Hy64] and will not be repeated here. As the previous work on this decay scheme relied on conversion electron and NaI(Tl) y-ray detectors, we felt that by using GeOLi) detectors we might be able to observe some new transitions in this decay. However, in our brief examination of PbZOHM several years ago, we failed to detect any new transitions, and the decay scheme remains essentially the same as that proposed by Fritsch [Pr56] in 1956 and shown in Figure III-9. This decay scheme is also consistent with the results of J. Cross in his study of 312°“ decay [Cr70]. However, Cross prOposed several states in szo“ that lie below the 2185.4-keV isomeric state and could possibly be populated in the decay of Pb2°“m. With this in mind, a re-examination of the szoum’decay scheme with the larger Ge(Li) detectors now avail- able may reveal some of these transitions and help to establish more definite spin and parity assignments for these levels. Although we were unable to detect any new y-rays in our study, we did make direct measurements of the xhconversion coefficients for the 899.2-kev E2 and the 911.7-kev ES transitions in Pb2°“m using a unique single-crystal Ge(Li) conversion coefficient spectrometer [Gr69]. 3.1.2. The G The a: the P‘..S.L'. frift device I: Has Mun“ :35 all Have Minted 1n 53'- 50! the The E16“30215 Her E in“?! uni 1.: diameter thread on Al .‘I efficiEnc listed in Tat manned PI 36 3.1.2. The Ge(Li) Conversion-Coefficient Spectrometer The Ge(Li) detector used in this experiment was manufactured at the M.S.U. Cyclotron Laboratory by C. Maggiore. It is a planar drift device with a drift depth of :71m» and an active volume of 2'0.4 cm. It was mounted in a conventional dipstick cryostat with a window of 0.25 mil Havar foil. The orientation of the detector and source is indicated in Figure III-1, The detector has a resolution of 1.8-keV FWHM for the 661.6-keV y-ray of Cs137. The relative efficiencies for both y-rays and conversion electrons were found to depend sensitively on source shape and position. To insure uniform reproducibility, the sources were vaporized through 3~mm diameter masks onto 0.25-mil Pt foil backings. These were mounted on Al rings which fit into the slot shown in Figure 111-1. The efficiency of the detector was calibrated by using the standards listed in Table III-1, whose conversion coefficients had been determined precisely. An efficiency curve was also obtained using theoretical conversion coefficients from the table of R. S. Hager and E. C. Seltzer [Ha68]. The intensities of the electron peaks were determined by first taking a spectrum of both electrons and y-rays, as shown for Ca137 in Figure III-2a. Then an A1 absorber was placed between the source and the detector to stop the electrons, and a spectrun*was taken'which contained only the y-rays, as shown in Figure III-2b. 'Ey overlapping the spectra and subtracting, the conversion electron intensities‘were easily obtained. The efficiency curve for the present detector, obtained by ---------é SLOT FOR souscs muss “LU“"W CW“ I summon son. . b ' some: ALUMINUM mm 3.28 clap— sLor FOR ems souscs mus assumes macros scum Figure III-1. The Ge(Li) conversion-coefficient spectrometer showing the orientation of the crystal and the source mount. COUN TS/ CHANNEL 38 I l I 1 I I j I"! IS? - Cs 6 t3 ‘5' . ‘Ww'. I“. ..I‘D- Io3 - ‘°"' .. I \ b a «VI-...". I o a! ‘0 1 W.~.*~‘~ ..., .I. \ 8 T‘ ~12 3 . 0 I 0 ’~. h . .°. 05"“... l i “I 1 w, . . . ., . ...,V. ° ° 4' - . 7va3.35 "9"" l : o/ll I02 ~ ' d ‘> o. o... .0: _ 3? ? ‘h‘.yay~UhaWWVV3%hfiflbrflozfi691;55wgufluvfii f\ to2 - be... ; /’l " 1 1 J I I i J l80 200 240 280 320 360 400 440 Figure III-2. CHANNEL NUMBER ’ Top: Cs137 spectrum showing the 661.6-keV y-rgy7and its K and L conversion electrons. Bottom: Cs spectrum taken with an A1 absorber between the source and the detector to block the electrons. By subtracting the bottom spectrum from the tap one a clean electron spectrum can be-obtained. i. Todd is sh efficiencies (\ meition, ( “ms and e] 5M Figure 1 Secau5c. the r efficiency cc Hays alane‘ :ffSEt Stale 3.1.3. ‘ w The we made by Free 39 R. Todd is shown in Figure III-3. It takes the form of the relative efficiencies for y-rays and electrons, i.e., D D e-) ’ C‘ vs. the y—ray energy, E7. To obtain the conversion coefficienct of a transition, one merely obtains an experimental intensity ratio for the y-rays and electrons and multiplies this by the corresponding ordinate from Figure III-4, i.e., o I C ff:_ . AY Because the efficiency for electrons varies very slowly, the above efficiency curve is very nearly the same as the efficiency curve for y-rays alone. The latter curve is presented in Figure III-3 with an offset scale for comparison. 3.1.3. Source Preparation The sources used for our conversion coefficient measurements were made by bombarding separated isotope Pb2°5(97.21) with 30 MeV protons from the M.S.U. cyclotron to induce the reaction Pb2°6(p,3n) 312°“. The 812°“ has a half-life of ll.2-h,decaying by electron capture to states in szo“ with 3132 of the decays populating the isomeric level. The B120“ activity was chemically separated from the Pb target (Appendix A), taken up in 6M'HCl,and loaded onto a heated 1.5 mm diameter by 5 cm long Dowex IXB ZOOemesh anionrexchange column. The B120“ activity stays on the resin,while the szown activity can be eluted with 0.3M"HC1. The szoum activy was allowed to build up ho IO- C= D (7V0 (6') '3 l0 :02 Figure III-3. ENERGY (keV) Efficiency curves. The solid curve represents the composite y—ray-electron efficiencies. Triangles are calculated from theoretical conversion coefficients, x's from experimental ones. The dashed curve shows a (displaced) y-ray efficiency curve. cl” ’b N i: the calm mmuoh 3.25 mil Pt were able t: mersion c the Y-rays a the With a there“ 01 ileum pe ‘ltttnm. "tabulated 91:. ickev hl in the column for 1 hour before being eluted. The few dQOps of activity obtained were then vaporized through 3dmm diamter masks onto 0.25 mil Pt foil backings. As the Bizou‘has an 11.2 h half-life, we were able to obtain many sources from one bombardment in this manner. 3.1.4. Experimental Results Figure III-4 shows two spectra of Pb2°“m taken with the Ge(Li) conversion coefficient spectrometer. The upper spectrum shows both the Ybrays and conversion electrons, while the lower spectrum was taken with an Al absorber placed between the source and detector. The areas of the 899.2- and 911.7-keV Ybray peaks and conversion electron peaks were obtained by hand stripping these from the upper spectrum. Using the efficiency curve shown in Figure III-3, we calculated the KLconversion coefficients of the 899.2—keV E2 and the 911.7-kcv E5 transitions. Our measured values are given in Table III-2, where they are compared with the theoretical values of Hagar and Seltzer [Ha68]. We also included the KIL ratio for the E5 transition, however, we couldn't extract a reliable X/L ratio for the E2 because of the complexity of this portion of the spectrum. ‘1 DUUN I a I vs Unvvu v...— l0 h2 204%". 5; Figure III-4. CHANNEL NUMBER szow" spectra obtained with the Ge(Li) conversion- coefficient spectrometer. Only the Pb2°“m transitions are labelled. Top: Spectrum showing both y-rays and electrons. Bottom: Spectrum taken through an Al absorber to remove the electrons. * L ‘5 ii; WJ UM’AMMM‘ l‘ \P P l 3”! L111}, 1;”: {gt-c \1 4R 5. - “awn A —. N Maw,“ a; b . 1\\'§=‘.‘\. git” I00 200 300 400 \ E Sulfide y-er h3 Table 111—1 Conversion Coefficient Calibration Standards Multi- “K a a Nuclide y-energy polarity experimental Ref. theoretical 30113 391.7 M4 b.38*O.OBXIO‘1 b 4.48 10"1 c813“ 795.8 E2 2.46*O.BOXIO’3 c 2.58 10'3 0313” 1365 E2 6.8 to.5 x10‘“ c 8.19 10‘“ 03137 661.595 M4 8.94*0.1OXI0‘2 d ~9.28 10'2 31207 569.6 E2 1.56:0.o7x10‘2 e 1.59 10‘2 81207 1064 M4 9.0 10.9 x10'2 e 9.70 10'2 Table 111-2 Pb2°“m Conversion Data Transition.MHulti- ex “K .K/L K/L 8 energy polarity experimental theoretical experimental theoretical (keV) 899.1 E2 o.oo72*0.0022 0.0065 --- --- 912.0 ES 0.0545to.0045 0.046 1.66*0.25 1.73 a R.S. Eager and E.C. Seltzer, Nuclear Data é§_(1968) l. b J.H. Hamilton, cited in C.H. Lederer, J.M5 Hollander, and I. Perlman, Table of'Isotopes, (J. Wiley and Sons, New York, 1967) 6th Ed. c R.A. Brown and G.T. Ewan, Nucl. Phys. 68 (1965) 325; W. Van Wyngaarden and R.D. Conner, Can. J. Phys.'§g_(1964) 42. d J.S. Merritt and J.G.V. Taylor, Anal. Chem. 21 (1965) 351. e K. Siegbahn, Nucl. Phys. 52§_(1967) 63. l.._ :nbabiliti :mMmt unperhaps iefcmatiOI iamdecapol xmubm smshm agesite pa mnuw mmhm :rs'aabilitj mmum M 3.2. P303" Decay andMé Transition Probabilities $2.1. Introduction Among electromagnetic transitions, the reduced transition probabilities of M4 transitions appear to be remarkably regular and consistent with the predictions of single—particle estimates. This can perhaps be explained partly by the fact that collective nuclear- deformations are not likely to contribute appreciably to a magnetic .hexadecapole field. Also, most known M4 transitions are isomeric transitions just below major closed shells and involve the high-spin , 9tates depressed by spin-orbit coupling from the next higher, _ opposite parity oscillator shells. Consequently, it is difficult for these transitions to be enhanced by suitable admixtures in the single- p‘r ticle states. When one encounters an M4 reduced transition prO‘bability that is abnormally large, one is thus tempted to look for ‘Q experimental explanation. Such was the situation with Pb203'". The decay of the ‘6.1 sec isomer Pb2°3m has been the subject of a number of investigations [St60] [Pr61] using conversion electron and Hal (Tl) spectrometers. These studies found only a single t"~‘Qnsition of about 825 kev, which was assigned an M4 multipolarity . on the basis of conversion coefficient ratios and the mean lifetime of the y—transition. The transition was assumed to go from a 13/2+ §tats to a 5/2- ground state, similar to those in the other odd-mass 1§Qmers of Pb. Because the MI: reduced transition probability was QBnormally large, Stockendal ISt60] suggested the possibility of an aSlclitional S-keV transition from the isomeric state to a state at 330 keV, whic‘ Relatively 11 ms undertake éifficulty oi Elf-life. 1 isvestigator: 77’2- [3058] transition 0 Sii-ke‘,’ 13/2 Std? V8 Obs B. “V? diffic- i" the 820 ._ 13113, the F F831}: "0“] After this 'ieta'g and l [313.“ I] do 1‘5 820 keV, which is populated in the electron capture decay of B1203 . Relatively little was known about the decay of 31203 when this study was undertaken, both because of its complexity and because of the difficulty of preparing B1203 free from B120” , which has the same half-life. Nevertheless, some progress had been made, and various investigators reported the spin and parity of the 820—keV state as 7/2— [N058] or 9/2- [St60]. If the level were 9/2—, the 5-keV transition would be an M2 and could possibly partially de-excite the In this 82S-keV l3/2+ level along with the 825-keV M4 transition. stl-Idy we observed such a branching decay. Because a 5—keV transition would be highly converted and Very difficult to observe directly, the easiest approach is to look f°1= the 820—keV transition that would follow the 5-keV transition. Thus, the presence or absence of an 820-keV transition in the decay of P5203” would imply the presence or absence of the S-keV transition. Af ter this work was completed, J. Cross [Cr70] made a study of B1203 dQcay and may have observed this S-keV transition directly using a th :ln-window Si (Li) detector. 3\~3.2. Experimental Method and Results Separated isotope Pb205 in the form of Pb(N03)2 was b0‘Inbarded with 40—uev protons from the Michigan State University chtor—Pocused Cyclotron to produce 312°3(t1_/2-12 h) through the r§8¢t10n Pb2°5(p,4n)312°3- The 31203 activity was separated chemically fTom the Pb target (Appendix A), taken up in 6M.HC1 and loaded onto ‘3 heated 1.5 m diameter X5 cm long Dowex 1X8 ZOO-mesh anion-exchange 201:0. Phi: every 6 sec : ma 7 013 3.7 keV my tesclve the inefully a] T} {filth Unav. bang place The SPGCtru flea“: shc dztaiued at 718M r918! i‘m Jen} and l Cam‘sim 516115 and internal c is Ca? ache 1| 94 the 355 t' . . ‘MltiO: h6 column. szomn was eluted with 0.3M HCl, at the rate of two drops every 6 sec for a 2-h period. Each drop was immediately transferred to a 7 cm3 Ge(Li) y-ray detector. The resolution of our system was 2.7 keV FWHM for the 825.2-keV y-ray from Pb2°3m, and this could easily resolve the 820.1 and 825.2-keV y—rays from B1203 decay and could hOpefully allow us to detect any 820.1-Rev y-ray from Pb2°3m . The spectrum in the region of interest of the 31203 activity (which unavoidably contained some B120“ with the same lZ-h t1/2) before being placed on the anion-exchange c'olumn is shown in Figure III-58- The spectrum of the szoam after separation, shown in Figure III-5b, clearly shows a peak at 820.1 keV. The areas of the two peaks obtained after stripping and correcting for the detector efficiency yield relativey-ray intensities of 10.611.02 and 89.411.01 for the 820.1 and 825.2 keVy-rays, respectively. Using the theoretical conversion coefficients of Sliv and Band [8165] for the K and L shells and those of Rose [R058] for the M shell to correct for internal conversion, the 13/2+ state is found to decay 82 by the 5.1- keV transition and 922 by the 825.2-kev M to transition. Our preposed decay scheme for szoam is shown in Figure 111-6. The previous uncertainty in the assignment for the 820.1-keV state can be removed. The 5.1-keV transition is anM2, making the state 9/2-. To check the possibility that some of the 81203 might have sz 0 3m and been eluted from the anion-exchange column along with the thereby have caused the 820.1-Rev peak, the drops of eluted activity were saved and counted after the szoam had died away. No B1203 activity was found in these drops. m < nm'O' 0 “ON! V221< 1 Km! 1 - w. men scans :omam maom mafiawmuaoo mansvfio>esav momam Am nwo aauuoonm mean» any .ucouma woman mug scum coaumumaom HmowEmno nouns Emomam A; "Aowwfilmaon aces mmmZDZ ..mzzlqlo 00m 000 02. com P # «v- .mIHHH ouawwm 000 fib h? 7238 1‘ 4 db t “9090'“ . eoa.n0fl no. 4 too. 'IBNNVHC) 83d SanOO Tfil. 8252 M 820.: 8252 M4 BZQI E2- 512—; AL 0 2‘)st 52.: h ’ Figure III-6. The decay scheme of Pb2°3m as determined from the Present study. Stc squares of tt in the odd-m2 They found t} than the eve Mable In 313.3 the mo transitions ml? in 11 regularly, a ’49 Stockendal and his co—workers [St60] [St55] calculated the squares of the radial matrix elements for the ixy24~fglz Mfi y—transitions in the odd-mass Pb isomers, using Mnszkowski's approximations [Mo53]. They found the IMIZ value for the transition in Pb2°3m to be 15% larger than the average of the others. Their later values [St60] are listed in Table III-3, along with those that we calculated in the same manner, using the most recent partial half-life values for all the MA y— transitions [By64]. The IMI2 value for the Pb2°3m transition now fits nicely in line with the others, indicating that this transition behaves regularly, as one would expect for.M% transitions. LA. ..— 50 Tab le II I-3 Radial Metric Elements for M4 y-transitions in Odd-mass Pb Isotopes w t 197 199 201 203 205 ' 207 11412 Stockendala 3.8*0.5 3.8*O.2 3.5:0.2 4.3¥0.1 3.710.1 Present ca1c.a 3.6*0.4 3.7to.2 3.5t0.2 3.8t0.2 (1812)b 3.6*O.1 a We have used our own experimental data for the szoam calculation and the most recent half-life values tabulated in ref. [Hy64] for the other calculations. b The value of [MI2 for szosm is inexplicably large, perhaps because of the presence of two unresolved lOlé-keV transitions, and is included only for the sake of completeness. See ref. [St60]. 3.3.1. Intrc Tl. Feeier, Raps .. 1'.“ '5 in th “‘3? and la :t-m‘ters ! thee as 31 mined Pith tentativgh. “ism th ‘0 A,“ e: ficw‘fi “310:1 3 51 3.3. The Decay of Pbgozm 3.3.1. Introduction The 3.62 h isomeric state in Pb202 was first reported by Maeder, Wapstra, Nijgh, and Ornatein [MaSé] in 1954. They produced szoyn in the bombardment of a T1 target with 254MeV deuterons. This study and later studies by Bergkvist and co-workers [BeSS], McDonell and co~workers [Mc57], and Johansson [J059] have resulted in the decay scheme as shown in Figure III-7 with the exception of those transitions marked with a crossed-dotted line and the 1552-keV level which were, tentatively, added in the present work. Although the decay scheme of szoz" is similar to that of szomn’ the isomeric level being 9- in both cases, it does differ from szom" in that some de-excitation by E decay to T1202 is observed. 3.3.2. Source Preparation In our initial examination of this decay in 1967, we tried to measure the conversion coefficients of some transitions in the decay of szozm using the conversion coefficient spectrometer described in Section 3.2. The sources were made by bombarding natural T1 foils with l7€MeV protons to induce the reaction T12°3(p,2n)Pb2°2. Our attempts to measure the conversion coefficients reouirad carrier-free sources. These were made using the chemistry described in Appendix D and vaporizing the carrier-free Pb202m onto 1 mil Al backings as described in Section 3.1.2. The y-ray singles measurements were done on sources made from both natural T1 foils and enriched T1203 (702) targets bombarded with l7-MeV protons. In this case we used the PbSOu precipitation procedure described in Figure III-7. S2 Decay scheme of szozm. The transitions marked with the crossed-dotted lines were added in the present study. The transition intensities given here are y-ray intensities only. The intensity of the 129.09-keV transition may seem improbable, however, as this is an E4 transition, it is very highly converted. eds Cl L at. e \ a 6 O 1nl .. .ll Nu m lfllflmt'laW I 7 Te... 53 Nm 8~ . . 1 ..Qunuld . ua.\1lq W q 3.“. a W: FE 2.3 flu. Alla 23 m a a It: .. .1 fl b: . a + . . “flu/r1 5o.» m e 1.". I. m _ mm ..- 1 1 won an m m m x. , Emow 1:de 5 t‘ 'e‘ ttl" P”? if :he ion-e. the precipit ”3. Etta N tea infomat ..i the spec tare alzost Ifiks cverla inestigati: fem that 53?. the de . ‘1‘ ‘ ..te ..eities i: the We; V? 531‘“ sq There III- 5h Appendix B to separate the szomn from the T1 targets. We have also recently prepared szom" from the decay of 1.6 h Bi202 using a combination of the ion-exchange procedure described in Section 3.1.3. (Appendix C) and the precipitation procedure described in Appendix B. 3.3.3. Experimental Results and Discussion The initial examination of this isomer in 1967 revealed no new information on the decay scheme, as the detectors used were very small and the spectra obtained using the conversion-coefficient spectrometer were almost impossible to interpret, as the broad conversion-electron peaks overlapped many of the y-ray peaks. We, therefore, abandoned our investigation of this decay scheme until several months ago, when we found that several of the y-rays from szozn decay overlapped with y-rays from the decay of szo1 (Chapter V), and we needed accurate y-ray intensities for szozm in order to subtract these from the intensities in the szo1 spectra. Much to our surprise, in this brief examination we found several new y-rays which seemed to have a =3 to 4 h half—life. Figure III-8 shows a typical spectrum obtained in this experiment using the 2.52 efficient detector. Table III-4 contains a list of the y-rays found in this examination along with their relative intensities in the 8ingles experiments. The 124.80-, 211.96-, 417.13—, 601.92-, 954.43-, and 1002.78-kev y-rays were previously unreported. Although we have yet t0 perfOrm coincidence experiments on this isomer, we have been able to ulake some speculations on the placement of certain of these y-rays. These y-rays are shown in the decay scheme, Figure III-7 as the crossed-dotted lines. Work on this isomer is continuing with both y—y and y-x-ray two- dimensional experiments planned. The y-x-ray coincidence experiment is feigned to a :tansition de 55 designed to allow us to assign the transitions either to the internal transition de-excitation or to the a decay branch to T1202. (I: 1.. 1 n a ... .1. ... - ....wz . a). 111.1 .6 .1, pet... 56 .ANoCmomfiw convince so cowuooeu Qaflmv m5 .3 wounmouo mos 8:30on ecu aHmun—o ou mom: .mousom one .uouomumu Aaavmo unusuaumm Nm.~ m an emeuoomu Ewownm mmmzsz mmzzolcm>m cooox 0:0 «0 mwamsom amuse mom mowumaoumwm mlHHH madman eoNndeN_ eoNede. oiijeli .----.o ol. .6 . mm mm mom mam meow . 1 . ...N m.mmmi..-..-..-- , .N 3 as Na .00 .2 @er , 4 ....m QEN" ,4 .e .5 .2 nvn mhmw m.mmm_--..--:_.;i- ...e e.nen_-...a,_-..._s.... ---...e .2 $33 «7 En now mm mm ..mmmTLdu1PEu .....v Na NNo .. e _ c . nON 9n mooNN-iL-§.-i..LIL Ema! - ism mq* . mo. EooNnn. Em hm EvoNaQ NONDQON_ O t lieo Nu .w¢ v;mml. - +N Nw NNv amen. meme. -..i R 56! activi 60 we set an upper limit of about 1 sec on the half-life of any new activity. We next attempted to populate an isomeric state in Pb from the decay of 81200. This procedure was similar to that used successfully in our study of 6.1-second Pbgoy" (Section 3.2). 35~minuta 81200 has an estimated electron capture decay energy of 6.5'MeV and a ground state spin of 7 and should therefore p0pu1ate high spin states in szoo. Biz00 was produced by bombarding separated isotOpe T12°3(7OZ) with a SO-MeV He3 beam to induce the reaction T12°3(He3 ,61)B12°° . The 81200 was chemically separated from the T1 target (Appendix C) and loaded onto an anion-exchange column. The Pb2°° activity was eluted with 0.3V HCl periodically and counted using the 0.421 efficient Ge(Li) detector. The rate of elution was varied for several different runs, the fastest rate being several draps every 5 seconds. However, we failed to detect any evidence of an isomeric state in Pb2°° longer than =1 second in these experiments. With the recent deva10pment of a helium thermalizer and jet transport system for nuclear recoils at the M.S.U. Cyclotron Laboratory [8069], we should be able to extend our search for szom" into the willisecond range. Delayed coincidence experiments, similar to those used to measure the 4-msec half-life of szosm [Be60], would be another method for reaching into the millisecond range. However, our failure to find an isomeric state with a half—life longer than :1 sec0nd indicates that a high spin state, such as the 7- state in szoem, falls below the 9- state, destroying any possibility for a long-lived isomeric state a of the ele 11d nucleu titanium a pmParties 513188 in :hdfaflerj CHAPTER IV THE ELECTRON CAPTURE DECAY or Pb200 4.1. Introduction In this chapter we present the results of our investigation of the electron capture decay of 21.5-h Pb2°° to states in the odd- odd nucleus 81T1112° This isot0pe and the other odd-odd isot0pes of thallium are in one of the most favorable regions for explaining properties of non-deformed odd-odd systems, for the single particle states in many of the neighboring odd-mass nuclei are reasonably well characterized. The first published study of the decay of Pb200 was by Bergkvist and his co~workers [Be55] in 1955. They bombarded natural T1 with protons and investigated the conversion electrons in the energy region 10-1600 keV with a double-focusing B spectrometer. They measured the half-life of Pb200 to be 21510.4 h and assigned 10 transitions to its decay. In 1956, Gerholm [Ge56] proposed a decay scheme consisting of four excited levels at 148.0, 257.3, 289.5, and 525 keV; these were placed on the basis of electron- electron coincidence experiments. Astrom, Johansson, and Bergstrom [As57] next measured the relative conversion-eleCtron intensities more precisely and also measured the half-life of the 148-keV state to be 8 nsec. (The half-life of this state was later determined to be 7.110.15 nsec by Johansson, Alvher,and Zuk. [J059]). They characterized the 168.0-kev transition to the ground state as a 61 wpure" 52 l I” 0- f4 assigned 2‘ there the an atom: fitted and lines 91:11 Imitation Electron c 15 transit for 10 of UOtters d; 257,15, 2‘ metal t1 decay of ' :fiained . ”in and 62 "pure" E2 transition, which allowed them to make an assignment of I 7r - 0- for the 148-keV state. (The T1200 ground state had been assigned 2- on the basis of its decay preperties [He57] to 38200, where the spin of 2 had been established previously by atomic spectra and atomic beams methods [M61]. The latest work on the decay of szoo was carried out by Wirhed and Herrlander [W162] in 1962, who studied the conversion lines with flat-field permanent magnet B spectrometers having energy resolutions as goOd as 0.1!. They also performed some electron- electron coincidence experiments. Conversion lines corresponding to 15 transitions in T1200 were found, and multipolarities were assigned for 10 of these on the basis 0f conversion-coefficient ratios. These workers devised a decay scheme including excited levels at 147.61, 257.15, 289.11, 289.92, 450.5, 525.6, and 605.3 keV. They also made “ever-‘31 tentative spin and parity assignments for these levels. Thus, although a reasonable amount of information on the decay of Pb200 had been obtained and assembled, many uncertainties mumZlned. These included the log ft's, several multipolarities, and 3P1“ and parity assignments for several states. One of the more facelusting problems concerned the verification of two closely spaced levels at 289.1 and 289.9 keV proposed by Wirhed and Herrlander [“162] but not reported by previous workers. In addition, as no Y‘ray gtudieg of Pb200 decay had been published, we felt that y-ray s pactroacopy using Ge(Li) detectors might allow us to find new r anaitions. This was a large order to fill, especially for the "“911 , poor resolution detectors used in the initial experiments, for an we were con where the 1 Eternal cc :bserved a: :anincing accessfuli 63 we were cmeting with B spectrometers in an energy region (<1 MeV) where the resolution of these devices are better than 1 keV and The fact that we internal conversion intensities are highest. observed any new transitions at all in this worked-over isotope is convincing proof of the ability of the Ge(Li) detectors to compete successfully with B spectrometers in this energy region. 5.2.1. It previous 1 My trar tantalum if [[1230 t atergy the mama: teal of ti M1203 a 3m f°un< ‘2‘), Phi 5f szmm :mer Vir . DUI 6h 4.2. Source Preparation 4.2.1. Introduction Since the decay of szoo had been the subject of many previous investigations, it was obvious that if we were to observe any new-y transitions from this decay we would need sources free from contaminating activities. This was espedially critical in the case of szo0 because all the y-rays belonging to this decay are lower in energy than 605 keV and the underlying Compton distributions fromt contaminant y-rays tended to hide very weak peaks. Therefore, a great deal of time and effort was spent trying to make impurity-free sources. 4.2.2. T1203(p,4n)Pb200 Our first attempts to make Pb200 consisted of bombarding natural Tl foils (29.52 T1203 70.5% T1205) with 34-MeV protons from the Michigan State University Cyclotron to induce the T12°3(P:4") Pb2°° reaction. Bombarding times varied from 1-5 hours depending on the beam current. This energy was chosen because it is just below the threshold for producing Pb199. However, we still produced in significant amounts all the possible Pb isotopes from (p,xn) reactions on T1203 and T1205, where 4ea21. y-rays from the following isot0pes were found in the freshly bombarded sources: Pb2°“m(67 min), Pb203 (52h), p62°2"'(3.6h), and Pb2°1(9.4h). Because of the short half-lives of Pb2°‘*m and szoz’" as compared to the 21-h szoo, it was an easy 'matter'virtually to eliminate these by aging the sources for 2 or 3 days. During this 2 or 3 day aging the 9.4-h Pb2°1 was also reduced to an acceptable level, although several of its more intense Y-rays could a were gr. 1 arobl since t centrib Itgion fie dec tactic 12 "rde Fer HZ “Pine for the half-11 65 could still be detected. However, y-rays from the 52-h Pb2°3 contaminant were greatly enhanced and became the dominant activity. The Pb203 was a problem not because of the number of y-rays emitted in its decay, 'since there are only three (279, 401, and 680 keV), but because they contribute vary significantly to the Compton baCkground in the energy region of interest and thereby tended to obscure the weaker peaks from the decay of szoo. As the Pb203 was produced mainly by the (p.3n) 7 reaction on T1205, we decided to raise the bombarding energy to 37 MeV in order to maximize the (p.471) reactions on both T1203 and T1205. For Tl203 this would enhance the production of 3.6-h szozfi at the expense of Pb203. Although we did go slightly over the threshold for the production of Pb199, this did not present a problem because the half-life of this isotope is only 1.5-h, and after 2 or 3 days any remaining Pb199 activity was not detectable. However, simply aging the sources did not eliminate our problems of source purity, for although we could eliminate the Pb parents, some of the daughter T1 isotopes are also y-ray emitters. These included 7.4-h T1199, 73-h T1201, and 12-d T1202. This would have been an easy matter to correct if these were the only Tl contaminants, for we could simply have performed a single chemical separation at the end of the aging period without fear of any more Tl activities growing in. As you might have guessed we were not so blessed, and indeed the daughter of Pb2°°, r1200, 1. unstable with a half-life of 26-h. This would not have been a significant problem if the T1200 decay produced only a few y-rays; but such was not the case and the Table of Isot0pes [Le67] reports me: 3656 I I i"! “« \ ’Ap, egu‘ 2f 66 61-Y-rays associated with this decay (actually, we observed several additional unreported-y-rays in just a cursory look at this decay). Because the T1?°° was constantly building up in our sources from the decay of szoo, it was necessary to perform a chemical separation periodically to get rid of it. The time between separations was generally one hour, but time periods up to 3-h were sometimes used. The chemical separation used is given in Appendix B. while the (p, (m) reaction on natural T1 was used for many experiments in the early days of this study, it still suffered from the presence of a large amount of Pb2°3impurity. Although we were stuck with this method of production for a long time, two new methods of producing Pb2°°were opened to us after the M.S.U. Cyclotron began accelerating He3 beams on a routine basis. 4.2.3. T1203(He3,6n) 31200 i Pb200 The first of these made use of a S8-MeV (70-MeV degraded with 20 mil of aluminum) He3 beam to induce the 112°3(He3,6n) 31200 reaction on natural or enriched (702T1203, 302T1205) Tl targets. The Bi200 has a half-life of about 37 minutes and decays by electron capture to szoo. As in the case of the (p,:m) reactions, there are many competing reactions in the (He3,xn) case also. In this case we produced the following Bi isotopes: B12°5(15.3-d), 312°"(11.2-h), B12°3(ll.8-h), Bizoz(l.6-h), and B12°1(1.8-h). If we had simply let the T1 target age for 2 or 3 days we would have ended up with essentially the same ratio of lead isotopes as in the direct approach using protons. However, if we examine the half-lives of the Bi isotopes we see that the B1200 has a short and mes hr rea 3i iso: 101’ 2 ‘ tile :c thei 369 10' In our 355 as 67 a shorter half-life than any of the others produced by this reaction, and most important, it is much shorter than that of B1203 (11.8-h). Our reasoning was that if we could separate the Pb isotopes from the B1 isotopes within a short time after the end of the bombardment, say 1 or 2 hours, most of the B1200 would have decayed to Pb200, while very little of the other Bi isotopes, especially B1203, would have decayed to their Pb daughters. This was accomplished using essentially the same ion-exchange method described in our search for pb200m (section 3&4). In our search for szoom we did occasionally use enriched T1203 targets and as a by-product of these experiments we obtained a few sources of Pb200 to use in this study. Although this method produced the purest sources used during this study, a great deal of effort was required to obtain sources of the required strength. 4.2.4. H32°2(He3,5n) Pb2°° The second method of producing Pb200 using He3 beams turned out to be the most satisfactory from the standpoint of both ease of preparation and reducing the Pb2°3 contaminant. This method made use of a so—uev (70-MeV degraded with 32 mil aluminmn) He3 beam to induce the Hg2°2(He3,5n ) Pb2°° reaction on natural Hg. At first glance this seems like a very messy way of making any Pb isotope because of the ’ many stable Hg isotopes. The stable isotopes of Hg are listed in Table IV-l along with their per cent abundances. Looking over these stable Hg isotopes it is apparent however, that the only isotope of Hg that could give rise to any significant amount of Pb203 from a He3 68 Table IV-l Stable Isotopes o§2§g_and Z Abundances Isotope Naturally occurring abundance a n3195 0.146% “8193 10.02 2 n3199 16.84 2 H3200 23.13 x 33201 13.22 2 33202 29.80 2 H320“ 6.85 z a As compiled in the Chart of Nuclides by the Knolls Atomic Power Laboratory, Ninth Edition (1966). q" n: ‘_ ”I re 69 been of SO-MeV is HgZO”.« Fortunately this has a relative abundance of only 6.85% compared to 29.82 for 38202 from which most of the szoo was produced. Actually, a significant amount of Pb200 also resulted from the reactions Hg2°1(He3,4n)Pb2°° and Hg2°°(He3,3n)Pb2°°. This gave us a favorable ratio of >7:l for the amount of Pb200 produced to that of Pb203. The remaining stable Hg isotopes produce Pb isotOpes with mass numbers below 200. However, these were not a prdblem since they all have half-lives of less than 2.5 hours and the sources were aged for several days before counting. HgO was found to be the most satisfactory form of mercury for these bombardments. The same chemistry used to separate szoofrom the T1 targets (Appendix B) was used to separate the Pb200 from the HgO targets, the only change being the addition of T1+++ in place of Hg++ as the hold-back carrier. Most of our final experiments, including the singles spectrum shown in Figure IV-l used sources prepared in this way. Even after performing the chemical separations, we did observe some contaminant peaks that we could not assign to any of the known Pb or T1 isotopes. However, these were shown not to originate from Pb2°° decay because of their differing relative intensities throughout the many singles spectra taken at different times and with different sources e 70 4.3. Experimental'Results 4.3.1. gy-ray Singles Spectra Energies and intensities of the Pb200 y-rays were determined using two five-sided trapezoidal Ge(Li) detectors having photopeak efficiencies at 1.332 keV of 0.422 and 2.52. Typical resolutions were 3.0 and 2.3 keV FWHM at the same energy. Both detector systems used room-temperature FET preamplifiers, low noise RC linear amplifiers with pole-zero compensation and near-Gaussian shaping, and 1024- to 4096- channel analyzers or ADC's coupled to the PDP-9 or Sigma-7 computer. The energies of the prominent y-rays were measured by counting the Pb2°° sources simultaneously with the energy standards listed in Table IVrZ. The energies of most of the weaker Pb2°0 y-rays were then determined by using the energies of the prominent Pb2°° y-rays as secondary standards. The centroids and intensities of the photopeaks were determined using the live-display computer program.MDIRAE [Meir]. Figure IV-l shows a y-ray spectrum obtained in 7-h with the 2.51 detector. In this figure only those y-rays assigned to the decay of Pb200 are labeled. Nineteen.y transitions were so assigned, having the energies and intensities listed in Table IV-3. Of these, the 155.29-, 139.39-, 348.23-,'377.92-, and 525.54-keV transitions had not been reported in the earlier studies. The energies of the 142.28-, 147.63-, and 368.23-kev y's were determined by stripping these peaks by hand and then calculating the centroids. The 348.23-kev 7 had to be stripped from an unresolved triplet that contained two contaminant peaks. Because even our best 1. 606....an one oownm mo hose—u 93 on wnwwnodon exude omen» .025 .uouuouov afiqvou uaowowmmo Nm.~ a new: a n :« moswwuao oownm mo anuuooem moamnwm maul» .HI>H seamen mumsSZ .5226.qu 71 com com ooe com . a _ . _ own owv own owm Geno. WU‘Q new; s . 0 so 0‘ ( ari?‘} .. . . o ...a 3;) O. . .. a h.. "filJfiflJ x) ,. a . a... m. a .m... .35)....) W n .. .. w w A. a m m_.a/ A! v t a‘ a m m m o o s); .5 a. .o a - M e ...m. .. is. air hid/1 e. r a. mm o. .. ea. _. e 3.] a ...t .1. c. i W T a s w a..." e an o. mm..oz.m ooaou * r .m u l o. h F _ P _ _ _ _ b _ 'IHNNVHO / SanOD 72 Table IV-2 y-Rays Used as Energy Standards “ m ‘.I m<.-<.--- -- —~ Nuclide . y-Ray energy (keV) Reference Am2“1 59.543:0.015 3 Co57 121.97 10.05 b 136.33 r0.04 b 0e139 165.84 10.03 c Pb2°3 ' 279.17 10.02 b ra182 100.104:0.002 d 152.435:0.003 d 156.387i0.003 d 179.393:0.004 d 222.109:0.005 d 229.322:0.008 d 264.072:0.009 _ d An198 411.795:0.009 e B1207 569.63 i0.08 b Cs137 661.595:0.076 f “r. Yamazaki and J. M. Hollander, Nucl. Phys. §§, 505 (1966). bJ. B. Marion, Gamma-Ray Calibration Standards, Univ. of Maryland Technical Report 653 (1957) cJ. S. Geiger, R. T. Graham, I. Bergstrom, and F. Brown, Nucl. Phys. §§, 352 (1965).: dAverage of: U. Cruber, R. Koch, B. P. Maier, and 0.w.B. Schult, Z. Naturforsch. 20a, 929 (1965) and E. J. Seppi, H. Henrikson, F. Boehm, and J.H.M. Dumond, Nucl. Instr. Methods 16, 17 (1962). eG. Murray,R. T. Graham, and J. S. Geiger, Nucl. Phys. 35, 177 (1963). fJ. S. Geiger, R. T. Graham, and F. Brown, Can. J. Phys. $9, 1258 (1962). 73 Table IV-3 Energies and Relative Intensities of 1 Rays from the Decay of szoo '::_-x Measured energies c»--.—.“ 3". (keV) Relative intensities Singles Integral y-y Anticoincidence coincidence K x-rays 3156 i 350 - - 109.54:0.04 14.5 22.0 370 12 142.28:0.03a 95.1 5.08 4900 55 147.63:0.03a 1133 2 308 21,500 1020 155.29:0.10 1.4 :0.5 - — 161.3210.04 9.1 11.0 650 6.6 193.39:0.10 1.0 10.4 — - 235.62t0.04 129 24.0 7000 81 257.19:0.03 134 :4.0 5100 100 268.36t0.03 119 25.0 6000 80 289.24i0.15a 32 i 10a 1950» 70 289.92:0.10a 51.6 r 103 302.9320.05 5.0 :1.0 125 315.60i0.08 ‘6.7 :1.0 260 348.23:0.08a 4.8 i1.5a 130 - 377.92:0.05 0.8 r0.3 9.7 — 450.56:0.05 3100 5100 2100 457.8010.07 3.5 i0.6 24 2.3 525.54t0.06 12.6 11.0 - 12 605.44:0.06 31.2 23 19 16.9 a Be sure to read the text for comments on how these .“ were obtained. a ‘- . i“‘.j -a‘- o . . . .: ans-.23: '3‘;°‘: : z 1:: - - energies and intensities AF. 145 - uu . i I stir: F...- ii 1.“... II fl 4‘ e s - -i ..VW. and: at e e . l1. ‘ 4 ‘1“ {MN .e a ‘5‘ “In ML. on. . . L 11h)- l 7h detector could not resolve the doublet consisting of the 289.24- and 289.92-kev‘y's well enough to allow us to strip these peaks, the energies for these transitions given in Table IV-3 were obtained from sum.and difference relationships among the other transitions. The intensities were determined on the basis of the relative contributions necessary to reproduce the energy of the doublet, "289.66-keV". Evidence for the doublet nature of the 289.7-keV peak.will be given in Section 4.3.2. The uncertainties in the energies listed in Table IV-3 are based on the uncertainties in the energy standards, the heights of the peaks above backgrounds, and the reproducibilities of the calculated energies from many different spectra. The relative intensities listed are averaged from.spectra obtained with both detectors. Their uncertainties are based on the reproducibilities of the intensities and the uncertainties in our experimentally-determined efficiency curves for the detectors. The K x-ray intensity for Pb2°° listed in Table IVr3 was obtained in the following manner. A szo0 source was aged until the 9.4-h Pb2°l was only a minor contaminant, thereby avoiding a correction for its x-rays. This left Pb203 and T1200 as the only major contributors to the total x-ray intensity. Since spectra of Pb203 and T1200 could be obtained relatively free from contaminants, it was an easy matter to determine how much each of these contributed to the total x—ray intensity in the Pb2°° spectrum. After subtracting out their contributions, the remaining K x-ray intensity belonged essentially to szoo. The three spectra taken to obtain this x-ray intensity are shown in Figure IV—2. ..ZzlxDC 1;... GMZ‘ICU. 10 10 10 10 10 10 H o H O S' H o H O N COUNTS PER CHANNEL H O .... 10 10. 10 10 10 10 Figure IV—2. In 75 23’s, + 1:~ 0 P6200 SINGLES FOR X-RAYS £F—_ 0 m Vl— IN No v .7 eN I H 05.0 (n m 0.001.750. X e I e e eNv A A N 0‘)th o m I .7 mmtplux o o .J H NNN ‘ N N — .O l I t: a. I I” t has. 289 — l450,6 .5 £— 579.1 (TIZOO) _ 368.0 3,47 5 1: L. -' .401.0 2?- Pb2°3 SINGLES FOR X-RAYS - 279,2 :2 v O (\ e o e- o ID V .‘f _ r- m H i no I \‘\ .. I m i m' khbwvdSJI| .. 3 200 53 TI SINGLES FOR X-RAYS g I C (q‘ :33 a: 3 I x: ‘9 '— I ... '3 H' | in m t m l'u.’ M ..-“ -‘/ .e..._ 1' I “WK . l I ..... ,MJ”, . f ‘ 1 "" \\~..4.A,.....-.I\,l.,_.,,.,...-.-. 3% .4 500 1000 15 00 CHANNEL NUMBER Low energy y-ray spectra of Pb2°°, Pb203, and T1200 taken with 0.422 efficient Ge(Li) detector and used to obtain the K x—ray intensity for Pb200 decay. 3:31.01. in u in a c £0 {at 92 76 4. 3 - 2. Coincidence Spectra In order to determine which y-rays appear in cascades and which are primarily e-fed ground-state transitions, we used the 0.422- efficient Ge(Li) detector in an anticoincidence experiment with an BXB-in. HaI(Tl) split annulus and a 3X3-in. NaI(T1) detector [Au67] The Pb2°° source was placed on top of the Ge(Li) detector and this inserted into the other end. The single-channel analyzers associated with each of the NaI (T1) detectors were set to accept all y-rays above 90—keV to eliminate the T1 K x-rays. A resolving time (21) of ~100 nsec was used to obtain the spectrum shown in Figure IV-3. The relative intensities from the anticoincidence experiment are listed in Table IV-3. The 147.63-, 450.56-, 525.54-, and 605.44-keV peaks are obviously greatly enhanced in the anticoincidence spectrum relative to their singles intensities. The 257.19- and 289.7-keV (289.24- and 289.92- keV doublet) peaks are also enhanced with respect to some peaks, such as the one at 235.62-keV. To complement the anticoincidence experiment and determine which y-rays are involved in strong coincidences, we performed an integral coincidence eXperiment, using essentially the same set-up ‘3 for the anticoincidence experiment except the 3XB-in. NaI(T1) “teeter was removed. The resulting spectrum is shown in Figure IV-4 811d the relative intensities are included in Table IV-3. From the integral coincidence and anticoincidence experiments alone it is quite aWarent that all of the Pb2°° y-rays are in relatively strong ceincidences except for the 450.56—, 525.54-, and 605.44-keV y's, II .. ‘5ll‘a(l ‘I({\Ilrfl“ (CO ~ .‘AH b c\ hV‘ .ooHoan sue moose oomph ou wmwmnoaon mason ease .vom nonuo osu moaxuoan nouoouov aflavHoz .mwumxm o nude seasons swans Asavaz .ouuwxw no ovens“ cocoae wouoouov Aaqvoo unowowmwo «no.0 o nuw3.oonweuno sum.auuuooem.mwma .meou > oomph mo assesses moaokuouoowuo< .mn>H seamen ”$0.232 ..ng one mum 000 . own CON mm. d d 1 4 Lihfxw. u. . .. 5.7.5.08.- . _u sf!%.. .5. . 3.. i. O. 1 0909 9‘33 019' i? J l "5 77 ‘IENNVHO 83d SlNflOO shout» Had «Ne.c a means he ooououno mm3.a=uuooem mash mumEDZ 4m22omo .msasnno madam AaHvHoz .ouuwxm no nave ooeoowoofioo 6H uouoouov Awgvoo unowowmmo .mmou > oomnm mo sensuous monovfiucwoo Houwounu .eueH seamen . ohm 02. 8 com mhn Com mm. _ a J 4 a .0— wmfifiaflw. r r1 «gym. sums... m . ngsrn 1N0— m A}... . m .../1 I. g . e r. O. m a A“ [j .43 .. n "1 mm 1 e m «m j ..1 AV0. a. .. s .5 mm mm .immz mozmoazzvo 1380th CON we r: "IBNNVHO 83d smnoo 79 which we can safely assume are primarily s-fed ground-state transitions. We can also deduce that the 147.63-, 257.19-, and 289.24— and/or 289.92- keV y's are partially e-fed ground state transitions that are also fed by y—rays from higher levels. These results are consistent with those of Wirhed and Herrlander [W162] . To aid in the placement of the remaining y-rays in a consistent decay scheme we employed several two-dimensional "megachannel" y-y spectrometer systems. For a more complete description of this two- dimensional system and the recovery program see Section 2.3. The first system employed was a Ge(Li)-NaI(T1) system using the 0.422 Ge(Li) detector in coincidence with a 3XB—in. NaI(T1) detector. The results of this experiment however were very difficult to interpret because of the poor resolution of the NaI(T1) detector. When another Ge(Li) detector of 0.672 efficiency became available we again tried the two-dimensional experiment, this time using both Ge(Li) detectors. This was the first successful two-dimensional Ge(Li)-Ge(Li) experiment Performed at M.S.U. Although the data obtained from this experiment far surpassed any previous gated coincidence runs using a Ge(Li)— Na1(1-1) system, it did suffer from poor statistics because of the small efficiencies of the detectors. The experiment was therefore repeated for the final time when two larpr volume (22 and 2.52) Ge(Li) detectors became available. The two detectors were placed 90° to each other with a graded Pb absorber bisecting the 90° angle to prevent Compton scattering between the detectors. The Pb2°° source was placed equidistant from the centers of both detectors and was replaced with a freshly saga: 0 II 50:5. obza 360.. a" 7's} ~Cks! Pic-7 V a udo__ \. I 80 separated source every 3 hours. During a 24-h period we collected a total of about 500,000 events. The two integral coincidence spectra obtained with the 2.02 and 2.52 detectors are shown in Figure IV-5. A selection of the gated coincidence spectra used in the construction of the decay scheme of Pb200 are shown in Figure IV-6. A summary of the coincidence relationships obtained from these spectra is given in Table IV-4. From these data we could confidently place all the observed transitions in a decay scheme, with the exception of the 155.29- and 193.39-kev y's, for which we had only weak coincidence data. And even these two transitions could easily be placed in the decay scheme between well-defined existing levels by using sum and difference techniques. In addition to helping us place the transitions, the two-dimensional experiments aided in identifying peaks that were part of the unresolved multiplets. As mentioned before, the 348.23-keV y is part of a triplet containing two long-lived contaminant peaks at 350.11 and 352.02-keV, as can be seen in Figure IV-l. We had failed to identify this peak as belonging to Pb200 decay until we looked at the results of the two-dimensional experiments and Observed a single peak at 348—keV that appeared to be in strong coincidence with the 257.19-kev transition (Figure IV-6) and fit nicely into the decay scheme. As mentioned in the introduction to this chapter, confirmation of the doublet nature of the 289.66-keV peak presented one of the most fascinating problems in this study. Our first attempts centered on detecting a broadening of this peak, using the 0.422 Ge(Li) detector 81 gem—hvmum I I” 0.5m: I m.om: I 4mZZ M . . . I \ I .{ ... 1 NI“ I IIIIIIIII'III I I «I III SD 3' BOB-KEV GATED 102'“- I 101+ _J LIJ Z 2 § 0 l E «a. BIS-KEV GATED 0. Mb $102+ 0:: °: 2 :r N m 3 “I g ‘23 53 l g I I 101‘” 1 348—KEV GATED N 1021’ I". ,3 B In .1' N ' I 1014. 1 ’ l . ‘ ‘ I 0 500 1000 1500 CHANNEL NUMBER Figure IV-6 (cont'd) COUNTS PER CHANNEL 10 10 "" 10 101+' LO 0 h 4' H I _. 1167.6 — 1h7.6 - 155.3 500 g 378-st GATE Y-DISPLAY ‘31 NO BACKGROUND SUBTRACTION [.— I I u I II 4 3‘5} 378-KEV HIGH BACKGROUND :0 — Y-DISPLAY m I— m V I IIIIUJ I LILLW 378-KEV LOW BACKGROUND Y-DISPLAY 368.0 (TI200) ”IIIII IL II 450-KEV GATED 457-KEV GATED Y-DISPLAY I]: I1 L I: ‘ 1000 1500' CHANNEL NUMBER Figurg IV-6 (cont'd) 87 Table IV-4 Results of y-y Coincidence Study Using 2-Dimensiona1 Ana1ysis Gated Energy Energies of y rays in Coincidence with Gate Strong Weak Very Weak 109.5 147,268 235, 348 — 142.3 147,235, 161 - 315 147.6 109,142,235, 161,348,378 155 268,303,315, 457 161.3 289.24 147 - 235.6 289.92,142, 109 - 147,257 257.2 268,348,235 193,315 155 268.4 109,147,257 - - 289'2 161,235,315 - - 289.9 315.6 142,147,289 257 - 348.2 147,257 - - 377.9 - 147 - 450.5 - - 155 457 147 - - 88 with a resolution of 3.0 keV FWHM at 1332 keV. Using a gain of 0.21 keV per channel we expected to see a broadening of the peak by several channels. However, no such broadening was observed. From energy sums and differences however, it appeared that a 235.62-keV y-ray p0pu1ated a state at 289.92—kev while a 161.32-keV y-ray populated a state at 289.24-keV. If this were indeed the case, we reasoned that if we could perform a gated coincidence experiment on the 161.32—keV y and without changing the gain of the amplifiers perform a similar experiment gating on the 235.62-kev y, we should observe a shift in the centroid of the 289.6-keV peak in the resulting spectra. In our first attempt at this approach we used a 3X3-in. NaI(T1) detector to set the gates and the 0.421 detector to obtain the coincidence spectra. The result of this experiment was inconclusive as one might have expected, since the 161.3— keV y-ray is not very intense and rides on top of a Compton distribution from the 235.6-keV y-ray. However, with the development of the two- dimensional Ge(Li)-Ge(Li) spectrometer system, our interest in this approach was revived. From the data obtained in the two-dimensional run we could easily perform gated coincidence experiments on the 161.3- and 235.6— kev peaks, and in addition perform a correction for the underlying Compton distribution. We carefully analyzed the resulting spectra, looking for a shift in the centroid of the 289.6-keV peak. This shift can be seen in Figure IV-7 thereby confirming the presence of two Y-rays. The energies obtained from this experiment were 289.22- and 289.79-kev, as compared with our adapted values of 289.24- and 289.92-keV obtained by sums and differences. 89 Figure IV-7. Results of the two—dimensional coincidence experiment used to confirm the doublet - nature of the 289.6-keV peak. A. y-side integral coincidence spectrum used for the gates. B. xbside integral coincidence spectrum showing the region near the 289.6- keV peak on an expanded scale. C. xbside spectrum in coincidence with the 161.3-keV peak (background subtracted). D. xbside spectrum in coincidence with the 235.6-keV peak (background subtracted). COUNTS/CHANVEL 9O '0“ -— -—- --- ___ .73.. _.._...__......-.-- - . . -- _ -- .--“--. .__._-- .-.“..- ... - ...- “...-....-- -.m. . - . . ._,_l A ~ '. A. sz°° Y - SIDE INTEGRAL COINCIDENCE ‘ } §EO I 'uz I I . :53 .. a. E ... a: '5 8 I I 'r 3 ‘3‘ I - O O. . O a?! ‘ 'T Ever. k I I- g: . 3279.2 (”2'“) I613 .3- 289.6 (DOUBLE T) . . . .. ‘ ‘ s- .. . .. 4 O . -‘ ‘ ”C . O 33I 2 r. o .. I .. E . . a E i .......... 8 '°°I' """""""""" ° ; ................................. (’I1 1, l I i I I ”I C Pb2°° X-DISPLAY IGHI ° . . eV GATED - . ”I. COINCIDENCE . , I NI . N; ‘ o. s o oo- o. o o a... . ... o .o o. o I ... ; o ..J... .. .. -1 --..-- .. ...-.J___. ' .=_J __- L . 3‘ I I .50.. o.sz°° x-DISPLAY 235-uev GATED - I . ' m. CCINCIDENCE ...! , $5. «I. i I I . ° -... .......... . OLL 1 - 1 A 4 L- m......._1 I 900 925 950 975 IOOO IOZS CHAN\EL NUVIBEFI’ Figure III-7 91 4.3.3. Conversion Coefficients Conversion coefficients for most of the transitions were determined using our photon intensities and the conversion-electron intensities of Wirhed and Herrlander [W162]. In order to normalize the two sets of data, we assumed that the 147.63—keV transition.was a pure E2 transition [As57] and that the theoretical conversion coefficients of Eager and Seltzer [Ha68] were correct for this transition. Table IV-S contains the transition data for P3200 along with the multipolarity assignments we propose for those transitions where both photon and electron intensities were available. The uncertainties in the experimental conversion coefficients represent the quoted uncertainties in both the photon [Table IV—3] and electron [W162] intensities as well as the uncertainty in the normalization factor. The normalization factor was taken as the average of those obtained using the K, LIILI. , and L111 conversion coefficients for the 147.63-kev transition. Figure IV—8 shows the theoretical Keconversion coefficients of Eager and Seltzer together with the experimental points. KFshell conversion coefficients were determined for all but the 155.29-, 193.39-, 348.23-, 377.92-, and 525.54-ke9 transitions, and L- and MLshell conversion coefficients were determined for many of the transitions. The multipolarity predictions all basically agree with those from the KYL and L-subshell ratios of Wirhed and Herrlander, although the latter are not very sensitive indicators. As can be seen from Figure IV-8 and Table IV-S, all of the transitions are Ml's with the exception of the 147.63-keV E2 and the 257.19-keV transition, which is 2401181 and :602 E2. TheseIMl transitions 92 Table IV -5 Transition Datijor Pb2°° ...-.' Multipole Energy Photon Conversion-electron Experimental Theoretical (keV) intensity intensitya conversion conversion order coefficient coefficientb 109.54 14.5 K 22 5.2:0.9 5.7 M1 LI 4'2 1130.2 0.93 M1 LII 0.42 142.28 95.1 K 69 2.510.2 2.7 M1 LI(LII) 14.2 0.51:0.06 0.48 M1 MI 3.4 0.1210.02 0.095 Ml 147.63 1133 K 100 0.30:0.03 0.34 I52 L (L ) 137 0.41:0.04 0.43 222 I II LIII 90 0.27:0.04 0.27 [32 MIIMIII 63 0.19:0.04 0.19 E2 161.32 9.06 K 4.5 1.73:0.3 1.9 M1 235.62 129’ x 26.7 0.70:0.05 0.65 Ml LI(LII) 4.6 0.12:0.01 0.11 M1 MI 1.35 0.036:0.004 0.023 M1 257.19 134 K 10.1 0.26:0.02 0.51 M1 0.091 E2 L (L 2.8 0.071i0.009 0.092 M1 I HJ 0.048 E2 LIII 0.5 0.013:0.005 0.00063 Ml 0.019 52 268.36 119 X 18.4 0.53:0.05 0.46 M1 LIMII) 3.1 0.089:0.013 0.077 M1 ”I 0.65 0.018:0.006 0.016 M1 289.24 32 K 3.7 0.39:0.13 0.37 M1 289.92 51.6 K 5.4 03510.08 0.37 M] 302.93 5.03 K 0.70 0.47:0.12 0.32 Ml L (L ) 0.12 0.08110.026 0.057 Ml H N H 93 Table IV-5 (cont'd) 315.60 6.69 K 0.68 0.3510.07 0.29 LI(LII) 0.20 0.1010.03 0.049 450.56 100 K 4.4 0.1510.02 0.11 LI(LII) 0.76 0.02610.003 0.019 .M 0.16 0.005410.0008 0.0044 457.80 3.49 K 0.40 0.3910.08 0.11 605.44 16.9 K 0.24 0.04610.009 0.051 L 0.04 0.00810.001 0.009 h -- bRef. [W162] Ref. [Ha68] 94 .Hmomm um» one woman mo mmoam> Hmowumuoosu moo uuw ou namuv ouw3 mowwmw muowumoam och .comnm mo amuse moo wowaoaaom moowuwmcmuu How H HWWNOU GOHmH0>fiOU HHUEQIN HMUHUUHOUSU “a“ HMUGOEHHQQKW .®l>H NHH—MHK «>3: >omw2w com com oov oon .oou , oo. _ In _ q I _ If: I ' l I l I l I l l l 1 I l I l l l "r. I I .Inmvhv. r 1 1| 1 l l I l l u I j ' l ' l "I u I _Io_. D I ... M l I .mu 1 l ' H I “ww- ' WI H mu .1 00. r! l l l I l l ' l m _¢¢ m l l n A I . .. .nx 95 all appear to have extremely little E2 admixing, which is somewhat surprising, considering the E2 enhancements that show up in some of the Pb isotopes much closer to the doubly closed shell. It would be helpful to have careful angular correlation experiments performed on this nucleus in order to improve the limits on E2 admixtures in the M1 transitions. On the basis of the experimental GK’ the 457.80-keV transition would seem to be an E2; however, for reasons mentioned later we have assigned it as an M1. 96 4.6. Decay Scheme Figure IV-9 shows the decay scheme we deduced from our 'experiments. Transition and excited-state energies are given in keV, with the adopted energies for the levels being weighted average based on our confidence in the respective cascade and crossover transitions. As mentioned in Section 4.3.1, the energies of the 289.24— and 289.92-keV levels are based on sum and difference relations. The Q6 of 3939 keV’was calculated from the "experimental" masses listed in the table of liyers and Swiatecki [uy65]. we have included for the sake of completeness the levels in T1200 populated by the decay [D163] of 37-msec T1200”; It can be seen that there is little overlap in the two decay schemes, so we forego further discussion of the high-spin levels. From the conversion coefficients in Table IV‘S and from the theoretical conversion coefficients of Eager and Seltzer where experimental values were not available, the total transition intensities, including conversion in higher shells, were calculated. These total transition intensities, in per cent of the total Pb2°° disintegrations, are given in the decay scheme. The total intensity for the 32.7-kev transition was obtained from the conversion intensity measured by Wirhed and Herrlander [Vi62]. This was corrected for the y-ray intensity by g using the L-shell conversion coefficients for an M1 transition, which they assigned on the basis of its L-subshell ratios. From.the measured K'x-ray intensity, KFconversion intensities, and.K fluorescent yield, 0.95, [F166] the total e-feeding intensity to the ground state was 97 an vounannon no: ouos_moumum oqnmluonwan omega “Ea .oounm mo meson c 0:» none o3 unuwu oaouuxo may u< .ounum onu mo unmwu on» on voumwa one oueum menu nonao> 9k moH one one sound some on mouse w name use may .mnowunumuounwmwv ocwnm on» no name use ma nm>aw mum nnoaufimanuu AHououv Han mo mowuamnounw may onu He no emcee use an ewuuflaeoe money» now .oomam mo «Boson known .mI>H «names . 5.8. Ne 08% dI l .N mARmdélOfi .I l - IN v R W b. #9 a w. z m m x 6 .7 .V “V . z a .w. I w v $§89..8§9 m 2 it. _ I _ . mlIO n e .w. n ®.NAO\O_oNv \l r I 8 i In ... a % mm a A _ 2 02x39 I I , _ I- .- an” chflmdv. .m N mwmnu D a m 3 C. I“ N . . qul. 5v ye mm.mw mw ..“mwwm mm m“ _. mm_ an 3389 . I. III. I I I .. m I " awn -v . 383395.03 m w o m Ins I. .0 Mb 0 . I £83 I I e w n I 3 >438 w, w. m m _ n: mw.mu mm mu I I. mnm a . Nah mENm \I O Nm at CON 0 cmv:N . 98 determined to be less than 0.51. This corresponds to a log ft > 8. The total e-feeding intensities for decays to each state were then calculated assuming that there was no ground-state feeding. These intensities are given in the decay scheme to the right of the energy levels. Log ft values based on them appear in italics at the extreme right of the levels. 99 4.5. Spin and Parity Asgiggents Ground state. Herrlander and Gerholm [He57] assigned In . 2- to the ground state of T1200 on the basis of a Kurie analysis of the positron spectrum resulting from the decay of this state to the 0+ ground 20°. (The T1200 ground state had a measured spin of 2 from state of Hg atomic spectra and atomic beams experiments [Hu61]. Our upper limit of 0.52 on direct a population to this state corresponds to a log ft > 8. This is in agreement with a predicted first-forbidden unique log ft =9 for such a 0+ + 2- transition. 147.63-keV state. The first excited state at 147.63-keV was previously assigned In - 0- by Astrom, Johannson, and Bergstrom.[As57] on the basis of the seemingly pure E2 nature of the 147.63-keV 7 transition and the strong e population of the state. Our work supports this 0— assignment, as the log ft of 6.2 lies in the range expected for a fairly rapid first-forbidden transition. 257.18-keV state. The log ft for e decay to this state was found to be 7.6. This could indicate either an allowed or a first- forbidden transition, which would populate 1+-and 0+ or 1— and 0- states, respectively. From the definiteIMl assignment for the 109.5-keV y to the 147.63-keV 0- state, we can eliminate the 1+ and 0+ possibilities on the basis of parity and the 0- state on the basis of observing the photons. The 257.2—keV Y to the 2- ground state is a mixture ofIMl and E2 multipolarities and also rules against the l+, 0+, and 0- assignments. We can thus quite confidently assign Ir - 1- to the .257.18-kev state. 100 The four highest-lying states. The log ft values for these states range from 6.3 to 7.1, thus falling into the range of both allowed and first-forbidden (noneunique) transitions. This implies choices of 1+, 0+, l-, and 0- for these states. Based on theIMl nature of the 289.92-keV y, we can narrow the choice to l- for the 289.92-keV state. This is also supported by the IMl nature of the 142.28- and 32.7-keV transitions. By analogous arguments the 450.56-keV state can also be assigned l-. TheIMl nature of the 268.36- and 235.62—keV transitions allows the assignment for the 525.54-kev state to be narrowed to 1- or 0-. As we were unable to determine the multipolarity of the 525.54-kev Y to the Z-ground state, we could not distinguish between the 1- or 0- choices from this; however, the fact that we observed photons at all in the 377.92—keV transition from the 525.54-kev state to the 147.63-keV 0- state strictly rules out the 0- possibility. Again we are left with a l- assignment. The 605.44—keV state can also be assigned 1- from the M1 nature of its ground-state 7 transition, and this is consistent with Nthe M1 nature of the 315.60-keV transition. The only inconsistency in this assignment arises from the 457.80—keV y, which goes to the 0- state and has a measured ox in the range expected for an.M2 transition. If this y-ray were indeed an M2, the 605.44-keV state would be 2+ and the a decay would be second forbidden, obviously inconsistent with the log ft of 7.1 as well as with the multipolarities of the other y—rays. Therefore, it appears that the measured a is in error, and the multi- .K polarity of the 457.80-keV y is undoubtedly M1 not M2. (‘x ‘3 to this stat transition I those of 1+, I to the gr he? 1- state leaIing us w 0f the 103 f discuss the I 101 289.24-keV’state. The determined log‘ft of 8.0 for s decay to this state is close enough to the range for a first-forbidden unique transition that we must include the possibility of a 2- assignment to those of 1+, 0+, 1-, and 0-. From the M1 multipolarity of the 289.24-keV Y to the ground state and also that of the 161.32-keV Y from the 450.56- keV 1— state, we can exclude the positive parity and 0— possiblities, leaving us with l-or 2-. We prefer the l- assignment slightly because of the log ft value but do not exclude the 2- possibility. We shall discuss the assignment for this state in the next section. 'I 81 closed shell shell at N c the odd-grow; nuclei. In I assumed to be extending it the states 1: the wave fIIng residual p-n we can use 5', assumption of 'I . . I I QI A PVPkrf'nh C Where the mat ”define of t 9101;0st by B m Crdering ‘- 'J P and in 81' from the adja are the ”bit Shell‘model a Pessiblt‘.) he I 102 4.6. ShellAModel Assignments and Discussion elTligg is an odd-odd nucleus one proton removed from the closed shell at Z - 82 and seven neutrons removed from the closed shell at N - 126. The simplest approach to such nuclei is to extend the odd-group model, as normally applied to odd-even and even-odd nuclei. In this model the properties of the nuclear states are assumed to be determined primarily by the odd group of particles. In extending it to odd-odd nuclei we assume that the wave functions for the states in the odd—odd nuclei are simple vector-coupled products of the wave functions of the two odd groups. .If we assume that the residual p—n interactions are weak compared to spineorbit forces [De61], we can use jj coupling, with its resulting simplications. With the assumption of jj coupling, a given proton and neutron configuration, £ 0 I pap where the nature of the residual p-n interaction will determine the injn>, can take on all integral spins, ij-jnl §_1'§_jp + jn, ordering of these spins. The modified Nordheim coupling rules proposed by Brennan and Bernstein [Br60] can be useful in predicting the ordering of the spins resulting from a given configuration. Here jp and jn are the single—particle total angular momenta obtained from the adjacent odd-mass nuclei, while 2p and in (assumed to be pure) are the orbital angular momenta obtained from standard single-particle shell—model assignments. In order to keep our analysis as simple as possible, we have assumed that both odd groups are of the lowest possible seniority. Explicit calculations show that in many cases the admixtures of higher seniorities in the wave function of a given lat-lying nuc not going toc Thc their effects The positions three of the 38200, at 366 ”use Of stat but the effec tucleus a lit This aWears in most of th CC-llfictivE e f III are diSCus have a high 1 temple“ Diet interacuons More Bensit correlation,3 ’ In this Odd-odd 103 low-lying nuclear state are quite small,_[0q59] so we are probably not going too far wrong here. The question also arises concerning collective states and their effects on the odd-odd states, perhaps even core-coupled states. The positions of the first 2+ quadrupole vibrational state is known in three_of the four nearest evenreven nuclei: szoz, at 961.4—keV [Ht57]; Hg2°°, at 368.0—keV [Sa65]; and Bglge, at 411.80—keV [Mu63]. The energy range of states we are considering in T1200 starts to overlap with these, but the effects of blocking in this odd-odd system should make the‘ “ nucleus a little more rigid, if anything, with respect to vibrations. This appears to be borne out by the lack of significant E2 admixtures in most of the M1 transitions, so in our discussion we do not consider collective effects explicitly. It must be remembered, however, that A we are discussing only a few low-spin states in a nucleus that must have a high level density even at low energies, so to obtain a more complete picture the effects of collective modes and configuration interactions will have to be included. As mentioned in Section 4.3.3, a more sensitive measurement of E2 admixtures, such as angular correlations, would be most welcome here. In attempting to assign the shell-model configurations in this odd-odd nucleus, we assume that the low-lying states should result from combinations of the lowest configurations in the adjacent oddf proton and odd-neutron nuclei. The proton configurations contributing to the low-lying states were assumed to be the ground and first two excited states in the adjacent odd-mass Tl isotopes. The resulting spins of l/2+, 3/2+, and 5/2+ are consistent with the shellemodel assignments odd-mass T1 the Z a 82 nuptising I consistentl} cmwist of t to the 31/2 Fc> Odd neutron you‘d or 10 °f the 113/2 10h assignments, 81/2, d3/2, and dS/Z' The spacing of these states in the odd-mass Tl isotopes is shown in Figure IV-lO. The last two protons of the Z - 82 closed shell should fill the 81/2 orbit, so it is not surprising that the ground state for the odd-mass Tl isotOpes is consistently l/2+. The first two excited states in these isotOpes then consist of the promotion of a proton from the filled d3/2 or d5/2 orbits to the 81/2 hole. For neutron members just below the N a 126 closed shell, the odd neutron can populate the p1/2, i13/2, p3/2, 0r.fS/2 orbits in the ground or lowest excited states. Because of the large pairing energy of the £13/2 orbit, it should be filled in preferentially by pairs and not by odd particles, which may account for the fact that no £13/2 ground states have been observed. In order to obtain an idea of the ordering of the low-lying neutron configurations contributing to the states in T1200, we would like to examine the ground and lowest excited states in the odd-mass isotones with 119 neutrons. Of particular interest are the states in Pb201 and Hg199. As can be seen from Figure Iv-ll, only the ground state is known in szo1 (excluding the i13/2 excited isomeric state). The In - 5/2- suggests an.f5/2 assignment as its primary component. We should be able to get some idea of the ordering of the excited states in Pb201, however, by looking at some of the systematics of the better-known odd-mass Pb isotopes, as shown in Figure IV—lZ. From these systematics, it would appear that the neutron states (holes) of interest in order of increasing energy are fs/z. p3/2. and.p1/2. The quasiparticle calculations of Kisslinger and Sorenson [K160] for Pb“1 agree with this ordering. However, as is apparent r Figure ENERGY (keV) A C) C) 01 C) C) no (3 C) 100 105 '+ 5 F 122 h- 3 + ’2 .. ‘.___ '. I. I h’jf 2 l93 l95 l9? l99 20! 203 205 ATI Figure IV-lO. Systematics of the low—lying l/2+, 3/2+, and 5/2+ states in odd-mass Tl isotopes. These should be relatively pure 81/2, d , and dS/z shell-model states. 3/2 700 '- 600 '- 500 - IOO .. a>mvv3 trmvmmzm 0.. FISH: 106 700 - I3,2+ 600 +- 500 *- .3} <3 <3 I ENERGY (keV) ‘6' 'CD *I 200 - 03/2" I00 *- o :— l I97 199 201 ' TBPtIIS angus sszus Figure IV-ll. Systematics of law-lying states in odd-mass isotones having 119 neutrons. These correspond to relatively ure ' and i shell—model states. p .p1/2’ fs/z’ 13/2 1000' 900 - h b O 0 O 0 0 0 any 6 5 m>®v¢ \rmvmmzm 800- 700 300L 200. FigL IOOO 900 800 N O O O) O 0 0| 0 O ENERGY (keV) .b O O 107 ’ w I 13 r- " /2 + a . 3,2- t 5,2— I/ .. 2 F' 5’2- 1;- 3/2- |/2 .- ' L 1 f 1 L 1 1 :97 I99 20: 203 205 207 APb Figure IV-12. Systematics of the f3,2, p1/2, p3/2, and i13/2 states in the odd-mass neutron-deficient Pb isotopes. fros Figurl probably 5: and Pb197, certain whe referring t ground stat and 5/2- (5' (Pa/2). TE and F3/2. naught-us. contributin C0111: igurati B "9 can We spin States Vithout am We do not the additio ' I Which are n 'I states and OUISeIVQS p and 2. Sta: the E deCav 108 from Figure IV-lZ, the spacing between the f5, 2 and p3/ 2 state is probably small in Pb 201, as the two states have crossed between here and Pb197,‘where the ground state is p3/2. It is not altogether certain.whether the ground state of Pb199 is fg/z or p3/2. Again referring to Figure IV-ll, we see that for Hg199 and Pt197.the ground state and first excited states are in the order l/2- (pl/2) and 5/2- (ft/2), with the second excited state in Hg199‘being 3/2- (Palz). These isotones therefore suggest an ordering of P1,2s.f5/2» and p3/2. It is obvious from this analysis that we cannot unambiguously assign an order to the single-neutron configurations contributing to the states in T1200. This will make the assignment of configurations more difficult. Based on the proton states and the neutron states available, we can immediately make the prediction that all the lowblying, lowe spin states in T1200 should have negative parity. This is borne out without exception in the experimental states determined in our studies. (we do not include the i13/2 state in the list of available states for the additional reason that it would lead only to higher-spin states, which are not p0pu1ated in Pb2°° e decay.) Table IV-6 contains a summary of all the possible spins resulting from p-n configurations using the 81/2, d3’2, and d5/2 proton states and the p5/2, p3/2, and f5], neutron states. We shall concern ourselves principally with the configurations giving rise to 0-, 1-, and 2- states, 88 these were the only states we observed populated in the 8 decay of Pb2°°. 3- 4. 109 Table 1v-6 Possible Configurations for Producing Some Low-Lying Odd-Odd States in T1200 a-‘rm ---—.‘---‘o-.n-' In (n,v) Configurations 0' (SI/zpl/Z) (d3/2p3/2) (dS/ZfS/Z) 1" (81/2p1/2) (81/2p3/2) (d3/2f5/2) (dB/zpl/Z) (dB/2p3/2) (d5/2p3/2) (dB/ZfS/Z) 2" (Bl/ZfS/Z) (31/21’3/2) (dB/ZPIIZ) (da/pfs/z) (d3/2P3/9) (d5/2pl/2) (dS/zpa/z) (dS/zfs/z) 3’ (surfs/2) (da/sz/z) (da/zpa/z) (dB/ZfS/Z) (ds/zpa/z) (ds/zpl/z) 1" (da/sz/z) (dB/ZPBIZ) 5- (ds/z-fS/z) “-‘ H ... «just-3:2 3. : ::=:—.:::..-::—.:.:‘ :f— 3 =38:3-:—.:-.x- 110 It has been suggested by Bergstram and Andersson [BeS7] that the 2- ground state of T1200 has the configuration, [(n31/2)(vf3/2)2-. This could be consistent with our analysis above. The "strong" coupling rule of Nordheim [N050] predicts that the 2- state will lie lower than the 3- state for this configuration. However, DeShalit and Walecka [Deél] have suggested a configuration of [(n81/2)(np3/2)]2- for this ground state. Because of the uncertainty in ordering of the neutron states in this region, this also could be consistent with our above analysis. The 2- state is also predicted to lie lower than a 1- state from this configuration according to Nordheim's "weak" rule and the modified rules of Brennan and Bernstein [Br60]. DeShalit and Walecka preposed their assignment on the basis of the systematics of the states populated by the decays of the isomeric states in the odd-odd Tl isotOpes. As shown in Figure IV-l3, the neutronrdeficient odd-odd Tl nuclei have 2- ground and 0- first excited states. However, in T1198 and T1196 we see two closely spaced excited states with In - 2— and 3— separated by 23 and 34—keV, respectively. The‘Ml transitions between these states compete very favorably with the much more energetichl transition to the ground state, which prompted DeShalit and Walecka to assume that these states have the configurations, [(usllz)(qf5/2)2_ and [(u31/2)(nf3/2)]3_. The 2- ground state of T1200 must then be primarily one of the other possibilities listed in Table IV-6, of which [(w31/2)(vp3/2)]2_ seems to be the best choice. Since the M1 transition, [(w31/2)(vf3/2)]3_ + [(n81/2)(vp3/2)]2-. is t-forbidden, whereas the M1 transition, [(n81/2)(Qf5/2)]3_ + [(n81/2)(vf3/2)]2- is not, this could explain the y-branching ratio for the cases of T1198 and T1196. Also, the 900 " 800 " 700 P 600 " 00F 5 .u.\lrnmvsy.uv .\.I 400 *- “1300 '- 200 } l00 * 0 e Figur 900 800 700 600 0| 0 O ENERGY (keV) 01 JD 8 8 200 I00 7+ I" sl - . __.s .. o ? ya- .I - . 2 - \O- o——-o——O—-—O-—0—-02- O O '- l l J l l l 1 I94 I96 I98 200 202 204 206 Figure IV—l3. ATI Systematics of some selected states in odd-odd Tl isotopes. The states connected by lines are assumed to be primarily the same configurations. keV state in T1200 is marked by a 7, for we have been unable to decide between 1- and 2- for its assignment. The 289.24- small split! consistent V such doub1e1 not conclus to accept e M to choose b component 0 This has be that M < Characteriz Shell‘model PIOtOn and my Very q simplistic for each of 30111there j CaIlnot use 1 for the 0. Rich lower pt Ju/z, P3/2 the lWest mclude t 112 small splitting between the two states of the latter configuration is consistent with theoretical predictions, and many other examples of such doublets are presented in Ref. [De61]. However, the arguments are not conclusive for T1200, and, based on our present knowledge, we have to accept either possibility for the ground state. Another potential source of information that might allow one to choose between the two above configurations for the primary component of the T1200 ground state is the magnetic moment of this state. This has been deduced from atomic spectral studies, where it was found that In] < 0.15“”. Now, relatively little has been done with respect to characterizing the magnetic moments of odd-odd states in terms of simple shellrmodel states, e.g., determining the damping effects of the odd proton and odd neutron on each other, so one can use such predictions only very qualitatively. Nevertheless, according to our somewhat simplistic estimates, one might expect the effective magnetic moment for each of the two above configurations to be almost the same, lying somewhere in the vicinity of +0.8uN, an odd-odd "Schmidt limit". One cannot use this as a basis for choosing between the configurations either. From Table IV-6 we find that there are three possibilities for the 0— first excited state. Because the 81/2 proton state lies much lower than the d3/2 and d5/2 states in this region, and the fglz, p3/2, and P3/2 neutron states are much closer together, we expect the lowest configurations to include the 81/2 proton state. .We therefore conclude that the [(w81/2)(up1/2)]0_ configuration is the best choise 113 for the first excited state. This follows the "strong" Nordheim rule, which suggests that the 1- state from this configuration will lie higher in energy than the 0— state. Since all the states above the first excited state that are populated by szoo decay have 1- assignments, with the possible exception of the 289.24-keV state, we cannot unambiguously assign the [(ns1/2)(up1/2)]1_ configuration to any specific one of them. However, one would expect to observe a strong Ml transition between the [(n81/2)(up1/2)]1- and [(w31/2)(up1/2)]0- states, which eliminates the 289.26-, 450.56-, 525.55-, and 605.44—kev states as contenders. This leaves the 257.18— and 289.92-keV states as possibilities for the major portion of the [(fl81/2)(vp1/2)1- strength. This is consistent with the prediction of a small splitting between the states of this configuration [Br60, 2e58]. Based on the fact that the 142.28— kever transition between the 289.92- and 147.63-keV states competes very favorably with the more energetic ground-state transition, we tend to favor the [(u31/2)(up1/2)]1_ configuration for the 289.92-keV state. However, if the ground-state configuration.were [(n31/2)(f3/2)]2- and the 289.92-keV state configuration.were [(n81/2)(up3/2)]1-, one could easily explain the weakness of the ground—state transition by its being i-forbidden. From Table Iveé'we see that there are seven possible configura- tions that can result in low-lying 1- states populated by the decay of Pb2°°.. Experimentally we have found six, possibly seven, such states. Cnnsidering the small energy differences between these states, one would expect configuration mixing to play a very important role in this nucleus. Thus, one should take the foregoing arguments as an outline of tLl' St ot‘ an pos so; fra CHE 11h the procedures to be followed in such assignments. And, of course, further attempts here to assign specific configurations to specific states could well be even more foolhardy, for the mixing could easily obviate simple selection rules for, say, the y transitions. As mentioned in Section 4.5, we have not been able to assign an unambiguous spin to the 289.24-keV state on the basis of the experimental data. There is some support for the 2— assignment in the systematics of the states in the other odd-odd Tl isotOpes. In the e decays of Pb198 and Pb195, 2- states are reported to be populated at 259.5 and 240.3—keV, respectively [Ju60], [8v6l]. A 2- state has also been tentatively observed at 205-keV in T119“ [Ju59]. Thus, we might expect to find a 2- state at about 280-keV in T1200. One possible configuration for this state could be [(n31/2)(vf3/2)]2-, as suggested by DeShalit and Walecka for the 2- states in T1198 and T1196. On the other hand, if the ground state were [(n31/2)(vf3/2)]2- and not [(n81/2)(up3/2)]2-, the 289.24-keV state might contain an appreciable fraction of the latter configuration. All other things being equal, one might expect the ground state of Pb200 to populate the latter configuration more strongly than the former, as the 31/2 protons are expected to lie closest to the Fermi surface in Pb. However, this is a very weak argument, and, in fact, if one reasons in terms of the a— decay probabilities, then a l- assignment is slightly favored for the 289.24-keV state, as was discussed in Section 4.5. Although we have included the states populated by (7+?) T1200” decay ID163] in Figure IV-9 for the sake of completeness, we have purposely refrained from including them in our discussion. We 115 expect very little overlap of these states with the ones we have just discussed, and to include them would have resulted in undue complexity. However, we would like to emphasize that odd-odd nuclei do provide one of the most convenient natural probes for studying the p—n residual interaction, so a complete study of states in odd-odd T1200 and the other odd-odd Tl isotopes would be most welcome. Now that high- resolution reactionrproduct spectrometers are coupled with moderate- energy, highest-resolution sector-focused cyclotrons, reactions such as (p,d) on T1203 and T1205 and (t,t) on the evenreven Hg isotopes could well be a very probitable study. CHAPTER v THE DECAY or Pb201 551. Introduction In this chapter we present the results of our study of the states in T1201 populated by the decay of 9.4—h Pb201. Because alTlig; is only one proton removed from the 2-82 closed shell and 6 neutrons removed from the N-126 closed shell, most of its lowblying states should be successfully described by the single-particle shell model. The systematics of this region are especially interesting as the neutron-deficient, odd—mass Tl isotopes provide one of the few series where we can observe the effects of successively plucking out pairs of neutrons on fairly pure single-particle states. Because of this and the similarity of the shell model assignments in T1201 and Tl199 we have postponed a discussion of the systematics and shell model assignments in this region until we have presented the results of our study into the decay of Pb199 in Chapter VI. The first published.work on the decay of P13201 was by Howland and co~workers [H046] in which they reported a half-life measurement of S-h for an activity resulting from the reaction T1203 (d,&n)Pb3°1. Neuman and Perlman [NeSO] in 1950 reported an Biz-h activity in Pb as a daughter of the 62-min and Z-h isomers of 31201. Using a B-ray spectrometer of 3! resolution, Wapstra and co-workers [Wa54] in 1954 observed several conversionrelectron lines from the decay of tho1 to which they assigned y-ray energies of 325 and 583 keV. 116 complete 5 of 0.22. about the oeasuremen Y-rays wer- states at . a half-11f nsec [1.160] "Wk of Fe anEula: Co several of couveI‘Sion Based on t scheme of 1723, and 1097‘: and by AaSa an B‘Spectrmn which! 331.22, 69I and 1479.7 311d 931g 117 In 1955 Bergkvist and co—workers [BeSS] published a more complete study of Pb201 decay using a Bespectrometer with a resolution ef 0.2%. They found 15 7 transitions belonging to Z9-h Pb201. At about the same time T. Gerholm [GeSS] showed by éBézcoincidence measurements in a double lens spectrometer that the 330- and 36l-keV Y-rays were in coincidence. Based on these measurements two excited states at 330 and 692 keV were proposed. In 1960 J. Lindskog, E. Bashandy, and T. Gerholm reported a half-life for the first excited state in T1201 at 330 keV of 0.07 nsecILi60]. The decay scheme was extended considerably in 1961 by the work of Pettersson and co-workers [Pe6l]. They performed gamma-gamma angular correlation and electronrgamma coincidence experiments on several of the stronger transitions. They also measured the K; conversion coefficients for the 330-, 361-, and 585—keV transitions. Based on their singles and coincidence results, they proposed a decay scheme ofa9 excited states at 330, 692, 907, 1097, 1277, 1406, 1618, 1728, and 2012 keV. (Of these we have confirmed only the 330-, 692-, 1097-, and 1277-keV levels.) The most recently published work on the decay of Pb201 was by Aasa and cbdworkers [Aa64] using an iron yoke double focusing B-spectrometer with resolutions as good as 0.17%. They assigned 32 transitions to the decay of prOI and proposed 10 excited levels at 331.22, 692.55, 753.2, 931.8, 1098.5, 1157.4, 1238.6, 1277.1, 1401.0, and 1479.7 keV. (We have confirmed all of these except for the 753.2- and 931.8-keV states.) of Pb201 w Andersson whose ener known whet excited st (3&361 ke‘ PW, sev: isomeric s1 isomer Came Ph0tonucleé l 33518 and 51 a half-11f based on H] 1111965 by direcuy by 58mg: 1,3: m)? 0.1 Thee 325110 ’ and State “as r 118 The only known Observation of positronmemission in the decay of Pb201 was made by Bergvist in 1957 and reported by Bergstrom and Andersson [Be57]. They merely stated that two pOSitron components, whose energy difference is 0.35 Mev,had been observed. It was not known whether the positron emission goes to the ground and first excited states (AE-33llteV) or to the first and second excited states (AB-361 keV). In addition to the levels in T1201 excited by the decay of PbZOI, several workers have reported the direct excitation of an isomeric state in T1201 of In - 9/2-. The first report of this isomer came in 1964 by K. Brandi and co-workers [Br64] who used the photonuclear reaction T12°a(y,2n)T12°1m to excite it. They measured a half-life of 2.1 msec for this state and observed two y-rays of 33518 and 597112 keV de-exciting it. They proposed a state at 927 keV based on these two y-rays. The second report of this isomeric state appeared in 1965 by Gritsyns and Forster [Gr65] who excited the isomeric state directly by bombarding Hg isotopes with protons to produce the reactions ng°l(p,n)r1201m and H320”(p,4m)1'1201m. They measured a half-life of 1.8:0.l msec for a level at 907 keV de-excited by Y-rays of 225:10, 325110, and 585110 keV. The most recent observation of this isomeric state was reported by T. W. Conlon in 1967 [C067]. They also used the proton bombardment of Hg to produce the isomer. They measured a half- life of 2.6510.2 msec for the isomeric state and y-ray energies of 330.6 and 583.3 kevswhich placed the isomeric state at 912.9i0.5 keV. It is apparent from this survey that a great deal of informa- tion had been accumulated on the decay of Pb201 and states in T1201. 119 However, if we examine the decay scheme which summarizes these data, Figure V-l, it is equally apparent that a great many gaps remained in the data. We felt that a y-ray study of this isot0pe using Ge(Li) detectors would go a long way in filling these gaps.and indeed it has, although it has also raised many new.questions. Where previously 32 y-transitions between 10 states in T1201 had been observed in the decay of 9.4-h Pb201, we report here 72 y-transitions between 20 states. We have also been able to assign multipolarities to 25 y-transitions where previously only 3 y-transitions had multipolarities assigned. We have also been able to make specific spin and parity assignments or at least put limits on these for all 20 levels. 120 .>oz as ao>ww mum moumum one mcoauumnmuu one wo moawuouo one .mvaum unmmoum 0:» ouomon neon: mm Homam mo osmnom Macon one .HI> ousmfim .m .8: en» 0 .NV. .. L ”I 83.0 «and w .NR L r w 9 w «Maud I? A .mxn INInIOMud IIIIIIIIII #8- 'I."' "1_.O'I..l.ll"lu'-l!l.l.vl" "'."III' ..II" 'I. ...'.l III"- .7 m 0 SEN Ida flag a... w “H3...— IIIIIIIIIIIIIIIIIIIIIIIIII l-il'l.'sl1 I-+UII.+I-¢1I-.UI' 1"- IlilII'III m a ism; ANS a-IOI'IJQI IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII g.|lva.llcnr.-l' I IIIIII Quaso 5H3. IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII rleuull- gnaum «0%; ...?“ 5.2.1. r12°3(;,3 The Pb? in this chapter w cyclotron t0 indu (70.51 r1205, 29, to maximize the ( amount of prOO. calculated thrash reaction, we did were aged Several activities by the 52-h pb203 and 3 Moon and 52-}! 01111205. The 67 Merely had to age undetectable . The Its half- 18 bouts (ii | 3sell number of “I 52~h pbzoa Was b‘l sources and aqua] szm. T111s is e] both T1203 by the I 121 5.2. Source Preparation 5.2.1. r1203(p,§n)Pb2°1 The Pb201 sources used in most of the experiments described 'in this chapter were produced using 27-MeV protons from the M.S.U. cyclotron to induce the T12°3(p,3n)Pb7-°1 reaction on natural T1 foils (70.52 T1205, 29.52 T1203). The proton energy of 27 MeV was chosen to maximize the (p,3n) reaction and yet not produce a measurable amount of Pb2°°. Although the energy of 27 MeV is slightly above the calculated threshold of 24.7 nev for producing pb2°° by the (p.4n) reaction, we did not observe any szoo in our sources even after they were aged several days. However, we did produce several contaminating activities by the (p,n) and (p,2n) reactions on T1203 giving rise to 52-h szo3 and 3.6-h -Pb202m respectively. We also produced 67 min Pb2°“m and 52-h Pb2°3 activities from the (p,2n) and (p,3n) reactions on T1205. The 67 min Pb2°“m presented no particular problem as we merely had to age the source for 12 hours after which it was virtually undetectable. The Pb202m contaminant did present a slight problem as its half-life of 3.6-h was long enough that aging the source for only 12 or 18 hours did not reduce this activity to an undetectable level. However, this was not a particularly troublesome problem because of the small number of well-known y—rays associated with this decay. The 52—h Pb2°3 was by far the most intense contaminant activity in these sources and actually accounted for more of the source activity than the Pb201. This is easy to understand,for we produced the Pb203 from both T1203 by the (p,n) reaction and13205 by the (p.3n) reaction. 122 1205 Of course the (19,31) reaction on T was by far the largest contributor.as Tl205 accounted for 702 of the target,and while 27 MeV 201 1203 protons maximized the production of Pb from T ,they also maximized the production of Pb203 from T1205. The Pb203 activity was 201 also enhanced with respect to the 9.4-h Pb by aging the sources. At the end of our experiments on Pb201 we did make one source using enriched T12°3(702) and the Pb203 activity was substantially reduced in this source. As in the case of Pb2°°.simply aging the sources did not eliminate the problem of source—purity.for the T1 daughters of szozm and Pb201, 12—d T1202 and 73-h T1201, are also Y-ray emitters. Because of their long half-lives and the small number of well—known Y-rays emitted in their decay, these did not present any major problem. However, we did perform a chemical separation of the lead activity from the T1 target at the end of the 12-18-h aging period for all the sources used in this study. We also performed a separation every 3-5 hours on the sources to keep the T1201 activity to a minimum. The chemical separation used is given as Appendix B. During most of the experiments, sources made from a given T1 target were used for only 5-12 hours of actual counting before being replaced by sources from another target bombarded 5-12 hours later. This was done in order to keep the ratio of Pb201 to Pb203 as high as possible while still allowing the sources to age for 12-18 hours. 5.2.2. 1.12030123157931201 S PbZOI When the M.S.U. cyclotron began accelerating He3 beams on a 123 routine basis we decided to investigate an alternate method of preparing Pb201 similar to that described in Section 4.2.3. for preparing szoo. In this case we bombarded natural T1 with a 48-MeV (70 MeV degraded with 40 mil aluminum) He3 beam to induce the T1203 (He3,5n)Bi201 reaction. 81201 has a half-life of 1.8 h and decays by electron capture to Pb201. Of course we also produced varying amounts of other Bi isotopes by the competing (Be3,Xn) reactions on both T1203 and T1205, where lngS. The major radioactive Bi isotOpes produced by these reactions were: lS-d B1205, 11.2-h B120“, 11.8-h B1203.and 1.6-h B1202. However, the half-lives of these Bi isotopes are all significantly longer than that of 81201 except for B1202, which has a comparable half-life. The T1 targets were bombarded for :1 hour with a 2-uamp He3 beam. The Pb activity was separated from the Bi activity and the T1 target by the ion exchange procedure described in Appendix C. In this case, however, we allowed the Pb activity to build up in the ion- exchange column for :4 hours before washing it off. The sources were aged for ~12 hours and the Pb activity was separated from the T1 daughters by the precipitation procedure described in Appendix B. Although sources prepared in this manner did have a more favorable Pb201 to Pb203 ratio than those prepared by the proton bombardment of natural T1,we did not use this preparation extensively because of the involved chemistry and the difficulty in making sources of the required strength. 5.3.1. y-ral' Si Pb201 two five'Sided t and a true coaxi of 3.0, 2.3. and All detector sys noise RC linear Gaussian shaping L096 channel ana ADC's coupled to M.S.U. Cyclotron The en counting the PbZ listed in Table sources to some energy standards determined by us Stand “51°13? computer Figure t he “21 detect t he (PM) react Sectio D 5.2 1 o . t9 prominent Peak h in detect”, assi; 12h 5.3. Experimental Results 5.3.1._g1-ray Singles Spectra Pb201 y-ray energies and intensities were determined using two five-sided trapezoidal Ge(Li) detectors (0.42% and 2.52 efficient) and a true coaxial Ge(Li) detector (3.61 efficient) having resolutions of 3.0, 2.3, and 2.1 keV, respectively, for the 1.332-keV y of Co“. All detector systems used room-temperature PET preamplifiers, low noise RC linear amplifiers with pole-zero compensation and near- Gaussian shaping. The data were recorded by a Nuclear Data Model 2200 4096 channel analyzer or by Norhtern Scientific 4096 and 8192 channel ADC's coupled to the Nuclear Chemistry group's PDP-9 computer and the M.S.U. Cyclotron Laboratory's Sigma-7 computer. The energies of the prominent y-rays were determined by counting the Pb201 sources simultaneously with the energy standards listed in Table V—l. The T1201 and szo3 were always present in our sources to some extent and were therefore very convenient to use as energy standards. The energies of the weaker Pb201 y-rays were then determined by using the now well-known y-rays from Pb201 as secondary standards. The centroids and peak areas were determined using the live- display computer program MOIRAE [Moir]. Figure V-2 shows a typical y—ray singles spectrum taken with the 0.422 detector. The source for this experiment was prepared by the (p,3n) reaction on natural Tl foils,and as was discussed in Section 5.2.1. the 279-keV y-ray from the Pb203 contaminant is the most prominent peak in the spectrum. From the data obtained with the 0.422 detector we assigned 45 y-rays to the decay of PbZOI. When the larger a?" // b (191 J.3‘ 125 Table V-l y-Rays Used as Energy Standards Nuclide y-Ray energy (keV) Reference T1201 Pb203 “198 A8110m 31207 MDSH Yea T8182 c060 135.34 $0.04 167.43 279.19 10.07 *0.008 411.795i0.009 446.77 620.22 657.72 677.55 686.80 744.19 763.88 818.00 884.67 937.48 1384.26 1505.01 569.63 1063.60 834.83 898.03 1836.13 1121.31 1189.06 1221.42 1173.23 1332.50 :0.04 $0.03 20.03 10.03 :0.03 $0.04 :0.04 220.04 £0.04 toot. 20.05 :0.07 £0.08 £0.06 £0.04 10.04 10.04 $0.04 10.04 10.04 i0.04 10.03 D O m a.n.n.n.n.n.n.n.n.n. 0 O tmrn Fh U‘O‘ Hsa‘ a C.J. Herrlander, R. Stockendal and R.K. Gupta, Ark. Phys. 11, 315 (1960) . b J.B. Marion, Nuclear Data A4, 301 (1968). 126 Table V—l (cont'd) c J.S. Geiger, R.T. Graham, I. Bergstrom, and F. Brown, Nucl. Phys. .68, 352 (1965). d S.M. Brahmavar, J.H. Hamilton, and A.V. Ramayya, Nucl. Phys. A125, 217 (1969). e R.E. Doebler, Nuclear Chemistry Annual Report, Michigan State University, COO-1779-13 (1969). f Average of b and g. 8 R. Gunnink, R.A. Meyer, J.B.Niday, and R.P. Anderson, Nucl. Instr. Methods 65, 26 (1968). 127 .HH Henson: so cowuomou Arm.mv emu mp nonsense mos asuuooem munu canons cu mom: mouaom may .uouomuov Aquoo ucoaoammo Nme.o m sues noxmu assesses anal» mmawsww Comma. .~n> ounmwm mmmSDz .52sz0 Ombn 00mm Comm OOOn . Omhm OOmN OmNN OOflW .mflffllfiflumaflt. .Jsi.uu : I. sis a s . d w $31.4. .. amoufifiufisguufl . ..4. iv . . 2 m u a O m m u t .u a I i 000 u w. z a n m. m .0 I .H 1 COO. 0mm. Com. own. 00: com 000 Can annew 1.1 ex _ _ W 0 mm m “ 8 1. A 9 9 “1| fill «l . rumm n s « umuu new u. .... mm » ... . two... V8 0 W. .1. fl .2 r. 2 F u) «e .1.“ km .7 w W Z .0 r a raw. n n We". m \i m. r .. m mm 1 .91.. .. . .. . l n I. 00....N T 1 90:6 a a $3026 .88 ... a T W 1 no...» _ _ _ _ — _ — 'IENNVHO 83d SiNnOO 128 volume Ge(Li) detectors became available we re-examined the decay and obtained the singles spectrum shown in Figure V—3. This was taken with the 3.62 detector during a 24-hour period using a source prepared by the (p,3n) reaction on enriched T1203. From the many singles spectra obtained with the 2.51 and the 3.61 detectors we have been able to assign a total of 72 y-transitions to the decay of Pb201. The energies and relative intensities of these 72 y-transitions are listed in Table V-2. As noted in Table V-Z, several of the 7— transitions have been corrected for underlying peaks from the decay of szozm. The energies and intensities for the 945.96- and 946.78-keV doublet were obtained from the two-dimensional y—y coincidence experiment. The method used to obtain these will be described in Section 5.3.5.d. The uncertainties in the energies listed in Table V-2 are based on the uncertainties in the energy standards, the heights of the peaks above the underlying Compton background, and the reproducibilities of the calculated energies from many different spectra. Uncertainties in the relative y intensities are based on their reproducibilities in many spectra and the uncertainties in the experimentally determined efficiency curves for the Ge(Li) detectors. The K'x-ray intensity for Pb201 listed in Table V-2 was obtained from the spectra of Pb201 and Pb203 shown in Figure V-4 in the following manner. A szo1 source prepared from enriched T1203 was allowed to age for ==15—h to reduce the szoum and Pb202m to an acceptable level. After this period the only major contaminant contributing to the total x-ray intensity was Pb203. A Pb203 source was then made from .ANONVMQNHH nonsense no coauomou Arm.mv ecu an voumemua mos asuuomem mwsu awsuno so new: mousse 05H .vowuue anew m magnum acuoouov Aquou usowofimmo ue.m m an wovuoomu asuuooem haul» mouwswm < 9mn> shaman $5.22,. 49.24.10 AXXH coon comm ooom com. 000. com _ _ _ _ d e _ 1 IL u mm . . w a... .; mm £233.” 7 I .m... e m .3; 3 T m m w a... .... a mu m 3 8.62m .88 .9 L8 as 'IENNVHO 83d SlNflOO 130 Table V~2 Energies and relative intensities of y rays from the decay of Pb201. Measured energies Relative intensities (keV) Singles Anticoincidence Intergal y-y coincidence K x-rays 4980 £250 - - 120.0 10.2 1.20: 0.3 - 30: 5 124.2 10.2 2.5 t 0.58 - 34: 5 129.95t0.1 6.4 i 0.68 - 114112 155.31:0.1 8.2 i 1.0 6.0: 1.0 179115 202.79t0.1 4.0 i 0.5 - 80:10 231.87io.l 6.5 i 0.8 1.6: 0.5 106111 241.02:o.os 10.0 t 1.08 2.1: 0.5 139:14 285.04i0.07 10.3 i 1.0 1.5: 1.0 166:17 302.7010.4 0.65 t 0.15 - 23: 5 308.93t0.15 2.3 i 0.3 - 251 5 322.4210.15 4.4 i 0.6 - 104:15 331.15i0.06 4550 $250 2800 i150 20,290i1000 341.51i0.08 6.8 i 0.8 1.41 0.6 85110 344.9510.07 18.3 1 1.5 5.0: 1.0 269:20 361.25:0.06 560 i 30 156 :10 67301400 381.29:0.08 12.9 x 0.7 3.5: 0.5 183:15 394.86:0.09 10.8 t 0.7 4.3: 1.0 213:20 405.96:0.07 120 1 6 33 1 5.0 17171150 464.9010.08 19.8 1 1.0 5.41 0.6 275:20 481.98:0.09 3.2 i 0.6 r 53: 8 510.7 10.20 6 t l 1.31 0.3 31: 5 514.38:0.09 9.1 i 2.0 1.6: 0.2 119: 15 540.90:0.09 16.2 1 1.0 4.5: 0.7 2381 25 546.28:0.09 16.5 1 1.0 4.4: 0.5 231: 25 562.81:O.10 1.8 i 0.4 - - 584.60:0.08 211 i 10 60.2: 3.0 2814: 200 Table V-2 (cont'd) 597.60+0.09 637.90:0.09 692.41:0.08 708.75:0.09 727.50:0.09 7s3.35:0.09 767.26:0.08 787.29:0.10 803.66:o.07 826.26:0.08 907.67:0.08 945.96:0.08b 946.78:0.4‘b 979.4 :0.3 999.23:0.o7 1010.3 :0.3 1019.8 :0.3 1062.79:0.15 1070.04:0.08 1088.85:0.09 1098.52:0.07 1114.73:0.08 1124.9 :0.2 1148.75:0.08 1157.45:0.09 1219.4o:0.15 1238.82:0.07 1277.11:0.07 1286.3 :0.2 1308.32:0.08 1330.50:o.15 1340.88:0.09 1381.4 :0.3 1+ 19.0 21.7 254 46.2 7.10 8.751 194 34 90 141 362 424 1+ 1+ H- H- I+ 1+ 1+ 1+ 1+ H- H- u: e-id la «n .. C O O I O c: 3‘ c> 03 hi H- H- l+ I+ I+ l+ ...a H U! h‘ u: N I+ H- 68.0 5100 3.75: 32.6 : 0.86: 26.9 : 1.1 : H- 131 1.0 1.0 12 0.7 0.8 10 20 30 10 0.3 0.2 0.4 0.5 4.0 3.0 6.0 0.6 0.1 2.5 0.4 0.1 4.0 0.2 1.6 0.15 1.5 0.2 4.4: 0.2 6.3:0.3 182 110 14.5: 0.6 2.3: 2.6: 0.2 0.2 75.5: 4.0 13 : 23.3: 54.9: 157 : 205 i 1.1: 17.0: 2.2: 35.5: 24.9: 108 : 4.42: 24.1: 8.64: 0.80: 71.9: 5100 3 1.0 3.0 8.0 10 0.4 1.0 0.2 2.0 1.5 6.0 1.0 1.2 0.7 0.2 4.0 1.90:0.3 17.0 :1.0 0.46:0.1 13.0: 0.6 303:25 295:25 1365:80 636:50 9:10 105: 15 1685:150 468': 100‘31 1184: 80“ 1240:80 2766:140 3460: 170 21:4 327:30 6.8:1.5 16:5 57:6 516:40 378:30 173:10 82: 10 318: 20 15:3 20:3 74:5 5100 7.2 192: 15 . 150:10 16:3 132 Table V-2 (cont'd) 1401.30t0.08 7.90 i 0.4 7.901 0.4 1334 1424.16i0.09 5.75 i 0.3 2.2 i 0.3 3016 1445.80i0.10 2.10 i 0.1 2.5 i 0.4 - 1479.91i0.10 10.4 i 0.5 10.4 i 0.6 9:3 1486.2030.12 - 1.1 i 0.1 - - 1550.5 10.4 0.27 i 0.04 — - 1587.6 $0.5 0.15 i 0.05 - - 1617.45i0.15 1.4 i 0.1 1.6 i 0.2 - 1630.9 10.6 0.14 i 0.04 - - 1639.1 $0.5 0.20 i 0.05 0.55i 0.10 ' - 1672.02i0.10 1.45 i 0.10 1.60: 0.2 — 1678.96i0.13 0.24 i 0.03 - — 1755.32i0.10 0.65 i 0.06 0.83! 0.2 - 1813.1 $0.3 0.26 i 0.05 - - These intensities have been corrected for the underlying peaks from Pb202m decay based on the y intensities listed in Chapter III. These energies and intensities have been determined with the aid of the 2-d experiment. See text for an explanation of how this was done. 133 "1 P0201 SINGLES FOR X-RAYS 106fir E: A ... :2: g :2 g; s ,§9 63 ' 3 10‘?— | V H N 03 no .0 (x m CL X N I to: I. ‘ I ON 10 -r— M I: 3 L‘A—v 104% “4&qu 10?: $101..- 2 Z < I 0 1‘“ 1 L {E j ' a. P6203 SINGLES FOR X-RAYS U) 1.. §§ :: £3 5 t:,. z 10 4- :2; N 133 ' x 10“ I r j 3 '3‘ '\ I 7 3 ~» *~ 10. we,” WMV E 2 a I 10 l 101.. l a , 500 1000 CHANNEL NUMBER Figure V—4. Low energy y-ray spectra of sz01 and szo3 taken with a 2.51 efficient Ge(Li) detector and used to obtain the .K x-ray intensity for szo1 decay. 134 enriched T1203 by the T12°3(p,n)Pb203 reaction. This source was aged :24-h before the spectrum shown in Figure V-4 was recorded. It was then an easy matter to subtract out the Pb203 x—ray intensity from the Pb201 spectrum. 5.3.2. Anti-Compton Spectrum The masking of y-rays by the Compton distributions of higher energy y-rays is one of the major limitations in detecting very weak y-rays in a singles experiment using Ge(Li) detectors. This was an especially bad problem in the case of our early singles runs on Pb201, for the Compton distribution of the 279-keV y-transition from Pb203 was very large as can be seen in Figure V-2. Even in the singles experiment in which we used the enriChed T1203 the Compton distribution from the 331-keV peak of Pb201 itself contributed to a substantial Compton background as is evident in Figure V-3. The conversionrelectron spectra of Aasa and co-workers [Aa64] contained several peaks below 331 keV that were masked in our y-ray singles experiments using natural T1 targets by this large Compton background. The only solution available to us at the time was to perform an anti-Compton experiment using the 8XB-in. NaI(T1) annulus in coincidence with the 2.52 Ge(Li) detector. The general experimental. setup for this experiment has been discussed in Section 2.2.1. The sources were made from natural T1 targets and counted for =30 h to obtain the anti-Compton spectrum shown in Figure V-s. Although it may not show up very well in this figure, we were able to obtain intensities for several low energy y-rays which 135 .vouooavaw omqsuonuo muons uaooxo mononma one Hausa mo house onu ou mnwwnoaon mason ease .Honnnu use no one menus on» no mousoo voumaaaaoo a saw: msasnsm Aaavaz .nwlwxm no mo use one spams“ voosae.uouoouom Awavoo unoaowmuo Nn.~ ecu new»: wheat» Henna mo snuuooee nouasounfiuc< .n1> shaman is. g5 89 m 08. 8» o 4 La WWSJJWOO gomem -8342 .88 p p p - 136 we could not even detect in our singles experiments using the natural T1 targets. Because of the reduced Pb203 in our last singles experiment using the enriched Tl203 targets, we also observed these weak, low-energy transitions in that experiment. The relative intensities obtained from a "true" anti-Compton experiment should be the same as those obtained in a singles experiment and, indeed, in this experiment the y-ray relative intensities were essentially the same throughout the spectrum as those obtained in the singles experiments. Of course the Compton distribution was not only reduced below 331 keV, but throughout the whole spectrum, which made all_the weaker peaks show up better,and we actually used the intensities from this experiment in arriving at the final relative intensities listed in Table V-2. 5.3.3. Anticoincidence Spectra In order to determine which y's are primarily e-fed ground- state transitions, we used the 2.52 detector in an anticoincidence experiment with an 8X8-in. NaI(Tl) split annulus and a 3X3-in. NaI(T1) detector as described in Section 2.3. A resolving time (21) of 100 nsec was used to obtain the spectrum shown in Figure V-6 . The relative intensities obtained are listed in Table V42. In order to compare the anticoincidence with the singles intensities and also the integral coincidence results,we have arbitrarily assigned a relative intensity of 100 to the 1277-keV transition in all three cases. This was chosen because it is a relatively strong transition and an essentiallleOZ c-fed ground-state transition (a y-transition depOpulating a level which is .mousowvna omwsuonuo .ouoas undone pudenda one Hownm mo >muov any on wcawnoaon «some mace .uau uoSuo may wnfixooan nouoouov AHHvHuz .nwlmxm m sues mnasnsm AaHvaz .sanwxw am ovumaw voosae usuoouov Aaavou unowowmuo an. N a means omens» ”omen mo sensuous ooaovwonaoowua<. .ol> shaman ”$952 ..wzzafo ’ ’ ' 80m 83 88 o8. 89 m ______ _ 1 _ _ . mm mu— mu— 137 r 823028 :24 .88 m 2.8 q 'IBNNVHO 83d SanOO 138 primarily fed directly by electron capture with little or no y-feeding from higher levels). The argument may sound a little circular here,as we did not know the 1277-keV level was primarily s-fed until after we analyzed the anticoincidence data, however, for the purpose of displaying the final results this met the above requirements. Based on the results of this experiment it appears that the 1755.32-, 1672.02-, l639.l-, 1617.45-, 1479.91-, 1445.80-, 1401.30—, 1277.11-, 1238.82-, 1157.45-, 1098.52-, and 979.4-keV transitions are all primarily c-fed ground-state transitions. The 692.41- and 331.15- kev transitions are not reduced in intensity as much as most of the other transitions. This leads us to believe that these are ground-state transitions partially fed by other y-rays from above. 5.3.4. Integral Coincidence Spectra To complement the anticoincidence experiment we also performed an integral coincidence experiment. In our first attempt we used the 0.421 efficient detector in coincidence with the 8XB-in. NaI(Tl) annulus. However, because of the poor efficiency and resolution of this detector we did not observe many of the weaker transitions present in the singles and anticoincidence spectra. Instead of repeating this experiment with a larger Ge(Li) detector and the NaI(T1) annulus, we used the integral coincidence spectrum recorded by the 2.51 detector in the 2—d fey coincidence experiment discussed in the next section to obtain the intensities listed in Table V42. This spectrum is shown in Figure VB7. The intensities obtained from the integral coincidence experiment using the 0.422 detector with the 139 .oumw osu a“ woesaoaa mums mmmuux M on» o>onm omens» HH< .HOuoouom Awavou ucoaowmuo no.m m spas menopauswoo aw nouuouov Afiavou uamwowwmo Nm.~ m magma he nonHMupo mos.a=uuooam mane .mxmulr Momnm mo asuuuoem oocovauuwoo Hmumouna .~I> ousmam mmmEDz ..mzz.COUJ¥ neuocm \ kmhflcm .ownh MON QUQQ fiOHufilGflhH cl) ls~F-I\IH. 17h H2V+ «m e«00.0 m«0.0 H00.0H 0H0.0 0.0 H0.« « H Hod n.0«0 «m 0n00.0 m«0.0 0H00.0Hq«00.0 «.0 Hm.H 0 H 00 «.000 .02 «000.0 m««0.0 m00.0H e«0.0 H H 0 0H H «0H m.«0« H2. m000.0 0«0.0 0H0.0H «00.0 «.0 Hm.H 0.0 Hn«.0 e.mn« H2. 0H0.0 «00.0 «00.0H HH0.0 n.0 H0.¢ « H «.0e 0.00« «m HH0.0 0m0.0 H00.0H 0H0.0 m.0 H0.m «H H on« q.«00 «m HH0.0 «no.0 000.0H 0H0.0 «.0 H0.0 0.H H 0.0a 0.«mn d2 nH0.0 0n0.0 000.0H Hm0.0 « H H« w H HH« 0.00m .02 «H0.0 000.0 000.0H 0m0.0 «.0 H0.H 0.H H n.0H n.0en H2. «no.0 02H.0 «0.0H «2.0 m H mm 0 H 0«H 0.00e H2. em0.0 00H.0 n0.0H «H.0 0.H H0.m «.0 H .0H 0.00m .02 «no.0 n«H.0 n0.0H «H.0 «.0 H«.¢ «.0 H .«H m.Hmm H2. «e0.0 H0«.0 «0.0H ««.0 0HH 0H« 0n H 0mm m.a0m H2. «20.0 m«.0 HH.0H H«.0 m.H He.h 0.H H n.0H 0Jmem «m.+.H2. Hn0.0 mm«.0 000.0H man.m 000A 0m«H 0nme «.Hmm .02 H00.0 Hm.0 mH.0H «0.0 0.0 H0.n 0.0 H n.« 0.000 H2. «s0.0 0n.0 00.0H «0.0 m.H Hm.“ 0.H H 0.0H 0.n0« «2 H2 Huvuo ousOHoHumooo uaoHoHumuoo oaoeHuasa A soHouu>soo soHuHo>sou hansousH huHusousH A>oxv vusmHee< Honuouoona HousuaHueexu somnuoesOUJM souonm museum _o«nh you damn sOHuHoduua ¢I> manna NOO0.0 QH0.0 It Noo.oH mHo.o O.H+. 175 H2 H2.+ «m H2V+ «m H2 2.0 2 + «.0 E S + 3 H2 H2 H000.0 0000.0 0000.0 «000.0 0000.0 0000.0 0000.0 «000.0 0500.0 0000.0 0H0.0 HH0.0 HH0.0 «H0.0 00H0.0 0H0.0 0000.0H0000.0 0000.0H0000.0 0H00.0H0000.0 «00.0H 0H0.0 HH00.0H0000.0 0H00.0H5000.0 «00.0HO0H0.0 «00.0H 0H0.0 «.0H 0.0 0.H H.0H 5.0 e H.0H 0.0 m.« 00.0H0«.0 0.0 «.0H «.H 0 H.0H 0.0 0 HH «H 00 0.HH 5.0H 0« +I +I +I +I +I +I +I +l 0.«0 00 0.50 0.0 HHH «5 ««0 «00 $3: dam a 305: due a 0.000H 0.00«H 0.00HH 5.0HHH 0.000H 0.050H 0.000 5.500 Co. usoov 0I> 0.2—cu. I/Km .°~ 176 00 mmsHm> HmOHuoHoonu mnu uHm OH semen mums mm>usu :uooam one .H00wm_ HouuHom 0:0 wamm .fiomnm mo 5woov.o:u waHBOHHom chHuHmsmuu How muaoHonmmou sOHwHo>=ou HHonmIM HMUHumHomnu 0cm HeadmaHHmexm .0HI> euame $9: >0mmzw con. 00: 000 005 000 com 00. trvvv I 4 fl 4 q d 1.111.] A Llllll l O. o. O. o. o. (TIBHS-M) 'O'O'l 5.4.1. Lev I studies, ed shown in F1 given in ke Vaighted a and crosso the final in Vhich t discussions Placement 0 Of Other hi in the orde reasoning 6 Singles SpI together) ’ energy ’ in 177 5.4. Decay Scheme of szo1 5.4.1. Level Placements The level scheme for Pb201 deduced from our coincidence studies, energy sums, and relative intensities of the transitions is shown in Figure V-15. Transition and excited-state energies are given in keV, with the adapted energies for the levels being a weighted average based on our confidence in the respective cascade and crossover transitions. Before we get into the discussion of our reasoning behind the final level placements, we should say a few words about the order in which these will be presented. We have tried to present the discussions in order of increasing level energies; however, the placement of some levels was very much dependent on the establishment of other higher levels. These cases will, therefore, be discussed in the order in which we originally placed them, so as to make the reasoning as straightforward as possible. 5.4.1.a. 331.15-keV Level As the 331.15—keV transition is by far the most intense y-transition in the decay of Pb2°1(its relative intensity in the Y-ray singles spectrum being several times that of all the others added together), we have no choice but to prOpose an excited state at this energy, in agreement with all previous studies. It also turns out to be the first excited state. Figure V-lS. 178 Decay scheme of PbZOI. All energies are given in keV and (total) transition intensities are given in percent of the szoo disintegrations. The percent 6 decay to each state and the log ft values for that state are listed to the right of the state. )Bt‘N .I to ..IIIMUIIU. \II N0 Iflm DC. I I.» I I I ...-Islam... . I mmmeM, ENJMMIIIMWIEIII 179 v.0 A g! 0.0 5’39 30.: 3.010. .gdv F. “313 .flnnfi. 32:2 2333 S nflu> masuHa 250.0 180 5.4.1.b. 692.4l-keV Level The second most intense peak in the sz01 y-ray singles is 'the 361.25-keV transition. This is also the strongest peak in the 331-keV gated coincidence spectrum. This, coupled with a strong 692.41—keV crossover transition that is somewhat enhanced in the anticoincidence experiment, confirms the previous placement of a level at 692.41 keV. 5.4.1.c. 1098.46-kev Level The results of the anticoincidence and integral coincidence experiments alone strongly suggest the placement of a level at 1098.46 keV. This placement is also supported by excellent energy sums, the 405.96 + 692.41 - 1098.37 and 767.26 +.331,15 - 1098.41 sums falling within 0.15 keV of the measured 1098.52—keV crossover transition. Finally, the very strong 692-, 361-, and 331-keV peaks in the 405-keV gated coincidence spectrum,along with the absence of the 692- and 361-keV peaks but presence of a 331-keV peak in the 767-keV gated spectrum put any doubts to rest. Again, this placement agrees with the previous decay scheme. Sal‘slede 1157e41-kev LBVGI. The arguments for the placement of this level are substantially the same as those just used above for the 1098-keV level. The 1157-keV y-transition was shown to be an essentially e-fed ground- state in the anti- and integral coincidence experiments. There are cascade and crossover transitions to both the 331— and 692-keV levels. These traI coincidenI for a 58.' 1098-keV I tentative and 946—1;. and 692-kc us to prep and 692-kg Present a doublet na Bated Coim 1157‘kev st 19ml; is 8 transitions IZiIiIIciden Ce Tible V~2 t Magnum. a grOUQd‘BL 181 These transitions are substantiated by the 465— and 826—keV gated coincidence spectra. Aasa and codworkers [Aa64] also found evidence for a 58.9-keV transition that fits very nicely between the 1157 and 1098-keV levels. Although we did not observe this transition, we have tentatively included it in the decay scheme. 5.4.l.e. 1238.82- and 1277.09-keV Levels The observation of a strong 331-keV peak in both the 908- and 945-keV gated coincidence spectra and the strong 331-, 361-, and 692-keV transitions in the 546- and 585-keV gated spectra leads us to propose states at 1238.82 keV and 1277.09 keV. The week 361- and 692-keV peaks in the 946-keV gated coincidence spectrum did present a problem in placing this transition before we revealed the doublet nature of the 946-keV peak [Section 5.3.5.d.]. The 2-d gated coincidence spectra also show a 120-keV transition feeding the 1157-keV state from the 1277-keV level. The existence of these two levels is also supported by the enhancement of the ground-state transitions in the anticoincidence spectrum and reduction in the integral coincidence spectra. These two levels have also been reported previously. 5.4.l.f. 1330.38-keV Level Although we observed a l330-keV y-ray in the singles and anticoincidence spectra, it appears from the intensities listed in Table V-2 that this peak is not primarily an e-fed ground-state transition. Of course, this does not mean the 1330-keV peak is not a ground-state transition, just that we were unable to pick this level out of the there are correspond are the 99 1,8, we fit respective keV. This as firmly 1 Spectrum a: mark this . °n these a: proPose a I the evident Figure V—9' data are a] d°P°DUlated lower level the 1401.21 as we had n also c(”Isis 182 out of the anticoincidence experiment. From energy sums we find Ithere are three relationships of the type EI+ELI 1330 keV where EL corresponds to the energy of a lower, well-established level and E1 are the 999-, 638-, and 232-keV transitions. Gating on these three Y's, we find they populate the 331-, 692-, and 1098-keV levels, respectively, substantiating the energy sums and the level at 1330.38 keV. This level is our first new addition to the level scheme and is as firmly based as any of the others already discussed. 5.4.1.g. Al401.21-kev Level The enhancement of the 1401—keV Y-ray in the anticoincidence spectrum and its great reduction in the integral coincidence spectrum mark this Y-ray as a probable E-fed ground-state transition. Based on these experiments and the energy sums alone, we would be forced to prOpose a state at 1401 keV. However, for those who would like all the evidence, we refer you to the gated coincidence spectra, Figure V-9, or Table V-3, where the 2-d results are summarized. These data are all consistent with the placement of a level at 1401.21 keV, depopulated by 1070-, 709-, and 124.2-keV transitions to well-known, lower levels. The very weak 302.70—keV transition.was placed between the 1401.21- and 1098.46-keV levels on the basis of its energy alone, as we had no coincidence data for this transition. This level is also consistent with the previous decay scheme. 5.4.leh. 1445.85-keV Level The first evidence for the 1445.85-keV level was in the anticoincidence and integral coincidence experiments where it appeared to be an peak in t‘ a level p shows the This indii Because 0 observe a a very we; the coinc: Placement! Previous c prosted a Clearly in coincidenc prOUUCe a SUbstantia in the ant supported six in the The Placem the 2‘d Co 183 to be an s-fed ground—state transition. The 331—keV y was the only peak in the lllS-keV gated coincidence spectrum and this also suggests a level placement at 1445 keV. The 753-keV gated coincidence spectrum shows the 331- and 361-keV transitions with almost equal intensities. This indicated the placement of the 753-keV Y 33 feeding the 692-keV state. Because of the weak nature of this 753-keV transition, we did not observe a strong peak at 692 keV in the 753_keV sated spectrum, although a very weak peak at this energy is visible in Figure V-9. Based on the coincidence evidence presented here we are very confident of this placement, even though all the peaks involved are relatively weak. The previous decay scheme [Aa64] did not have a level at 1445 keV but proposed a level-at 753 keV fed by the 345—keV transition. This is clearly in opposition to the results we obtained from the 753 gated coincidence spectrum. The 345—keV gated spectrum also failed to produce a peak at 753 keV. We have, therefore, eliminated the 753.2—keV level proposed by Assa. 5.4.1.1. 1479.87- and 1639.47-keV Levels The evidence in support of these two levels is quite substantial. Both have been shown to be e-fed ground-state transitions in the anticoincidence and integral coincidence experiments. Both are supported by numerous cascade transitions to lower, well-known levels, six in the case of the 1479-keV level and five for the l639—keV level. The placement of these cascade transitions have all been confirmed by the 2-d coincidence data,and we refer the reader to Figures V-9 and V-15 and T involved i nature of populates 1817618. B: anticoinci 692-keV lex In additior Populate t 1672-kev 1 1“ any of Itn d Of these le transition came from t The relativ. the 803‘kev level at 11‘ I 285‘. 18h V—15 and Table V-3 for the specific transitions and cOincidence results involved in these placements. We should mention here again the doublet nature of the 946-keV peak and that it is the weaker component that populates the 692-keV. ,5.4.l.1g 1672.00-keV and 1755.31-keV Levels. These two levels complete our highest cohfidence group of levels. Both have E-fed ground-state transitions as shown by the anticoincidence experiment, both have transitions to the 331— and 692-keV levels which are supported by the 2-d coincidence experiments. In addition, the 514-, 395-, and 341-keV transitions were shown to p0pu1ate the 1157-, 1277-, and 1330—keV levels, respectively, from the 1672-keV level. Although these two levels have not been reported in any of the previous studies, we are quite confident of their correct placement. 5.4.l.k. ll34.81:,71290.05-, and 1420.00-keV Levels Placing these levels was the most frustrating problem in this decay scheme and, as we indicated above, we do not feel as confident of these levels as the ones already discussed. These are three of the four levels where we did not observe a direct ground-state transition. Our first piece of evidence for the 1134.81-keV level came from the 803-keV gated coincidence spectrum shown in Figure V-9. The relative intensities of the peaks in this spectrum indicate that the 803-keV 7 directly populated the 331-keV level and the resulting level at 1135 keV was populated directly or indirectly by the 155—, 130‘, 285-, and 345-keV transitions. This speculation was supported by the 341 peaks of e at 331 anc‘ depopulate already cc p0pulate t level at 1 The strong “‘1? the 3. also gated peaks Of a 692-keV st; the Placem. 1420 0 00~kel 1420‘ and I that the SI of the 28S wally Cam IEVEI, there IIe have 812 Ilith the 8C the ll35‘kel Med qI Show the ID 185 by the 345-keV gated coincidence spectrum which contained only two peaks of equal intensity (after correcting for detector efficiency), at 331 and 802 keV. This would indicate that the 345—keV y-ray depopulates a l479-keV level.which is quite possible as we have already confirmed a level of this energy. As we postulated above, the 285-keV transition may directly populate the 1135-keV level. If this were true, we would expeCt a level at 1420 keV and there is evidence to support such a level. The strong 1089-keV transition was shown to be in coincidence with only the 331-keV transition in our 2-d experiment [Figure V-9]. We also gated on the weak 727-keV y and observed only 331- and 361-keV peaks of almost equal intensity, indicating the 7274keV y p0pu1ated the 692-keV state. These two pieces of evidence strongly support the the placement of a level at 1420.00 keV. Thus far we have quite confidently placed the 1134.81- and 1420.00-keV states. After placing the 285-keV transition between the 1420- and 1135-keV on the basis of the coincidence data, we noticed that the sum of the 129.95- and 155.3l-keV y-rays is within 0.22 keV of the 285.04-keV transition. If the 130- and 155-.-keV Y-rays were really cascade transitions from the l420—keV level to the 1135-keV level,there would have to be an intermediate level at 1265- or 1290—keV. We have already mentioned the fact that the 130- and 155-keV y's are in coincidence with the 803-keV transition and therefore populated directly or indirectly the 1135-keV level; however, this does not tell us whether or not they are in a cascade thru one intermediate level. In Figures V-lO and V-ll we show the low energy portion of the 130— and lSS-kev gated coincidence spectra . indeed in direct cas gated spec the S98-ke 361-, and the depOpu far the st the S98-ke a level at transition placement I Based on t] at 1290.05 the ones dI they rePI‘EI 8“Peorted II spec t mm ’ or integra 18 3180 SU which Comp. he? Mk 1 experin 186 spectra. These spectra show that the 130- and 155-keV transitions are indeed in coincidence with each other, although not necessarily in a direct cascade. However, we did observe a 598—keV peak in the 130-keV gated spectrum but not in the lSS—keV gated spectrum. Then gating on the 598—keV peak, we observed,in addition to the l30—keV peak,33l-, 361—, and 692-keV y's in exactly the same intensity ratio as found in the depopulation of the 692-keV level. As these three peaks were by far the strongest in the coincidence spectrum, we are convinced that the 598-keV transition populates the 692-keV level directly, placing a level at 1290 keV. The fact that we did not observe the 155-keV transition in the 598-keV gated spectrum is consistent with the placement of this transition between the 1290- and 1134-keV levels. Based on the evidence presented here we feel certain there is a level at 1290.05 keV, although we do not feel as sure of this placement as the ones discussed earlier. 5.4.1.1. 1550.5-, 1617.45-, and 1712.5-keV Levels. The evidence for these three levels is somewhat limited and they represent our lowest confidence group. The 1550.5-keV level is supported by the observation of a y-ray of this energy in the singles spectrum, although we did not observe it in either the anticoincidence or integral coincidence spectra because of its very low intensity. It is also supported by the sum relationship, 1219.40+33l.15 - 1550.55, which compares very well with the measured 1550.5-keV y—ray. The 1219- keV peak is very weak, but we were still able to gate on this in the 2-d experiment. we observed a single peak at 331 keV in the resulting coincide tion to that was have the to the me transitio Figure V- at 331 Re keV level. included 1 transition kev gatEd I M Peak. 1712-5, 815 trans 1 tion Spectra . 'I hWever’ We of this lee in on: SingIl more than 9' of °°1ncide experiment . 187 coincidence spectrum. Therefore, we do have some coincidence informa- tion to support this level. The 1617.45-keV level is supported by a ground-state transition that was shown to be e-fed in the anticoincidence experiment. We also have the sum relationship 1286.3-I-33l.15 - 1617.45, which is identical to the measured 1617.45-kev ground-state transition. The 1286.3-keV transition is also very weak and the gated coincidence spectrum is Figure V-9 is not very convincing, although it does show a weak peak at 331 keV. From these pieces of evidence we have proposed the 1617.45- keV level. The 1712.5-keV level is by far the most speculative we have included in our decay scheme. As we did not observe a ground-state transition, our first indication of this level came from the 1381.4- keV gated coincidence spectrum in which we observed only a weak 331- keV peak. The energy sums, 1019.8+692.4 - 1712.2 and l38l.4+331.l - 1712.5, also suggested the level. Unfortunately, the 1019.8-keV transition was so weak we did not observe it in any of the coincidence spectra. This concludes our discussion of the level placements; however,we would like to add a few parting words as to the completeness of this level scheme. 'We have included 64 of the 72 y-rays observed in our singles experiments in this decay scheme and these account for more than 99.952 of the total transition intensity. The vast amount of coincidence data we obtained from the 2-d y-y coincidence experiment is consistent with the decay scheme as shown in Figure V-15 188 with very few exceptions, and these few were weak coincidence observations which are questionable. The 8 y-rays left unplaced are all very weak and our assignment of these to szo1 decay is also some- what questionable. . 5.4.2. 8+-feeding In our singles experiments we observed a very weak peak at 510.7 keV which appeared to be slightly broader than the Y-ray peaks next to it. Considering the energy of the peak and its possible broadening, we have assumed that this peak arises frcI.positron annihilation in the B+¥decay of Pb201. Based on the Q: of $2 nev, calculated from the "experimental" masses listed in the table of Myers and Swiatecki [1065] , we find that 8+-decay is energetically possible in the decay of Pb201. Using the 511-keV Y and the.K xrray intensities listed in Table V82, we have calculated an upper limit of 0.0361 for the total B+¥feeding in the decay of Pb201. From theoretical ratios of 3000/5+ [Vs59], the calculated Q6 of 22 nev, our spin and parity assignments, and our e—feeding intensities, we predicted the B+Ffeeding intensities listed in Table ves. These three levels are the only energetically possible ones for B+¥feeding from .the ground-state of Pb201. The predicted B+-feeding to the groundé state was calculated assuming 1.322 e-feeding [Section 5.6.3.]. . The 511-keV peak was also visible in the integral coincidence spectra of the 2-d Y-Y coincidence experiment and we were.able to. obtain the 511-keV gated coincidence spectrum shown in Figure V‘9. The only r-ray that we could definitely observe to be in 189 Table V¥5 8+;feedingin Pb201 Decay Energy of Level Experimental Theoretical Calculated Observed (keV) s-feeding sK/g+ B+¥feeding B+efeeding Ground State <1.3zz 260a 0.00461 331.15 51.8 1 340b 0.12 z .50.034z 692.41 6.53: 9000b 0.0058: a This is a first-forbidden unique B-transition. These are first-forbidden nonunique B-transitions. 190 coincidence with the 511-keV peak was at 331 keV. Although the 361-keV Y shows up in the 511 gated spectrum in Figure V-9, it was also very strong in the coincidence spectra of the adjacent background, and because of the poor statistics we could not definitely determine whether the 361-keV peak in the 511-keV gated spectrum.was legitimate or due to chance and incomplete background subtraction. We also observed the 511-keV peak in the 331-keV gated coincidence spectrum. A.very weak 511-keV peak was also visible in the 692-keV gated spectrum while in. the 361-keV gated spectrum it was either absent or hidden in the Compton background. The only previous report of positrons in the decay of Pb”1 indicated two branches approximately 350 keV apart [BeS7]. Nothing was mentioned concerning their relative intensities or which states they populated. Based on the results of our experiments, there appears to be a small amount of B+¥feeding to the 331-keV level and some weak evidence for B+Ffeeding to the 692-keV state. As these two states are 361-keV apart, this is at least consistent with the previous observa- tions. A triple y-coincidence [511 keV-511 keV-any] experiment [Au67] was performed using the 8x8-in. NaI(Tl) split annulus with the 3.62 Ge(Li) detector in an attempt to substantiate the tentative B+Efeeding results of the 2-d experiment. However, because of the extremely weak nature of the 511-keV peak and the wide energy gates used in the NaI(Tl) detectors, we could not draw any conclusions concerning the B+-feeding from this experiment. 191 When more and larger volume Ge(Li) detectors become available, it would be interesting to repeat the triple-coincidence experiment using three of these. This would be even more valuable in connection with a 3—dimensional data-taking system similar to our present 2- dimensional system. One could then perform a background subtraction on the two 511-keV gates, which would be very desirable in this case considering the intensity of the Compton background under the weak Sll-keV'peak. 5.4.3. Egg it's The total transition intensities, including internal conversion in the K, L, and.M'shells, were calculated from our singles intensities and the theoretical conversion coefficients of Hager and Seltzer [Ha68]. We assumed an.Ml multipolarity for all transitions except those shown to be 32 or mixed.Ml + E2 from the measured K- conversion coefficients listed in Table Ve4. These total transition intensities, in percent of the total prOI disintegrations, are given in the decay scheme, Figure V-lS. From the measured K xeray intensity and Keconversion intensities and the calculated K fluorescent yield, 0.95, [F166] the total e-feeding intensity to the ground-state was determined to be <1.32%. This upper limit on the feeding to the ground has taken into account the uncertainty in the x-ray intensity listed in Table Ve2. If we use the actual x-ray intensity listed in Table VBZ, we obtain a net negative feeding to the ground-state, and as this is somewhat unorthodox, we assume the actual ground-state feeding to be somewhere in the range 02 - 1.322. This corresponds 192 to a log‘ft58.6. The total e-feeding intensities to each state in T1201 were then calculated assuming 1.322 ground-state feeding. These intensities are given in the decay scheme to the right of the energy levels. Log ft values based on these e-feeding intensities, the 9.4-h half-life of Pb201, and the calculated Qs appear at the extreme right of the levels. 193 5.5. Spin and Parity Assignments 5.5.1. Ground and 331-keV States The T1201 ground-state has a measured spin of 1/2 from both atomic spectra [Hu6l] and atomic beam resonance [L158,Ha58] experi- ments. This is consistent with the predicted 81/2 proton shell model assignment, giving us a spin and parity (In) of l/2+. Our upper limit of 1.322 on direct 8 population to this state corresponds to a log;ft1_8.4 which is in agreement with a predicted first-forbidden log;ft :_9 for a 5/2- to l/2+ transition. However, as this transition would be z"131/2 +vf5/2, which makes it l-forbidden as well as spin forbidden, the log‘ft is probably quite a bit higher than 9. The 331-keV state was previously assigned.In-3/2+ by Pettersson and co-workers [Pe6l] on the basis of the mixed.Ml +-E2 331-keV Y- transition and the strong 6 population of this state (51.8% of our decay scheme). Our work supports this 3/2+ assignment, as the log ft of 6.6 is in the range expected for a first-forbidden transition. 5.5.2. 692- and 1277-keV States The 361-keV y-ray depopulating the 692-keV level was previously assigned a multipolarity of M1 based on the measured K—conversion coefficient [Pe6l]. Our value for this conversion coefficient also suggests aan multipolarity. This would allow In-l/2+, 3/2+, or 5/2+. However, the angular correlation results of Pettersson and co- workers [Pe6l] rule out all but the 5/2+ possibility. This assign- ment also agrees with the log ft of 7.3 which lies in the range of a first-forbidden transition. 19b The angular correlation results of Pettersson [Pe6l] require a spin of 1/2 or 3/2 for the 1277.99-keV state. These spins are both consistent with the‘Ml multipolarity calculated for the 945.96—keV y to the 3/2+ 331-keV state which, in addition, suggests a positive parity. TheIVl multipolarity of the 584.60-keV transition moreover is incompatible with a l/2+ assignment, which leaves us with the 3/2+ assignment for the 1277-keV state. The log‘ft of 6.5 also sup- ports this assignment, as a l/2+ assignment would be first-forbidden . unique log ft_>_9 . 5.5.3. l098.46—keV State The calculated log‘ft for e decay to this state was 6.9. This indicates either an allowed or first-forbidden nonunique transition, which could populate 3/2-, 5/2-, and 7/2- or 3/2+, 5/2+, and 7/2+ states in T1201. From the definite M1 assignment for the 405.96-keV y to the 692-keV state (In-5/2+), we can eliminate the negative parity assignments. The M1 multipolarity of the 767.26-keV Y to the 331- keV state (3/2) narrows the choice to 3/2+ or 5/2+. The 32 multi- polarity we have assigned to the 1098-keV ground-state transition is based on the measured 9K listed in Table Veb. This would strongly suggest a 5/2+ assignment for the 1098.46-keV state; however, we cannot positively eliminate the 3/2+ assignment on this basis for the transition could possibly be a mixed Ml + 5'2 with the E2 component strongly enhanced. However, as there are very few mixed Ml + E'2 tran- sitions observed in this decay scheme, and we might not expect such a strong collective enhancement at this energy, we feel the 5/2+ assignment is the better choice. We have indicated this in the 195 decay scheme by putting the 5/2+ assignment first and the 3/2+ assign- ment in parenthesis. 5.5.4. 1134.81- and 1290.05-keV States The log;ft values calculated for the 1134.81- and 1290.05-keV states allow the.In assignments 3/2+, 5/2+, 7/2+, l/2+, 9/2+, 3/2-, 7/2-, and possibly even 5/2—. Based on the measured qK's we have assigned both the 803-keV’y from the ll36-keV state to the 331-keV state and the 597-keV y from the 1290-keV state to the 692-keV state as E2's. These.E2 transitions eliminate the negative parity pos- sibilities as well as the 9/2+ possibility for the ll34—keV state. This leaves us with 4 possible assignments for the ll34—kev state and 5 possible assignments for the 1290-keV state. If we assume the 803— and 597-keV Y's are mixed‘Ml +-EZ transi- tions we obtain the following possibilities, l/2+, 3/2+, and 5/2+ for the lle-keV state and 3/2+, 5/2+, and 7/2+ for the 1290-keV state. A l/2+ assignment for the llBé-keV state is somewhat questionable on the basis of the log ft, since this would be a first-forbidden (unique) transition, log ftz9, whereas the measured log f%-8.l. For a l/2+ assignment we‘might also expect to see this level populated from the 1277-keV level (3/2+) or see a transition to the ground-state (l/2+). For the llBé—kev state we still have the possible assignments 3/2+ and 5/2+. Although the log ft's are in agreement with these assign- ments, the question again arises as to why so few Y's populate the ll34-keV level and why there is no Y-transition to the 1098-keV (5/2+), 692-keV (5/2+), or ground (l/2+) states. These same argu- ments hold for the 1290-keV state, but even.more so. Under the 196 assumption of an Ml admixture in the 597-keV y, we have only the possible.[n's 3/2+, 5/2+, and 7/2+. The l29C-keV state is populated by only one transition out of all the higher lying states, many of which we will show later have.In's of 5/2+ and 3/2+. In addition, this level feeds only the 1134- and 692-keV states. If the-TH were 3/2+ we might have expected at least some branching to the ground or first-excited states. Having discussed the possible spins arising from a mixedle + E2 assignment for the 803- and 597-keV y's let us consider the alternative. If we assume noJMl admixture in the 803-keV Y, we have the additional possibility of a 7/2t assignment for the ll34-keV state. This assignment does raise some questions as to the y-ray branchings, namely, why is there no transition to the 692- or 1098-keV states, which, being 5/2+ states, could be populated by‘Ml transitions. However, a 7/2+ assignment provides the best explanation for the apparently "pure" E2 multipolarity of the 803-keV transition. Assuming no.Ml admixture in the 597-keV Y gives us the two additional possibilities of 9/2+ and l/2+ for the 1290-keV state. The l/2+ state might be expected to populate the l/2+ ground-state or the 331-keV level (3/2+), while the 9/2+ assignment would not be expected to populate these. In addition, the 9/2+ assignment provides the best explanation for the apparently "pure" E2 nature of the 597-keV y. A 9/2+ assignment would also be consistent with the population of the ll34-keV state if we made a 7/2+ assignment here. The spin and parity assignments most consistent with the above arguments are 7/2+ for the 1134.81-keV state and 9/2+ for the 1290.05- 197 keV state. These assignments are also consistent with the theo- retical predictions of Alaga and Ialongo (A167). We will discuss these theoretical calculations in more detail in Chapter 6. How- ever, we can not definitely eliminate the 5/2+, 3/2+, and l/2+ pos- sibilities for the ll34—keV state or the 7/2+ possibility for the 1290-keV state and these have been placed in parentheses following our preferred assignments in the decay scheme. 5.5.5. 1157.41- and 1479.87-keV States The log ft's for these two states (7.3 and 6.8) fall within the limits for allowed and first-forbidden transitions. However, we can eliminate any negative parity possibilities on the basis of the M1 + 32 multipolarities of the 826.26— and 1148.75-keV y's. This leaves us with the 3/2+, 5/2+, and 7/2+ possibilities. However, since the 826- and 1149-keV v's to the 331-keV state (3/2+) have strong.Ml components, the 7/2+ assignment is not possible, leaving us the 3/2+ and 5/2+ possibilities to choose from. On the basis of the y-ray branching ratios, a notoriously poor criterion but the only one available, we feel the 5/2+ assignment is the more probably for the 1157.41-keV level, although we would definitely not eliminate the 3/2+ possibility. In the case of the 1479-keV level we again prefer the 5/2+ assignment on the basis of the Y- branching ratios but we also have an M1 Y-transition, 345-keV, to the 7/2+ lle-keV level. If we were more certain on the spin and parity of the ll34—keV state, we could be more definite in our assignment of the 1479-keV state. However, as mentioned in. 198 the last section, this 7/2+ assignment is somewhat shakey. Considering all the evidence we have assigned the 1479.87-keV level as 5/2+ with a 3/2+ second choice. 5.5.6. 1238.82-keV State Because there are Ml transitions to the ground and first two excited states, we were able to assign this state as 3/2+ without much question. The log‘ft of 6.8 agrees with a 3/2+ assignment, falling within the range of a first-forbidden transition. 5.5.7. 1401.21 and 1445.85-keV States The log ft's for these two states, 7.0 and 7.8, fall within the range of both allowed and first-forbidden transitions. However, both haveJMl transitions to the 331— and 692-keV levels, eliminating the possibility of an allowed e transition to these states. A 7/2+ assignment is also inconsistent with the observed.M1 transi- tions to the 331-keV state (3/2+). We now are left with a choice between a 5/2+ and 3/2+ assignment for both states, with only the y-branching ratios on which to base our decision. In both cases the 331- and 692-keV states are populated more strongly than the ground-state, which would be the case if the ground-state transitions were "pure" single-particle E2 transitions. Based on this we might predict a 5/2+ assignment for both states. However, this argument is very weak as collective effects could enter strongly into these states and we have only to examine the y- branching ratios for the 1238- and 1277-keV y's to see how erroneous these predictions could be. Nevertheless, we have placed the 5/2+ 199 assignment first in the decay scheme but with the 3/2+ assignment following it having almost equal probability. 5.5.8. 1420.00-keV State The log;ft of 7.1 falls within the range of an allowed or first- forbidden a transition, and thefill nature of the 285-keV transition to the 1134-keV state rules out the negative parity states. Again we have a first-forbidden 2 transition, resulting in the 3/2+, 5/2+, and 7/2+ possibilities. The 285-keV1Vl transition to the 7/2+ state also narrows the choice to the 7/2+ and 5/2+ possibilities. The 7/2+ assignment would definitely be the better choice based on the y-branchings observed in this case. The first thing we notice is that the l420-keV state populates only states with spins of 9/2+, 7/2+, 5/2+ and in one case 3/2+. The fact that there wasn't a trace of the ground-state transition in the singles spectrum would also support the 7/2+ assignment, as this would be aanB transition. Of course, the 7/2+ and 9/2+ assignments were not definite for the 1134- and 1290-keV states but we feel that the internal consistency between the 1134-, 1290-, and thO—kev states lends support to our spin assignments for all of them. 5.5.9. 1550.5-keV State The log ft of 9.3 for the 1550.5-keV state is the result of an extremely small a feeding intensity, 0.0071. While this log ft is still in the range for a first-forbidden transition, it is definitely in the range of a first-forbidden (unique) transition. These give rise to the possible In's l/2+, 3/2+, 5/2+, 7/2+, and 200 9/2+. Based on the fact that this state was found to populate only the grand (l/2+) and first-excited (3/2+) states, we can limit the spins to l/2+, 3/2+, or possibly 5/2+, with the l/2+ being the most probable. We have also included a possible 3/2- assign- ment for this level since the log ft is close enough to that for an allowed: (Z-forbidden) 5 transition, that we cannot absolutely rule it out. This assignment would also be consistent with the y-branchings. Of course, an allowed (Z-forbidden) 6 transition would also allow 5/2- and 7/2- possibilities, but we have ruled these out on the basis of the y-branchings. $5.10. 1617.45- and 1712.5-keV States The log ft's for a decay to the 1617.45- and 1712.5-keV states are 8.0 and 8.2. These could indicate In's of the l/2+ through 9/2+, 3/2-, 5/2-, and 7/2-. The 1617.5-keV state is depopulated by only a ground state transition and a transition to the 331-keV State. This narrows the most probable assignment to l/2+, 3/2+, 5/ 2+, or 3/2-. The 1712.5-keV state is depopulated through the 692- and 331-keV states with almost equal intensities, there being no ground-state transition. The y-branchings in this case make a 1/2+, 9/2+, or 7/2- assignment unlikely. .This leaves us with In - 3/2+, 5/2+, 7/2+, 3/2- and 5/2- as possibilities for the 1712.5-keV state. $5.11. 1639.47- and 1672.00-keV State Both the 1639.47- and 1672.00—kev states have a log ft =- 6.8. The resulting I1! possibilities are 3/2i, 5/2i, and 7/21. The 201 negative parity assignments can be eliminated in both states on the basis offill transitions to lower positive parity states. A 7/2+ assignment for the l639-keV state is not compatible with the 1308- keVMl y to the first excited state and we are left with the two possibilities 3/2+ and S/2+. We can rule out a 7/2+ assignment for the 1672.08-keV level on the basis of the 394-keV1Vl transition to the 3/2+ 1277.09-keV state. Again we are left with the two possibilities 3/2+ and 5/2+. 5.5.12. 1330.38- and 1755.31-keV States The calculated log‘ft's are 7.5 and 7.3 for the 1330- and 1755-keV states respectively. The possible.In's for these log ft's are 3/2i, 5/2:, and 7/2:. We have eliminated the positive parity' possibilities for the 1330—keV level on the basis of the 308-keV.M1 transition from the positive parity l639-keV state. The observation of ground state transitions from both of these two states rules out the 7/2 spins as these would be.E3 or‘M3 transitions. For the 1330-keV state we are left with the 5/2+ and 3/2+ possibilities, and for the l7SS-keV state we are left with the 5/2+, 3/2+, 5/2-, and 3/2- possibilities. 5.5.13. Summary of Spin and Parity Assignments Of the 20 preposed states in T1201, we have made unique spin and parity assignments for 5 of these. We have narrowed the assign- ments of 10 more states to two possibilities and narrowed the pos- sible spin and parity assignments of the remaining 5 levels to four. As mentioned at the beginning of the chapter, we will postpone 202 a discussion of the systematics and theoretical calculations of the odd~mass Tl isotopes until the end of Chapter 6. 203 CHAPTER v1 ms DECAY or Pb199 AND STATES IN ODD-MASS Tl ISOTOPES 6.1. Introduction This chapter contains the experimental results of our :study of the states in T1199 populated by the decay of 90 min Pb199, «as well as a discussion of the systematics and shell model assignments 10f these levels and those of the other odd-mass Tl isotopes. Pb199 was first reported by Neuman and Ferlman [Ne50] in 1959 as the daughter of 24 min B1199 (made by proton bombardment of in). By measuring the yield of 7-h Tl199 obtained at 60 minute intervals from a Pb199 source, they determined the half-life of 7Pb199 to be =80 min. The second and most recent study of Pb199 decay was made ‘by Andersson and co~workers in the mid 1950's [An55, An57]. They produced Pb19g sources by proton bombardment of natural Tl, followed in some cases by mass separation of the resulting Pb activities. Using a B-spectrometer of 0.31-0.42 resolution, they assigned several conversion electron lines to the decay of Pb199 and measured the half-life to be 90:12 min. These electron lines corresponded to y-ray energies of 352.8, 367.0, and 721.0 keV and the K/L ratios of the 353 and 367-keV transitions indicated they were both of mixed.Ml+E2 multipolarities. The 353- and 367-keV transitions were also shown to be in coincidence in an electron-electron coincidence experiment and based on this, and the observation of the 721-keV 20h cross-over transition, they proposed two excited states at 367 and 720 keV. (These levels are in agreement with our work.) Assuming a l/2+ ground state, they prOposed In assignments for these two excited states of 3/2+ and 5/2+, respectively, based on multipolarity assignments and the systematics of the other odd-mass Tl isotopes. (The ground state spin of 1/2 was later confirmed in atomic beam resonance [Br57] and atomic spectra [Hu6l] experiments.) In addition to the three y-rays already mentioned, for which a definite assignment was made, they also made tentative assignments to Pb199 of several electron lines corresponding to y-transitions of 267 and 1132 keV (also observed in our study). Andersson and co~workers [An57] also looked at the 8+ spectrum of a complex mixture of isotopes, including Pb199, and after extensive corrections, obtained a maximum end point energy of 2.8 MeV, which they tentatively assigned to Pb199. In addition to the levels in T1199 excited by the decay of Pb199, several workers have reported the direct excitation of an isomeric state in T1199. The first report of a possible isomeric state in T1199 was published in 1957 by Leipunskii and co~workers [Le57]. Using a pulsed beam of 20 uev protons on Ego and a NaI(Tl) detector, they observed a 370i20—keV y peak decaying with a half-life of 42:5 msec. In 1963 Diamond and Stephens [D163] produced an isomeric state in T1199 by bombarding Au197 with 22 uev o's to induce the reaction Au197(a,2n)T1199m. They measured the electron spectrum 205 following the decay of Tllggm using a single wedge-gap B-spectrometer and assigned the following five y-transitions to its decay: 29, 353, 367, 382.1, and 720 keV. They measured the half—lives of these transitions to be 2714 msec and proposed a level at 749 keV de-excited by the 29- and 382.1-keV transitions to the 720— and 367-keV levels respectively. The K/L ratio and half-life indicated an.E3 multipolarity for the 382-keV transition and an In of 9/2' for the 749-keV level. A y-Y angular correlation experiment confirmed the possibility of a 9/2 (E3) to 3/2 (M1+E2) to 1/2 spin sequence. Additional studies of T1199m were made by Demin et a2. [De63], Gritsyna and Forster [Gr65], and Conlon [C067]. The results of these studies generally supported the decay scheme proposed by Diamond and Stephens [Di63]. Demin at al. obtained a half-life of 28.9iO.6 msec for the isomeric state, Gritsyna and Forster found a half-life of 26.6il.4 msec, and Conlon, using a Ge(Li) detector, observed y-rays of 367.010.3 and 382.8:0.3 keV decaying with a half-life of 29.2i2.0 msec. From the survey of previous studies presented above, it is apparent that very little was known about the decay of Pb199 before the present study, especially when one considers the fact that the Pb199 decay scheme should be at least as complex as that of 1%”1 (Chapter V). Indeed, where previously 3 v-transitions between 3 states in T1199 had been observed in the decay of Pb199, we report here 89 y-transitions placed between 29 levels. 206 6.2. Source Preparation 6.2.1. ngoogHe3,41)Pb199 Our first attempts at producing Pb199 consisted of bombarding natural HgO with He3 beams. We made bombardments at various energies from the coulomb barrier up to 70 uev in an attempt to optimize the production of Pb199. However because of the large number of stable isotopes of Hg (Table IVbl), we could not obtain a source at any energy that didn't contain large amounts of contaminating activities. Aging the sources, as was done in our preparation of Pb200 by a similar reaction, does not help in this case because the half-life of Pb199 is much shorter than many of the contaminants and comparable to most of the others. Therefore, we decided very early in our study of Pb199 that the use of separated isotope targets was a necessity. We obtained separated isotope Hg2°° (99.91) in the form of HgO from Oak Ridge National Laboratory. After a rough excitation experiment, we found a He3 beam of =35 rev (70 rev degraded with 56 mil aluminum) maximized the (He3,5n) reaction on Hg200. However, this energy is slightly above the calculated threshold of 29 nev for producing Pb198 from the (He3,6n) reaction and we did observe small amounts of Pb198 and its daughter, T1193, in most of our sources produced at this energy. We also observed Pb201, Pb200, and Pb199m in these sources. The Pb200 and Pb201 were in very small amounts and did not cause any major problems during the first few half-lives of Pb199. However, 207 because of their longer half-lives they became more of a problem as the Pblggdecayed away. Therefore, we seldom counted a source longer than 5 hours from the time it was made. We did make a very significant amount of Pb199" and it could have been a serious problem; however, only one y-ray is known to be emitted in its decay to Pb199 and its half-life is only 12 minutes. In order to reduce this activity in our spectra, we usually waited about 15 minutes from the end of a bombardment before starting the chemical separation. The separation took an additional 30 minutes and, therefore, by the time the source was ready to count, the Pb199m had decayed by almost four half-lives and was no longer considered a problem. In the course of this study we found we could obtain Pb199 sources of useful intensity by bombarding a barely visible amount of the HgO with a highly focused He3 beam of several uamps for 15 to 30 minutes. The lead activity was then chemically separated from the T1 and Hg contaminants using a procedure similar to that used in separating Pb from T1 targets (Appendix B), the only change in the procedure being the addition of Tl+++ in place of Hg++ as the hold-back carrier. ‘Most of the sources used in our study of Pb199 were made using this method. 6.2.2. T12°3(2,5n)Pb199 The second method used to produce Pb199 made use of the reaction T12°3(p,5n)Pb199. In our first attempts, we bombarded natural Tl foils with 44euev protons. Although 44 MeV is slightly above the 208 calculated threshold of 41 MeV for the T12°3(p,6n)Pb198 reaction, -we did not observe any y-rays from either Pb198 or its daughter, T1198, in these sources. However, the sources made with natural Tl targets (29.52 Tl203 70.5! T1205) did contain a large amount of 9.4h Pb201 produced by the (p.5n) reaction on T1205. The Pb201 was a particularly serious contaminant because of the large number (72) of y—rays emitted in its decay. Using enriched (701) T1203 targets, we obtained sources with greatly reduced amounts of Pb201. However, these sources still contained a much higher level of szo1 than those made from the He3 bombardments of HgZOO. In addition these sources also contained Pb199m’ Pb200’ Pb202m. ”3203, and PbZOW. These sources were chemically purified before counting using the procedure given in Appendix B. Pb-Tl separations were also performed every 45 minutes to reduce the T1 contaminants that kept growing-in. 209 6.3. Experimental Results 6.3.1. -ra Sin les 8 ectra Pb199 y-ray energies and intensities were determined using the 2.52 and 3.61 efficient Ge(Li) detectors. The y-ray singles spectrometer systems used in this study were described in sections 2.1. and 5.3.1. and will not be repeated here. The energies of the prominent y-rays were determined by counting the Pb199 sources simultaneously with the energy standards listed in Table VI-l. The energies of the weaker Pb199 y-rays were then determined by using the now well determined y-rays from Pb199' as secondary standards. Figure VI-l shows a typical y-ray singles spectrum obtained in 45 minutes with the 2.52 efficient Ge(Li) detector at a resolution of 2.3 keV FWHM for the l332-keV'y-ray of 006°. The source used to obtain this spectrum was made by the (He3,4n) reaction on separated Hg2°° as described in Section 6.2.1. From the many singles runs performed during this study, 117 y-rays have been found to belong to the decay of Pb199. The energies and intensities of these y-rays are listed in Table VI-Z. During many of the singles experiments, and also in the anticoincidence experiment (Section 6.3.2.), we recorded a series of spectra, usually at 45 minute intervals, over several half-lives of a given source. (To improve the statistics of the weaker y-rays, we added spectra obtained with other sources during corresponding 45 minute intervals.) In most of these experiments we performed Pb-Tl separations on the source every 45 minutes to remove the T1 and Hg 210 Table VI-l x-Rays Used as Energy Standards Nuclide Y-ray Energy (keV) Reference Ir192 205.782 i 0.014 a 295.938 i 0.009 a 308.429 i 0.010 a 316.486 i 0.010 a 468.053 1 0.014 a 588.557 i 0.017 a 604.385 1 0.017 a 612.435 1 0.017 a 0056 846.78 i 0.06 Average of b, c, and 1037.89 1 0.07 Average of b, c, and 1238.30 i 0.05 Average of b, c, and 1360.25 i 0.05 Average of b, c, and 1771.43 1 0.05 Average of b, c, and 2015.37 i 0.06 Average of b, c, and 2034.93 i 0.06 Average of b, c, and 2598.58 i 0.06 Average of b, c, and 3253.63 1 0.06 Average of b, c, and D-DaG-O-O-D-G-O-D- a G. Hurray, R. L. Graham, and G. S. Geiger, Nucl. Phys. 63.353 (1965). b R. Cunnink, R. A. Heyer, J. B. Niday, and R. P. Anderson, Hucl. Instr. Methods _6_5_ 26 (1968). c M. a. Phelps, 0. c. Sarantites, and w. c. Winn, Nucl. Phys. 4149 647 (1970). d R. A. Meyer and D. Camp, private communication, Lawrence Radiation ' Laboratory, Livermore, California (1970). 211 .voaoooa one amuse mo amuse ecu ou wcawcoaoo mason one no Has no: asuuuoom menu ma moaumauoum coon onu mo monsoon one oo>uomno shout» mo noose: owuoa ecu mo monsoon an) mm 1N odouoma commandos co coauomou Are.mo:e mnu mo nonmeouo on: season mmfiom new .ooauom sue me m menace uouoouoo Awgvou ucowofiwmo Nn.~ m %n novuooou madam uo aSuuuoem host» moawcwm < $249236 ORE .RXB” .XKN .... i.1.....5..............1. .5 5...... a . . . n . m . .HuH> daemon 252% $53 ...an m 212 Table VI-2 Energies and Relative Intensities of x-rays From the Decay of Pb199 Relative Intensities Measured Energies Integral y-y (keV) Singles Anticoincidence Coincidence K X-rays 14501150 Cl 120.54:0.15 1.0$0.3 0.3 --- 130.73$0.2 0.5:0.2 --- --- 152.14$0.20 1.1$o.4 0.5 --- 202.2 10.3 0.5$0.2 --- _ --- 222.83$0.10 1.2t0.5 0.3 21 240.3 $0.2 2.8t0.2 --- 64 267.6 $0.2 6.9$0.49 3.2 116 312.3 $0.7 0.5to.2 --- e-- 319.2 r0.4 1.6$0.5 --- --- 344.0 $0.7 0.65:0.25 --- --- 353.39$0.06 169 $8 17 4750 361.4 $0.6 6.0t2.0“ 1.4 199 366.90$0.06 790 :40“ 400 13,000 390.3 $0.4 3.0to.s 0.7 as 400.54$0.08 23 :2 2.3 q 600 430.9 $0.3 3.5$0.3 0.3 121 433.2 $0.3 3.0$0.8 0.4 --- 476.9 $0.2 3.2iO.8 --- 131 494.89$0.10 6.6$1.0 --- 190 503.15$0.2 1.9$0.6 0.3 --- 510.90$0.10 29 $3 2.0 1400 521.23$0.07 7.5$1.s 0.4 285 537.0 $0.2 1.2t0.3 -—- -—- 574.9s$0.15 1.8t0.3 0.7 --- Table VI-2 (cont'd) 605.8 10.6 a 641.3 10.4 685.2 10.2 720.24:0.06 724.5 10.4 735.4 $0.3 753.92:0.08 76l.98i0.07 777.20:0.1S 781.48i0.07 792.5 10.4 833.83iO.10 838.68i0.10 874.77:0.09 911.8010.15 937.8910.08 984.4 10.5 995.6 10.4 1005.13:0.08 1029.21i0.09 1048.09:0.09 1052.66i0.09 1115.1 10.4 1121.00:0.7 1135.0410.08 1161.27:0.09 ll70.70:0.09 1177.2 i0.4 1187.23:0.10 1209.6010.10 1215.2 10.3 1239.12i0.10 O.7.tO.4 1.1:0.2 1.7i0.2 116 :5 2.0:0.4 2.0:0.4 28.4il.S 40.0:2.0 5.410.5 33.2:2.0 l.0:0.3 3.3:0.5 l6.0il.2 29.2:1.5 6.6:1.0 37.7:2.0 1.0:0.4 2.1to.4 24.0:1.2 28.9il.5 5.2:0.8 5.0:0.8 ll.3t1.0 26.8:1.S 140 :7 15.4:1.0 5.510.5 l.2:0.3 8.3:0.8 4.6iO.3 l.8:0.3 37.8:1.5 213 556 1060 220 630 47 390 564 111 730 78 470 540 130 110 240 185 1400 335 106 140 64 610 Table VI-2 (cont'd) 1265.4 10.3 1291.5010.10 l311.2810.10 1325.7 10.3 1328.3 10.3 1358.6 10.3 1382.7110.09 1401.9410.10 1481.2 10.6 1502.0410.08 1506.2 10.4 1517.1210.10 1524.1010.15 1531.2310.10 1553.3 10.3 1563.3010.15 1577.5 10.5 1592.5810.15 l602.6110.9 1610.6710.10 1631.8 10.3 1647.2 10.6 l658.4310.09 1695.2810.10 1725.3 10.5 1749.7010.10 1768.4810.15 l793.1010.2 1840.0 10.4 1859.3 10.3 1891.3 10.3 1898.7 10.6 3.210.3 5.10.5 7.011.0 3.310.5 3.310.5 6.210.8 51.012.0 18.0il.0 2.410.4 38.311.5 3.310.4 8.310.6 2.910.? 9.210.6 1.510.3 1.410.3 l.010.2 4.710.5 7.310.6 10.210.7 1.610.3 2.010.4 100 6.0iO.5 l.210.4 41.412.0 4.0:1.0 4.010.? 1.410.2 2.310.5 7.210.S 1.510.5 0.69 63 75 131 87 46 494 220 107 24 29 E100 215 Table VI-2 (cont'd) 1930.69$0.20 2.0$0.5 2.0 --- 1959.5010.20 1.210.2 1.2 5.. 1973.5 10.3 1.3$0.2 1.5 --- 2000.6110.15 3.3$0.3 0.3 33 2019.60:0.15' 1.7$0.3 1.3 --— 2031.4 $0.5 4.2$0.3 4.2 --- 2042.6 10.3 2.310.6 4.4 23 2046.3 $0.6 2.310.6 2.3 --- 2062.5010.20 1.310.2 1.7 --- 2066.95$0.20 1.210.1 -- 20 2073.4 $0.2 . 1.310.2 0.31 20 2090.20$0.20 . 2.910.3 2.3 --- 2100.3 $0.3 0.3:0.2 -- .17 2153.6 $0.3 0.310.2 0.33 16 2130.2 $0.4 0.77$o.2 --- --- 2206.5 10.3 ' 1.5:0.3 1.3 --- 2226.7 10.3 0.510.3 0.4 --- 2237.2920.10 11.010.7 _ 10.3 14 2244.3 $0.3 0.6:0.2 0.4 --- 2303.7 10.3 0.9$0.2 1.0 --- 2341.6 10.3 2.7$0.2 2.9 --- 2361.9 10.3 1.6i0.3 1.6 --- 2367.0 10.5 1.5$0.3 1.5 --- 2399.2 $0.3 1.4to.2 1.4 --- 2433.1 $0.3 2.7$0.2 2.7 --- 2547.6 10.4 0.510.2 0.7 --- 2566.3 0.4 0.510.2 0.5 --- 2643.2 10.4 0.610.2 0.3 --- 2751.9 10.7 0.310.2 0.3 -- f aThese intensities have been corrected for underlying peaks from the decay of other isotopes. 216 contaminants that kept growing-in, while in a few experiments we allowed these contaminants to accumulate over several half-lives. From the relative intensities of the y-rays observed in these various experiments, it was then an easy matter for us to separate y-rays belonging to Pb199 from those belonging to other Pb isotopes as well as T1 and Hg contaminants. As an additional check on our assignment of these 117 y-rays, we required that the y-ray singles relative intensities observed in sources made by the (p.5n) reaction on enriched Tl203 agree with those observed in sources made by the (He3,4n) reaction on Hg200. The uncertainties in the energies listed in Table VI-2 are based on the uncertainties in the energy standards, the heights of the peaks above the underlying Compton background, and the reproducibilities of the calculated energies from the many singles spectra. Uncertainties in the relative y-intensities are based on their reproducibilities in many spectra and the uncertainties in the experimentally determined efficiency curves for the Ge(Li) detectors. The K x-ray intensity listed in Table VI-2 was obtained from the singles spectra in the same manner as the y-ray intensities, although in this case we had to correct for the K x-ray intensities contributed by the Pb200 and Pb201 impurities in their a decay. Pb200 and 96201 were the most abundant impurities in most of our sources made by the (He3,4n) reaction on separated isotope ngoo; although even these were present in rather small amounts as is evident in.Figure VI-l. 217 6.3.2. Anticoincidence Spectra In order to determine which y's are primarily s-fed ground- state transitions, we used the 2.52 detector in an anticoincidence experiment with an 8x8-in. NaI(Tl) split annulus and a 3x3-in NaI(Tl) detector as described in Section 2.2.1. Figure VI-2 shows an anticoincidence spectrum obtained in 8 hours using a coincidence resolving time of 250 nsec. In order to reduce contamination from 7.4 hour T1199, which was constantly growing-in, a fresh source, free of T1 and Hg was prepared every hour, and a new target was bombarded every four hours. The relative intensities obtained are listed in Table VI-2. In order to compare the anticoincidence with the singles intensities and also the integral coincidence results, we have arbitrarily assigned a relative intensity of 100 to the l658-keV transition in all three cases. This was chosen because it is a relatively strong transition and an essentially 1001 s-fed ground-state transition (a y-transition depopulating a level which is primarily fed directly by electron capture with little or no y-feeding from higher levels). The argument may sound a little circular here, as we did not know the 1658-keV level was primarily s-fed until after we analyzed the anticoincidence data, however, for the purpose of displaying the final results this met the above requirements. The results of the anticoincidence experiment indicate a great number of apparent e-fed ground-state transitions, 33 in all, and these are listed in Table VI-3 as possible states in T1199. 218 .oouoc ouwfiofio muses uaouxo “ceases.” was .302. Momma—m ou wmuwmoauo mason 3:0 .660 some can woes—00.3 nouoouov 35:2 .5 man a 5.3 madness thumz d.“ mum on 032: pounds— uOuoouoo 3.53 ucoaowwwo N.m.~ m not; oucwmuoo shout» woman no guooea «ensues—$0035 .NIH> one»; E2? 02.» L4I||Jl4l1oJoulnl .|._lr..:: .4J1 4441 «till. II! I Ila-..IIII .11. . 11.11....» ...... ...... €51 ...: ‘ u m m m e .m w *2 l nus I 9 bill it'll le— .48.! m I“ I 2588 geese m u .- WB‘NVHO 83d SLMWOO 219 Table VI-3 Possible Levels in T1199 Indicated by Apticoincidence and Integral Coincidence Experiments EY (keV) Ey(keV) EY (keV) 1121.00 1959.50 2244.3 1502.04 1978.5 2303.7 1602.61 2019.60 2341.6 1631.8 2031.4 2361.9 1647.2 2042.6 2367.0 1725.3 2046.8 2399.2 1749.70 2062.50 2433.1 1768.48 2090.20 2547.6 1891.3 2206.5 2566.8 1898.7 2226.7 2643.2 1930.69 2237.29 2751.9 220 6.3.3. Integpgl:goincidence Spectra To compliment the anticoincidence experiment and determine which y-rays are involved in cascades, we wanted to perform an integral coincidence experiment. However, instead of doing an entirely separate experiment using a Ge(Li) detector in coincidence with the 8x8-in. NaI(T1) annulus, we used the integral coincidence spectrum recorded by the 2.52 detector in the 2-d Y-y coincidence experiment (Section 6.3.4.) to obtain the intensities listed in Table VI-Z. This spectrum is shown in Figure VI-3. The results of this experiment were entirely consistent with the results of the anticoincidence experiment. 6.3.4. 2-d 1:1 Coincidence Experiment From the results of the anticoincidence experiment we placed 33 y-rays as possible e-fed ground-state transitions; however, we were still left with 84 unplaced y-rays. The placement of most of these remaining y-transitions in the Pb199 decay scheme was aided greatly by a 2-d y-y coincidence experiment of 4096 x 4096 channels, using the 2.52 and 3.62 efficient Ge(Li) detectors. The general experimental setup for this experiment was described in Section 2.3. and will not be repeated here. During the 24 hours of counting 22.2 million coincidence events were recorded. The two integral coincidence spectra obtained from this data are shown in Figure VI-4. The Y-side spectrum was recorded by the 3.61 detector and the Xeside by the 2.52 detector. Sources for this experiment were made by the He3 irradiation of separated isotope ngoo, however, in this case no 221 .uooaoumoaoo as» nouns uowumu oowwm on» so oesuouaon em: nowusumeoe Hmofiluno on menu menu cu .ouuaoo man on sea as nouoouoo Awavow usofioaumo uc.n a no“: oumoouoowoo cw nouoeuoo Aquou usuaoummo Nm.~ a mafia: he vooasuoo shawl» «mush no sauuoeeo eosoowuouoo HammouaH .mIH> enough g 135.5 - 5:11.. I . - . 1: W .111...“ e £8882?ng Ian M 1WD 83:! Slmoo 222 p 3&8 and «5 .3 33%» 33.x 2% mum om» mm: flfihwommm quanta use .am—nm co pnuawhmuxm m mocmuwocfioo >3» Hacofimcmawvuoap map sauna cmuuooon auuoomm moaucwonfioo adummwun .wuH> whamam mwmzzz szz “67.. H. __ . m». 4m 4% 00 8 I - — u .. .v h.u .b 0.! . _ L .m 80 .l 69 I I _ a SS 9 S I. I .u M n“ M. c. _ _ I ._ _ _ . 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IIS '— 89Z _ EZZ '- 866 — 00*! — SEh '- 199 .. 89E _. _ _ 7w 89 88 I 688 — L98 _ ESE oH oH oa o" o~ o— 13NNVHO 83d SlNflOO COUNTS PER CHANNEL 10 10 10 10 10 10 10 10 10 10 237 0 725-KEV GATED 735-KEV GATED " 367 "‘ l.12 (TIISB) 1H 754-KEV GATED - 367 762-KEV GATED as m m 3 " :5: E l 3 | I ‘ ”“HMIIIHI n l . i . 500 1000 1500 2000 CHANNEL NUMBER Figure VI-S (cont'd) IHU 238 777-KEV GATED 78 l -KEV GATED o~5.| oom.l mmm.l “111 11 f D W AENoNan: m S... I v E .h 8 7 33.: m3 I Azflouomy NN:.I ll Ll 828-KEV GATED Ill IL, I]. Ill, 1 0 101 F ..wzz.:I— S I V I 500 1000 1500 2000 CHANNEL NUMBER Figure VI-S (cont'd) COUNTS PER CHANNEL 10 10 10 10 10 253 Y E III] --367 _ 367 - 367 2067-KEV GATED N0 BACKGROUND SUBTRACTION I III- I 2078-KEV GATED NO BACKGROUND SUBTRACTION II III I ZIOO-KEV GATED NO BACKGROUND SUBTRACTION '- 367 2I08-KEV GATED NO BACKGROUND SUBTRACTION 2I59-KEV GATED NO BACKGROUND SUBTRACTION II I I II 500 1000 1300 2000 CHANNEL NUMBER Figure v1-5 (cont'd) 25% Table VI- II Results of Y-x coincidence Study of Pb199 Using 2-dimensiona1 Maia Gated energya Energies of 1-razs in Qincidence with Gate (keV) Strong Weak 159 (11199) 248, 284, 300, 334. 375 353. 367 174 (2) 762 320. 353. 367 177 (7) 575 248, 367 208 (11199) 198, 248, 284, 1013 542 224 (7) 412 367 227 (2) 386 368, 1135 235 (215200) 290 241 267. 353, 367. 720. 875 521, 400 248 (11199) 208 257 (28200) 235. 268 396 267 (Pb199+Pb2°°) 257, 353, 367. 762 361 284 (11199) 208 290 (Pb2°°+Pb199) 235, 365. 575 331 (Pb201) 361, 405, 585, 767, 907, 652 946 336 (T1198) 412 344 (Pb2°1+Pbl99) 331 353 173, 175, 223, 241. 267 361. 688. 690. 367, 400, 438, 440, 477 1439, 1577, 1647 495, 511. 521, 762, 777, 834, 912, 938, 1005, 1029, 1053, 1171, 1210, 1239, 1311, 1327, 1506, 1517 361 (Pb199+Pb2°1) 267, 331. 585 400, 440, 495, 637. 688 w 367 (Pb199+T12°°) 173, 175, 223, 241, 259. 361. 416, 622, 735, 267, 290, 353, 382, 390, 745, 834, 887, 912, 400, 425, 432, 457, 477, 1647, 1891 495, 511, 521, 579, 661, Table VI-3 (cont‘d) 374 382 386 388 390 400 412 416 422 425 431 448 455 477 482 495 511 521 579 (prOHm) (7) (7) (7) (T1198) (7) (PbZOZm) (7) + 433 (7) (7) (7) 255 685, 725, 754, 762, 776, 781, 828, 839. 875, 922, 938, 947. 976, 1005, 1029, 1053, 1115, 1135, 1161, 1171, 1187, 1207, 1210, 1226, 1239, 1265, 1274. 1292, 1309, 1311, 1327, 1358, 1361, 1383, 1402, 1408, 1480, 1525, 1517, 1531, 1593, 1603, 1610, 1695, 1724, 1793, 1859, 1975, 2001, 2181 899 579 267, 368 511, 1239 223 376, 637, 899 306 353, 223 367, 875 353, 521 353, 361, 367, 511, 720, 387, 425, 440, 526, 777 754, 839, 1029 587, 637, 677, 1204, 223, 1422, 1444, 1470, 1312 1732, 2042, 2193 268 922, 875 657, 787, 961 277 754, 1161 295, 912, 353, 367 353 207, 227, 284, 562, 781 241, 267, 353, 368,720 754. 1029, 1115 762 208, 268, 412, 720, 762, 353, 368 241, 353, 367, 720, 762 449 277, 331, 353, 368, 400 267, 320, 521, 961 412, 511, 720, 754, 1121 1239 241, 312, 353, 367, 390, 412, 448, 511 720 368, 828 Table VI-3 (cont'd) 587 629 639 652 657 661 676 685 701 720 725 735 754 762 777 '781 787 828 834 839 875 886 912 938 961 1005 1013 1029 1048 1053 (pb201+'r1198) (T1200) (pb199+q1198) (7) (prOZW) (11200) (11198) (11200) (111,202)?!) (Pb201+T1200) (T1200) (pb202 ) (T1199) 331, 637 368, 227, 422 412 839 661 267, 781, 1517 367’ 267, 720 353. 277, 422, 331, 367, 241, 368 353, 353, 422, 353, 353, 353, 367, 256 361, 412, 490, 579, 579 412, 587 390, 938, 400, 521, 762, 1005, 1029 777 353, 376, 477, 495, 400 367, 720 367. 353, 961 579 685, 367, 1053 390 720 720 367, 367, 787 367, 720 367, 367, 839 720 720 762 490, 295, 368 367 367, 368, 353, 1327 353, 367, 361, 286, 482, 223, 579 246, 353, 241, 312 276 319, 267, 388, 448 637, 720, 1005 1053 482 433 367, 1171, 1311, 367, 292 412 433 422, 579 320 433 260 367 277, 400 440 290 440, 754 Table VI-3 (cont'd) 1100 1115 1121 1135 1161 1171 1187 1207 1210 1226 1239 1265 1274 1292 1311 1326 1328 1358 1368 1383 1402 1422 1481 1506 1517 1524 1531 1593 1603 1611 1631 1658 (7) (T1200) (T1200) (T1200) (11200) (T1198) (pb199+11200) 267, 511, 223, 367, 353. 367 368 353, 368 353, 367 368 367 353, 367 367 368 367 367 412 257 367 433 367 432, 504 367 367 367, 720 367, 720 353, 367 353, 367 367 367 368 367 367, 720 367, 438, 875 477, 495 438 344 412 367 212 412 320, 511 258 Table VI-3 (cont'd) 1695 367 1750 312 1768 412 1793 367 1859 367 2001 367 2041 (7) 412 2067 367 2078 367 2100 367 2159 367 a 'Hany of the gates set in analyzing the y-y coincidence experiment included y-rays belonging to isotopes other than Pb199 and these are identified in parenthesis following the energy of the gate. In cases where the origin of the gated y-ray was not known or un- certain, we have used the notation (?) following the gated energy. 259 6.3.5. Conversion Cogfficients m mltimlarity Agsignments Because internal conversion data were available for only three y-transitions, we were unable to obtain conversion coefficients, and hence multipolarities, as we did in our previous studies of Pb200 and Pb201. However, we will discuss here the small amount of data ' that is available. Andersson and co-workers [An57] reported conversion electron intensities for only the 353- 367-, and 720-keV transitions. They determined the multipolarities of the 353- and 367-keV transitions to be mixtures of 752 M1 + 252 5'2 and 301 M1 + 702 5'2, respectively, based on the internal conversion K/L and LI + LII/LII! ratios. For the 720—keV transition they obtained only the K-conversion intensity and therefore couldn't determine its multipolarity. However, using this K-conversion intensity and our measured photon intensity, we can obtain the X-conversion coefficient and hence the multipolarity, provided we can normalize the two sets of data. Unfortunately, there are ' no directly measured conversion coefficients or even a transition of known "pure" multipolarity which we could use, and we had to normalize the twa sets of data to the mixed Ml-I-EZ, 367-keV transition. From their measurement of the electron spectrum of 28 msec T1199", Diamond and Stephens [D163] obtained a multipole mixture of 232 M1 + 772 E2 for the 367-keV transition, in fair agreement with the results of Andersson and co-workers mentioned above. Using the average multipole mixture of these two studies, 26! M1 + 742 3'2, and the theoretical conversion coefficients of Eager and Seltzer [Ha68], 260 we obtained a K-conversion coefficient of 0.011 for the 720-keV transition which is in good agreement with the theoretical value of 0.0095 for an E2 transition. We also calculated the Keconversion coefficient for the 353-keV transition and obtained a value of 0.144. This corresponds to a multipolarity of 60%.M1 + 401.32 compared to 752IM11+ 252 E2 obtained by Andersson.[An57] from conversion co- efficient ratios. 261 6.4. Decay Scheme of Pb”9 6.4.1. Level Placements The level scheme for Pb199 deduced from our coincidence studies, energy sums, and relative intensities of the transitions is shown in Figure VI-6. Transition and excited-state energies are given in keV, with the adopted energies for the levels being a weighted average based on our confidence in the respective cascade and crossover transitions. Before we get into the discussion of our reasoning behind. the final level placements, we should say a few words about the order in which these will be presented. We have tried to present the discussions in order of increasing level energies; however, the placement of some levels was very much dependent on the establishment of other higher levels. These cases will, therefore, be discussed in the order in which we originally placed them, so as to make the reasoning as straightforward as possible. 6.4.1.a. 366.90—keV Level The large relative intensity of the 366.90-keV transition along with its coincidence behavior leads us to place a level at 366.90-keV. This placement is in agreement with the previous study of Pb199 decay [An57] and all studies of 28 msec T1199” decay [0163, De63, C067]. This also turns out to be the first excited state. 6.4.1.b. 720.26-keV Level The second most intense peak in the Pb199 v-ray singles Figure VI-6. 262 Decay scheme of Pb‘gg. All energies are given in keV and (total) transition intensities are given in percent of the Pb2°° disintegrations. The percent 8 decay to each state and the log ft values for that state are listed to the right of the state. 263 a law it ’3 3.5: .NC I: 3% 3.: In; 3.: 1.3 $2.23“ :5 5 a. :4" «aka 5.! ; 3.:‘5 3. fig 355 ”gun“ a! g.— 2556 _Q.t I: 3.: I96 oIH> seamen 3,519.0 \Ii 203 oIH> muawwm 8 QR S ICON 3.: fled .0.» .3 3.: a $3” 7.: In; 3.: IO... :4; find an. 1N4 mam“... 2.13! ; .08.!3 .3. $8.. 35 I'd ”gum.“ .0... $5.. ...:Seo ...: a as \IfiflaI :5 no.6 fitted} Wm. >62 «mac 5: No £8 Ne .90 sEom 26h spectrum is the 353.39-keV transition. This is also the strongest 'peak in the 367-keV gated coincidence spectrum. This, coupled with a strong 720.24-kev crossover transition that is somewhat enhanced in the anticoincidence experiment, confirms the previous placement {An57, D163] of a level at 720.26 keV. 6.4.1.c. 1120.90-keV Level. The results of the anticoincidence and integral coincidence experiments alone strongly suggest the placement of a level at 1120.90 keV. This placement is also supported by excellent energy sums, the 400.54 + 720.26 - 1120.80 and 753.92 + 366.90 - 1120.82 sums falling within 0.22 keV of the measured 1121.00-keV crossover transition. Finally, the very strong 720-, 353-, and 367-keV peaks in the 400-keV - gated coincidence spectrum, along with the absence of the 720- and 353-keV peaks, but presence of a 367-keV peak in the 767-keV gated spectrum put any doubts to rest. 6.4.l.d. 1241,67-kev Level. The level at 1241.67-keV was placed on the basis of gated _ coincidence results and energy sums. The 521.28-keV transition was strongly in coincidence with the 353-, 367- and 720-keV transitions . while the 875-keV transition was in coincidence with the 367-keV transition but not with either the 353- or 720-keV transitions. 'The 120.54-keV transition was placed between the 1241.67- and 1120.90-keV levels on the basis of energy sums only, as our coincidence data did not extend below about 160 keV. 26S 6.4.l.e. 1482.25-keV Level The presence of this state was first suggested by the results of the anticoincidence experiment in which the weak 1481.2- keV transition appeared to be a partially e-fed ground-state transition. The presence of this state was confirmed by gated coincidence spectra which showed the 24l-keV transition in coincidence with the 521- and 865-keV y's, the 762-keV transition in coincidence with the 720-, 353-, and 376-keV y's, and the 1115.1-keV transition in strong . coincidence with the 367-keV y. The 361-keV transition was found to be in weak coincidence with the 400-keV transition and based on this and the energy sum, we placed this transition between the 1120.90- ' keV and 1482.25-keV levels. 6.4.1.f. 1502.00:, 1632.00- and 1658.47-keV Levels The evidence in support of these three levels is quite conclusive. All three were shown to be e-fed ground-state transitions in the anticoincidence and integral coincidence experiments and all three are supported by cascade transitions to the 366.90- and 720.26- keV levels. The 1632.00-keV level also feeds the 1241.67-keV level via a 390.3-keV transition. The placement of these cascade transi- tions have all been confirmed by the 2-d coincidence data. We refer the reader to the decay scheme (Figure VI-6), the gated coincidence spectra (Figure VI-S), and the summary of the 2-d coincidence results (Table VI-4) for the specific transitions and coincidence results involved in these placements. 266 6.4.lég, 1725.4-, 1749.6-, 1768.51, and 1891.1-keV Levels The evidence for these four levels is essentially of the same nature as that presented for the previous three levels. The l725.3-, 1749.70-, 1768.48- and 1891.3-keV y's have been shown to be shfed ground-state transitions in the anticoincidence experiment and all four levels are supported by cascade transitions to the_first and second excited states. The results of the 2-d coincidence experiment as summarized in Table VI-4 confirm the placement of these cascade transitions. 0n the basis of the energy sum alone we would probably place the 222.83-keV transition between the 1725.4- and 1502.00-keV_ levels. In the 224-keV gated coincidence spectrum, Figure VI-S, we see a strong 412-keV peak and a weak 367-keV peak, indicating a doublet, the more intense and higher energy component belonging to the decay of T1198 and the weaker component belonging to the decay of Pb199. Although the observation of the 367-keV peak in this spectrum does not conflict with our placement of the 223-keV transition, it does not really pin it down either. However, the observation of a strong 223-keV peak in the 353-, 367- and 1135-keV gated coincidence spectra, as well as a weak 223-keV peak in the 781-keV gated spectrum, provides the best evidence for our placement of this transition. The placement of the 267.6-keV transition between the 1749.6- and 1482.25-keV levels was based on the energy sum, 1482.25 + 267.6 - 1749.8-keV, and the 2-d coincidence results, which leave‘ little doubt as to the certainty of this placement. 267 6.4.1.b. 1898.1-, l959.45-. and 1977.8-keV Levels These three levels have been quite firmly established in our Pb199 decay scheme. Again, the behavior of the ground-state 4 transitions from these levels in the anticoincidence and integral coincidence experiments was a primary piece of evidence. Strong cascade transitions from all of these levels to the 366.90-keV level,' as well as cascade transitions to the 720.26-keV and 1482.25-keV levels from both the 1949.45- and 1977.8-keV levels, were confirmed by the 2-d coincidence data. The only inconsistency we found between these placements and the coincidence data occurred in the 477—keV gated spectrum. As summarized in Table VI-4, the 477-keV gated coincidence spectrum contained strong 241-, 267-, 353-, 367-, 720?, and 762-keV peaks and weak 754-, 1029-, and 1115-keV peaks, all consistent with our placement except for the strong 267-keV peak and the weak 1029-keV peak. However, as we did not observe a 477-keV peak in the gated spectrum.of either the 267- or 1029-keV y's, we have assumed that these peaks in the 477-keV gated spectrum are spurious (they may arise from incomplete background subtraction). We also have good coincidence data to support the placement of the 777.20-keV transition between the 1898.1- and 1120.90-keV levels. The. 319.2-keV transition was placed between the 1977.8- and 1658.47-keV levels on the basis of the Energy sum and the observation of a weak 319-keV peak in both the 1658- and 938-keV gated coincidence spectra. 6.4.1.1. 2226.5— and 2367.3-kev Levels These two levels complete our highest confidence group of 268 levels. Both have e-fed grhund-state transitions as shown by the anticoincidence experiment, and both have transitions to the 366.909 end 720.26-keV levels which are supported by the 2-d coincidence experi- ment. In addition to these transitions, the 2367.3-keV level may be depOpulated by a 641.3-keV transition to the 1725.4-keV level and a 735.4-keV transition to the level at 1632.00 keV. We have some very weak coincidence evidence for our placement of the 641.3-keV y in that weak 720— and 1005-keV peaks were observed in the 639-keV gated. coincidence spectrum (actually this gate included two y's, one from T1198 of 637-keV and the weaker 64l-keV y from Pb199). However, we did not observe the 64l-keV y in any of the other gated coincidence spectra including the 720- and 1005-keV gates and we have no explana- tion as to why it is absent other than poor statistics. The 735.4-keV transition has been placed solely on the basis of the energy difference between the 1632- and 2367.3—keV levels without any supporting coincidence evidence. We have indicated our uncertainty about this placement by using a dotted line for the 735.4-keV transition in the decay scheme, Figure VI-6. 6.4.1.1. 1554.10-keV Level Although we are including the 1554:10—keV level in our intermediate confidence group of levels, the evidence for this level is so strong that we could probably have included it in our highest confidence group. The strongest piece of evidence for this level is the observation of a single strong 367-keV peak in the 1187-keV gated coincidence spectrum. This coincidence relationship is confirmed 269 by the fact that the 1187-keV peak appeared in gnly_the 367-keV gated spectrum. Another piece of evidence supporting this level comes from the 834-keV gated spectrum which shows only two weak peaks at 353- and 367-keV, indicating that the 833.83-keV y feeds the 720.26-keV level. This coincidence relationship is also supported in the 353— and 367- keV gated spectra. Although we observed a 1553.3-keV y in the singles spectra, we are not positive that this 7 is the ground-state transition frOm the 1554.10-keV level. The most obvious reason for our questioning of this placement is the poor energy agreement of the 1553.3 keV y with the energy sums 366.90 + 1187.23 - 1554.13 and 720.26 + 833.83 - 1554.09. A second problem with this placement is the behavior of the 1553.3-keV transition in the anticoincidence and integral coincidence experiments. In our decay scheme, the 1554.10-keV level is fed almost entirely by direct e-decay from Pb199(>902) with only weak feeding from higher levels in T1199, and yet from a com- parison of the measured intensities of the 1553.3-keV y in the singles, anticoincidence and integral coincidence experiments, this y-transition behaves as if it were in relatively strong coincidence with other y's. However, considering the very weak nature of this transition and the resulting poor statistics obtained in the coincidence ex- periments we are somewhat skeptical of any conclusions based on these experiments. The 433.2-keV transition was placed between the 1554.10- and 1120.90-keV levels on the basis of the energy sum 433.2 + 1120.9 - 1554.1 and the observation of a 433-keV peak in the 1121- and 754- 270 keV gated coincidence spectra. The 312.3-keV transition was placed on the basis of the energy sum 1241.7 + 312.3 - 1554.0 and its presence in the 875- and 521-keV gated spectra. Considering all the evidence presented for the placement of a state at 1554.10-keV, it seems to us to be quite firmly established. 6.4.1.k. 1930.4- and 2031.5-keV Levels The evidence we have to support these two levels is typical of that which we required to include a level in our intermediate confidence group., Both levels are depopulated by e-fed ground-state- transitions as shown by the anticoincidence and integral coincidence experiments and both have at least one transition to an established level which is supported by the results of the 2-d coincidence experiment. In this case both the 1930.4- and 2031.5-keV levels feed the 720.26-keV level. The 1209.60-keV transition which depopulates the 1930.4-keV level is observed strongly in the 367- and 353-keV gates but not in the 720-keV gate*while the 1311.28-keV transition 'which depOpulates the 2031.5-keV level is observed in all three of these gated spectra. These coincidence relationships are confirmed in the 1210- and 1311-keV gated coincidence spectra. Although the evidence we have presented for our placement of these two levels is fairly conclusive, it is also somewhat limited and we couldn't place these levels in our highest confidence group. These two states complete our intermediate confidence 271 group of levels. Again, both levels were shown to have e-fed groundf state transitions in the anticoincidence experiment. The second strong piece of evidence for the 2237.4-keV level comes from the 1517-keV gated coincidence spectrum which contains only 353-, 367- and 720-kev peaks. The 353-, 367-, and 720-keV gated spectra support these coincidence relationships and shew a strong 1517-keV peak, and in addition, these are the 2211 gates in which the 1517-keV peak was observed. From these coincidence results it is fairly certain the 1517.12-keV transition directly feeds the 720.26- keV level, and the energy sum 1517.12 + 720.26 - 2237.38 is in excellent agreement with the ground-state transition of 2237.29-keV. The remaining three transitions we show depopulating this level were placed solely on the basis of the energy differences between the levels, as we don't have sufficient coincidence information on these transitions to make definite placements. This is especially true for the 605.8- and 735.4-keV transitions which could be placed elsewhere in the decay scheme on the basis of energy sums and, in fact, both transitions appear twice in our decay scheme, Figure VI-6. In addition to the e-fed ground-state transition of 2433.1 keV, the 2433.7-keV level is depopulated by only one other transition, the 2066.95-keV transition to the first excited state. This transition is supported by the observation of a 367-keV peak in the 2067-keV gated coincidence spectrum. Although we did not observe a 2067-keV peak in the 367-keV gated spectrum, this could easily have been missed because of the muCh higher background in the 367-keV gated spectrum. 272 6.4.1.m. 1528.2- and 1695.2-keV Levels These are the first of six levels we placed in Our lowest confidence group, which means they are based on very limited and sometimes conflicting evidence. However, we still required one piece of solid evidence, such as the observation of an e-fed ground-state transition, plus at least one weak supporting argument, such as a cascade transition placed solely on energy sums. The principal piece of evidence for the 1528.2—keV level came from.the 1161-keV gated coincidence spectrumuwhich contains strong peaks at 367, 432, and 504 keV. However, the 367-keV peak is by far the most intense, suggesting that the 1161-keV transition directly populates the first excited state giving rise to a state at 1528.2-keV. Although no other transitions were found to depopulate this level, it appears to be fed from the well-established 1959.45- and 2031.5-keH levels via the 430.9- and 503.15-keV transitions. The 1695.2-keV level is supported mainly by the observation of a single strong peak of 367 keV in the 1328-keV gated coincidence spectrum. This evidence is supplemented by the excellent agreement between the energy sum 366.9 + 1238.3 - 1695.2 and the 1695.28 -ray observed in the singles spectra. However, there is a conflict between our placement of the 1695.28-keV transition in the decay scheme and the results of the anticoincidence and integral coincidence experiments. As the decay scheme now stands, the 1695.2-keV level is populated only by direct electron capture from Pb199, with no feeding from higher energy levels 273 in T1199. This means that the 1695.28-keV2trdnsition intensity should be enhanced in the anticoincidence and reduced in the integral coincidence spectra, or in the case of Table VI-Z, the intensities listed in the singles, anticoincidence, and integral coincidence columns should be equal within experimental error. Unfortunately this is not the case, and the 1695.28-keV transition behaves as if it were in coincidence with another transition (or transitions) having an energy greater than =lOO keV (low energy cutoff for the anticoincidence experiment). Although we did observe a 1695-keV transition in the 367-keV gated coincidence spectrum, the 1695-keV gated spectrum contained only a very weak 367-keV peak, in fact the 367-keV peak was just about as strong in the gated spectrum.of the background adjacent to the 1695-keV transition. Based on these observations we canIt satis- factorily account for the behavior of the 1695.28-keV transition in the anticoincidence and integral coincidence experiments unless it consists of an unresolved doublet. This possibility will be discussed later in. Section 6.4.1.p. In our decay scheme, the 1695.28-keV transition is shown as a dotted line to indicate our uncertainty about its placement. 6.4.1.n. 2159.3-keV Level The 2159.3-keV level is based on evidence quite similar to that presented in support of the 1695.2-keV level. The principal support for this level comes from.the observation.of a single strong 367-keV’transition in the l793-keV gated coincidence spectrummwhich is verified by the presence of the l793-keV transition in only the I 3674keV’gated spectrum. Additional evidence for this level consists 274 of a possible ground-state transition and a possible cascade transition to the 1554.10-keV level. The placement of the 605.8-keV transition is uncertain as we have already placed a transition of this same energy between the well-known 2237.4- and 1632.00-keV levels, although neither placement is supported by coincidence results. The 2158.6-keV transition presents us with the same problems we faced in placing the 1695.28-keV transition in the previous section. According to our decay scheme, the 2159.3-kev level is fed entirely by C-decay. However, in the anticoincidence and integral coincidence experiments, the 2158.6-keV transition does not behave as the other totally e-fed ground-state transitions, its intensity is reduced in the anticoincidence experiment and enhanced in the integral coincidence experiment. Although we observed a weak 367-keV peak in the 2158-keV gated coincidence spectrum, it was so weak that it could easily have been the result of chance coincidence events and we did not observe the 2158-keV peak in the 367- keV gated spectrum. All things considered, we have serious reservations about the placement of the 2158.6—keV transition in our decay scheme but much less doubt as to the placement of the 2159.3-kev level. 6.4.1.0. 2206.7-, 2547.45,,and 2643.2-keV Levels All three of these levels were based on the observation of e-fed ground-state transitions in the anticoincidence experiment and at least one cascade transition to an already established level. Unfortunately these cascade transitions were all too weak to observe in the 2-d coincidence experiment and were placed solely on the basis of energy sums. 275 For the 2547.4-keV level we were able to find only one possible cascade transition, the 2180.2—keV transition to the first excited level. The 2643.2-keV level was also found to have only one possible cascade transition, the 984.4-keV transition to the 1658.47- keV level. In addition to the 2206.5-keV ground-state transition, we have placed possible cascade transitions from the 2206.7-keV level to the 366.90—, 1482.25-, and 1632.00-keV levels. Again, all of these placements are tentative and were based only on energy sums. 6.4.1.p. Possible Additional Levels Although we have concluded our discussion of the levels shown in the decay scheme, Figure VI—6, we would like to add a few words here concerning some possible additional levels. As we mentioned in Section 6.4.1.m., the 1695.28-keV transition may be a doublet with one component being the ground-state transition from the 1695.2-keV level while the second is in coincidence with the 367-keV transition. This would mean a possible state at 2062.18 keV. Such a state is supported by the observation of an e-fed ground-state transitiOn in the anticoincidence experiment. In Table VI-3 we have listed 33 possible levels based solely on the anticoincidence and integral coincidence experiments. Of these 33 possible leVels, 19 have been supported with some additional evidence and are included in our decay scheme and another, the 2062- keV possibility, is discussed above. Although the remaining 13 possible levels could not be supported by any cascade transitions and ‘were left out of our decay scheme, they still remain good possibilities. 276 6.4.2. Log fr's In this section we will explain how we obtained the e-feeding ‘ intensities and corresponding log ft values for the levels in T1199 populated from the e—decay of Pb199. Because the 8+-decay of Pb199 is weak compared to e-decay, we can ignore it in the following calculations without significantly affecting the results. We will discuss the B+-decay of Pb199 in Section 6.4.3. The total transition intensities, including internal conversion in the K, L, and M'shells, were Calculated from our singles intensities and the theoretical conversion coefficients of Hager and Seltzer [Ha68]. We assumed M1 multipolarities for all transitions 'except the 720-keV transition which we assumed to be E2 and the 353- and 367-keV transitions which were mixed.Ml+E2 (See Section 6.3.5. for discussion of these multipolarities). The total relative transition intensities are given in the decay scheme, Figure VI-6. Using our measured K x-ray intensity, the calculated K- conversion intensities, and the calculated K fluorescent yield of 0.95 [F166], the total e-feeding intensity to the ground-state was found to be 354%. This corresponds to a log ft of 6.4. Such a low log f% is in serious conflict with the generally assumed first- forbidden (unique) character of this 8 transition and leads to the . conclusion that the x-ray intensity given in Table VI—Z is too high. The only other possibility is that the ground-state In of Pb199 is 3/2- and not S/2-; as. is generally believed. (The ground-state spin of T1199 has been measured to be 1/2 in atomic beam [Br57] and atomic 277 spectra [Huél] experiments.) However, from the systematics of the Pb isomeric states, the 5/2- ground-state assignment seems fairly well established and it is more likely that the error here is due to our measurement of the x-ray intensity. Considering the systematics of the other odd-mass T1 isotopes and the first-forbidden (unique) nature of the ground-state 8 transition, we can fairly safely assume that e-feeding of the ground state is very small, probably <12. Therefore, for the purpose of calculating the percent e-feeding to the excited states and their log fi's, we have assumed gg.ground-state feeding. The resulting e-feeding intensities and log f%'s appear in the decay scheme to the right of the energy levels. 6.4. 3. Bit-feeding In the y-ray singles spectra.we observed a moderately strong peak at 510.9-kev which was definitely broader than the y-ray peak- next to it. Considering the energy of this peak and its obvious broadening, we have assumed that this peak arises from positron annihilation in the B+Fdecay of Pb199. This assumption is verified by the results of the Z-d coincidence experiment which shows a strong 511-keV peak in the 511-keV gated coincidence spectrum. Based on the Pb199 Qt of 33.2 nev, calculated from the experimental masses listed in the table of liyers and Swistecki [Hy65], we find that B+-decay - is energetically possible to levels in T1199 up to 32180 keV. From theoretical ECHO/B+ ratios [Wa59], the calculated Q; of 33.2 uev, our e-feeding intensities, and theoretical subshell ‘ ratios for electron capture [Wa59], we predicted the B+-feeding 278 intensities listed in Table VI—S. We also had to make the assumption that all B-transitions to these levels were allowed or first-forbidden nonunique, as the spins of most of the levels are unknown (See Section 6.4.4.). However, based on the log‘ft's calculated in the preceeding section this is a good assumption. Using the intensity of the 511-keV peak listed in Table VI-2 and the calculated K xrray intensity due to s-decay assuming no s-feeding of the ground state, we have calculated an upper limit of 1.4: for the total B+-feeding in the decay of Ph199. The 511-keV peak was fairly strong in the integral coincidence spectra of the Z-d Y-Y coincidence experiment and we were able to obtain the 511-keV gated spectrum shown in Figure VI—S. In this spectrum we observed fairly strong coincidences with transitions de-exciting the 1120.90-, 720.26-, and 366.90-keV levels. We also observed a weak 511-keV peak in several gated coincidence spectra of transitions depopulating the 1241.67- and 1658.47-keV levels. How- ever, based on the 511-keV gated coincidence spectrum, it appears .that the 366.90-, 720.26-, and 1120.90-keV levels receive the bulk of the B+¥feeding. This conclusion is supported by the theoretically calculated B+Ffeeding intensities listed in column four of Table VI—S. From the intensities of the 367-, 353-, 720-, 400-, 754-, and 1121; keV transitions in the 511-keV gated spectrum and the measured upper limit of 1.42 for total B+¥feeding of T1199, we were able to make a roush calculation of percent B+Ffeeding to the 366.90-, 720.26-, and 1120.90- keV levels. These B+Ffeeding intensities are listed in column five of Table VI-S. Considering the rough nature of these calculations, the 279 Table VI-S B+-feeding in Pb199 Decay Energy of Level Experimental Theoretical Calculated Observed (keV) e-feeding EK/B+a 8+-feeding 8+-feeding Ground State Assumed - OZ -- ' --l -- 366.90 24.82 9.3 2.22 0.94%. 720.26 3.01 18 0.14% 0.18% 1120.90 4.8% 56 0.07% 0.27% 1241.67 2.6% 80 0.03% -- 1482.25 3.52 250 0.012 -- 1502.00 16.82 290 0.05% -- 1528.2 0.74% 320 0.002% -- 1554.10 1.32 390 0.003% ~- 1632.00 1.0% 640 0.001% -- 1658.47 11.12 800 0.012 -- 1695 . 2 <03> through 26.01 to 0.0082 -- 2159 . 3> 0‘5 _. ._..__._ Z =2.5% 2 '1.4% 8 Assumed all B-transitions to be either allowed or first- forbidden unique. 280 agreement between the predicted B+Ffeeding and that observed is better than we expected. The worst agreement is between the observed 8+- feeding of 0.272 to the 1120.90-keV level and that predicted by the. theoretical calculation, 0.072. This is somewhat anomalous in that all of the other B+-feeding intensities we have observed in Pb199, as well as Pb201 (Section 5.4.2), decay are either approximately equal to the theoretical predictions or much less. This anomalous intensity could be explained by the existence of a 511.10-keV y-transition between the 1632.00- and 1120.90-keV levels. In our decay scheme, we show this possible transition as a dotted line. Based on our 2-d coincidence data, we were unable to rule out such a transition. A 511-511-Y triple coincidence experiment using the 8x8 in. NaI(T1)I split annulus would be of great help in resolving this question as 'well as providing better quantitative data on the B+-feeding of all the levels in T1199. 281 6.5. Spin and Parity Assignments Before getting into the discussion of the specific spin and parity (In) assignments for these levels we would like to make a few quite general statements concerning the criteria used in making these‘ .assignments. As we discussed in Section 6.3.5. we have multipolarity assignments for only three y-transitions, and as these are all connected with the ground and first two excited states, they are only of help in assigning the In's of the 366.90- and 720.26-keV states. Therefore, we discuss the assignments of these two states in a separate section (Section 6.5.2.) following a brief discussion of the ground state (Section 6.5.1.). The In's of the remaining 26 excited states were based entirely on the calculated log fr's, y-branching ratios, and the systematics of the odd-mass Tl isotopes, particularly 131,201. The log f% values we calculated for the excited states in T1199 all fell hemeen 6.3 and 7.7 which is well within the range of . both allowed (AIiO, :1 An -no) and first-forbidden nonunique (AIiO, :1 An ~yes). Although a log ft of 7.7 is somewhat low for a first forbidden unique transition (AIiZ, Am-yes), we have somewhat arbitrarily and conservatively included such a possibility for those, states where our calculated log j%:was 37.5 and rejected this possibility for log f%'s 57.4. Based on this difference in log f%, we have divided our discussion of the 26 remaining excited states between Section 6.5.3. (log ft's _<_7.4) and Section 6.5.4. (log ft's _>_7.5). The y-branching ratios were generally used only to limit the 282 transition multipolarities to M1, E2, E1, or M1+E2. The systematics of the odddmass T1 isotopes were useful only for the lowest energy levels, although we did use the results of our study of the levels in T1199 in a general way to support our preference for positive parity assignments. Our In assignments for the levels in T1199 are summarized in Table VI-6. In general, where more than one In assignment was possible for a level, we have listed the preferred assignments first. However, for most of the levels, this means only that the positive parity possibilities are listed before those with negative parity and the ordering of the positive parity possibilities is generally by increasing spin and does not indicate a preference for low spin assignments. 6.5.1. Ground State The ground-state spin of T1199 has been measured in both Iatomic spectra [Hu61] and atomic beam resonance [Br57] experiments to be 1/2. This is consistent with the predicted 81/2 proton shell model assignment, giving us a spin and parity (In) of l/2+. Based on an f5/2 ground state for Pb199, direct s-population of the T1199 ground state would involve the transition :fl81/2+Vf3/2, which is Z—forbidden as well as spin forbidden and one would expect the log to be quite a bit higher than 9. This supports our assumption made in Section 6.4.2. of 302 e—feeding to the ground state. Spin Assigggents for Levels in T1199 Populated 283 Table VI-6 in Pb199 Decay Level Level (keV) In (keV) In 0.00 172+ “1891.1 372+, 572+, 372- 366.90 372+ “1898.1 372+, 572+, 372- 720.26 572+ b1930.4 372+, 572+, 372-, 172+ a1120.90 572+, (372:) “1959.45 372+, 572+, 372- “1241.67 572:, 372+, 772+ “1977.8 372+, 572+, 372- “1482.25 372+, 572+, 372- “2031.5 372+, 572+, 372- “1502.00 372+, 572+, 372- 1’2159.3 372+, 572+, 372-, 172+ b1528.2 372:, 572:, 772+, 172+ “2206.7 372+, 572+, 372- “1554.10 372+, 572+, 372- 82226.5 372+, 572+, 372- b1632.00 372+, 572+, 372-, 172+ “2237.4 372+, 572+, 372- '“1658.47 372+, 572+, 372- “2367.3 372+, 572+, 372- b1695.2 372+, 572+, 372-, 172+ “2433.7 372+, 572+, 372- “1725.4 372+, 572+, 372- b2547.4 372+, 572+, 372-, 172+ -“1749.6 372+, 572+, 372- b2643.2 372+, 572+, 372-, 172+ “1768.5 372+, 572+, 372- “ The log ft's for these levels were <7.4 and first-forbidden unique transitions were not allowed. '— b transitions were considered possible. The log fr's for these levels were >7.5 and first—forbidden unique 284 6.5.2. 366.90- and 720.26-keV States The 366.90-keV state was previously assigned In- 3/2+ by Andersson and co~workers [An57] on the basis of the mixedlMl+E2 367? keV Y-transition and the strong 6 population of this state (24.82 in our decay scheme). The mixed Ml+E2 multipolarity of this transition has been substantiated in all succeeding studies. (See Section 6.3.5. for a discussion of the multipolarity assignments.) Our work supports this 3/2+ assignment as the log f% of 6.6 is in the range expected for a first-forbidden B-transition. * The 353.39-keV transition depopulating the 720.26-keV level was also found to be mixed.M1+E2 multipolarity in the previous studies, and our calculated 0K supports this multipolarity assignment. This would then allow an In assignment of l/2+, 3/2+, or 5/2+ for the 720.26—keV level. The calculated log ft of 7.4 for a decay to this state rules out the l/2+ possibility, as this would then be a first- forbidden unique transition, log ft 38. This leaves only the 3/2+ and 5/2+ possibilities. From the measured mg of 0.011 we were able to assign an E2 multipolarity to the 720.24-keV ground-state transition (Section 6.3.5.). While this strongly suggests a 5/2+ assignmenchere, ‘we can't positively eliminate the 3/2+ assignment on the basis of this multipolarity as the transition could be mixed.Ml+E2 with the EZ component strongly enhanced, although we would be surprised to see such a strong collective enhancement in this case. Based on the systematics of the odd-mass Tl isotopes, a 5/2+ assignment is also the best choice for the 720.26-keV state. 285 6.5.3. Remainigngtates with Log f§'8 57.4. The log ft's of the 19 levels to be discussed in this section range from 6.3 to 7.4, all of which are well within the ranges for allowed (AI-0, 11 An-no) and first forbidden nonunique (AI-0, :1 An-yes) B-transitions and significantly below the log ft's usually found for first forbidden unique (AI- :2 An-yes) transitions.- (These 19 states are identified in Table VI-6 by a superscript a preceeding the level energy.) Based on the log fi's alone, we have six possible In's for each of these 19 states, 3/2:, 572:, or 7/21. However, for all but one of these states, which we will discuss below, we were able to eliminate the 5/2-, 7/2+, and 7/2- possibilities on the basis of the y-branching ratios. In order to avoid repetitious arguments for these states we will discuss a typical example, the 1658.47-keV level, and let the reader go through the arguments for the other similar levels. The 1698.47-keV level is depopulated by three Y-transitions, the 1658.43-keV y to the ground state (In-l/2+), the 1291.50-keV y to the first excited state (In-3/2+), and the 937.89-keV level to the second excited state (In-5/2+). Based on this y-branching from the 1658.47-keV level we can clearly rule out the 7/2+, 7/2-, and 5/2- possibilities, as these assignments would mean M3, E3, or M2 ground- state transitions competing with M1,E2, or E1 cascade transitions. (While a few examples of such competing transitions are known, they are extremely rare.) We are now left with the 3/2+, 5/2+, and 3/2- possibilities and this is as far as we can go with the data presently 286 available. As we mentioned earlier, we do prefer the two positive parity possibilities based on systematics and we listed the 3/2- possibility last in Table VI-6. Looking over our decay scheme, Figure VI—6, we note that, with the exception of the 1241.67-keV state, all states with a log ft 37.4 have transitions to the 1/2+ ground state plus an additional transition to either or both the 3/2+ first-excited or 5/2+ second- excited states. The same y-branching argument used above can be applied to all of these states giving us the final In possibilities of 3/2+, 5/2+, and 3/2-. From a comparison of the levels in T1201, Chapter V, with those reported here for T1199, we believe the 1120.90-keV level in T1199 corresponds roughly to the 1098.46-kev level in T1201. (The 1098.46-keV level in T1201 was assigned a preferred spin of 5/2+ with (a 3/2+ spin also considered possible.) Based on this similarity, we have tentatively assigned the 1120.90-keV level in T1199 an In of 5/2+. However, we can't definitely rule out the 3/2+ or 3/2— possibilities on this weak argument alone, and so we have listed them in parentheses in Table VI-6 following the preferred 5/2+ assignment. The 1241.67-keV state has a log f% of 7.2 which allows In's of 3/21, 5/21, and 7/21. However, no groundwstate transition was observed in this'case, although this state did populate the 366.90-, 720.26- and 1120.90-keV levels. Based on our In assignments for these three levels, Table VI-6, we see that,of the six In possibilities given above, only the 7/2- possibility is inconsistent with the y-branching 287 from this level. Therefore, we are left with five possible In assignments for this level, 3/2i, S/Zi, and 7/2+. 6.5.4. States with loghj%'s 27.5. The log ft's of the remaining 6 levels placed in our decay scheme range from 7.5 to 7.7. (These 6 states are identified in Table VI-6 by a superscript b preceeding the level energy.) While these log ft's are well within the range for allowed (AI-0,11 Anuno) and first forbidden nonunique (AI-0,11 An-yes) B-transitions, they are high enough that we decided to include first forbidden unique (AI=2 An=yes) transitions as additional possibilities. Therefore, based on the log ft alone, we have 8 possible In assignments for these states, 3/2i, 5/21, 7/2i, l/2+, and 9/2+. 0n the basis of the y-branchings from the 1632.00-, 1695.2-, and 1930.4-, 2547.4-, and 2643.2-keV levels we can eliminate the 5/2-, 7/2:, and 9/2+ possibilities. This leaves us with the final possible In assignments of 3/2+, 5/2+, 3/2-, and l/2+ for these 5 levels. Although we can't definitely eliminate any of these possible assignments we can say that, based on the log f%'s, l/2+ assignments for these states are the most unlikely, and we have indicated this by placing this In possibility last in Table VI-6 for these 5 states. The log f% of 7.7 for the 1528.2-keV state allows In's of 3/21, 5/22, 7/2:, 1/2+, and 9/2+. However, unlike the states discussed above, this level is depopulated by only one transition, the ll6l.27-keV transition to the first excited state (3/2+), and based on this observation, we can eliminate only the 7/2- and 9/2+ possibilities. 288 This leaves us with 6 possible In assignments for this level, 3/21, 5/21, 7/2+, and 1/2+. Although the 1528.2-keV level is fed by the 1959.45- and 2031.5-keV levels, this information does not limit the .In assignments beyond the choices given above. This concludes our discussion of the spin and parity assignments. As is evident from Table VI—6, we were unable to assign unambiguous Spins or parities for very many levels, and the few states for which we have made a unique assignment were known before the a present investigation was undertaken. The lack of measured conversion electron intensities and hence conversion coefficients with which to make multipolarity assignments has severely limited our ability to assign unique spins and parities. A careful measurement of the conversion electron spectrum of Pb199 should, therefore, be of highest priority in any future investigation of this decay scheme. 289 6.6. Theoretical Description of Odd-mass Tl Isotopes Because our studies of the states in T1199 and T1201 are of an "experimental" nature rather than "theoretical", it is easy for us to become obsessed with the details and results of the experiments leading to a decay scheme and lose sight of the principal reason for obtaining the data, which is to provide a test for present and future nuclear models. Indeed, during the ~2 years we spent in obtaining and analyzing the data, very little time was devoted to the theoretical studies of these isotopes. While we will now attempt to examine the odd-mass Tl isotopes, particularly T1201 and T1199, in terms of some of the current models, we do not pretend to have the sophistication of a theoretician and will make no attempt to predict a future direction for the theoretical calculations. However, as several calculations have been made of the levels in T1199 and T1201 we do feel that a critical comparison of these with our results will be of some help in future calculations. 6.6.1. Shell Model Description of Odd-mass Tl Isotopes As we are in the region of the double shell of Z-82 and ”=126, it seems quite reasonable to start our description of the levels in the odd-mass Tl isotopes in terms of the single-particle shell model. Figure VI-7 shows the shell model states available in this region. Of all the odd-mass Tl isotopes Tl207 should provide the best test for this model as it has a closed neutron shell (u=126) and, like all Tl isotopes, a single proton hole in the z-82 closed shell. The excited states of T1207 up to 2.0 MeV are shown in Figure VI-8, along with 390 Figure VI-7. e j 1772 5/2 2:11 2 719/2 g 7 9 2 / 2:82 N3126 3 'Opl/Z l 2 / ‘fs /2 3/2 3/2 i h __._._. ... 13/2 1/2 5/2 "" f7/2 9. , ~— h 1/2 9/2 Proton Neutron Orbitals Orbitals Shell~model orbitals near the Ne126 closed shell and the z=82 closed shell. The order of these states may change somewhat as the total number of protons or neutrons change. The order shown here is that observed near the double shell closure at pb208. 82 126 Figure VI-8. 291 Systematics of states in neutron- deficient odd-mass Tl isotopes below 2.0 MeV. All states populated by radioactive decay and nuclear reactions have been included except for the high spin states in T11 9 observed by Newton et al. [Ne70] in their study of the reaction Au197 (a,2ny)T1199. .wuH> enemas Heeoz .moeoae _meae_h _esaml .eoem_ .eeae_ Heeesm seeeeeea .aeeum schemes. .neenl _mean_ mamas mamas «owes domes moses amass mmsaa +~\H +~\H +N\H +N\H+II+17 +~\H.:s:|| +~\H.|vasu. +~\H.|IIII +N\m-::||. +~\m ..III. +~\m ...lrt- $214.! $32!... A+~\C........l A+~5 Ill As~\mv.-;:.x +~\m. Aum\ov.:l|ll +~\m Ill. +29 +~\m III. eu~\a.|+:++. Au~\mv 2 w e351... ...... in». ...--. re. .---.. .+~\mv;+:nnls a: II ..S ---... I--- +~\m. -~\HH .1111: +~\m munulu “Mmmnu I l --.~...- -.....—. — - .... ... --. o.H m.a o.~ ABN u; Kfiiaug Iahaq 293 selected states of lhe other neutron-deficient odd-mass Tl isotopes. The presence of a l/2+ ground state and excited states with In's of 3/2+, ll/2-, 5/2+, and 7/2+ in T1207 are in agreement with predicted shell model states between Z=50 and Z=82, namely the s d , h , 1 3/2 11/2 9 and g . (The level in T1207 lies at 3.48 MeV and is not 7 97/2 . (flyz /2 shown in Figure VI-8.) Moreover, the electromagnetic properties, beta transition rates and single neutron transfer reaction spectroscopic factors all indicate that these are fairly "pure" single-particle states. As we go from T1207 to the lower mass Tl isotopes, we are breaking up the N-126 closed shell. However, for the single-particle shell model we assume the even numbers of neutrons are paired-up to give a resultant spin of zero, and, therefore, the ground state spin is predicted to be l/2+ for all the odd-mass Tl isotOpes, and looking at Figure VI-8 we see that this is indeed the case. Based on the d3/2 1207 first excited state in T , the single-particle shell model would predict a d3 first excited state for all the odd-mass Tl isotopes 72 and we see in Figure VI-8 that this prediction also appears to be correct. Based on the similarity in the energies of the d3/2 single- particle state in T1207 and the 3/2+ states in the other Tl isotopes, we can probably assume these are also fairly "pure" d states. 3/2 The second excited state in T1207 is the 7211/2 shell model state at 1341 keV and although possible 11/2- states have been observed in some odd~mass Tl isotopes close to this energy [Ne70], a number of 29h states appear between the d3 2 states and ~l300 keV in all these isotopes except, of course, for T1207. The second excited state for T1205 through T1197 is 5/2+. These 5/2+ states could be explained as being the dS/Z shell model state, which is indeed, the next state in T1207. However, the d state lies at 1674 keV in T1207 and we 5/2 would be hard pressed to explain the lowering of this state to 615 keV in T1205 where the first 5/2+ state appears. These 5/2+ states have been interpreted in the past as d shell model configurations [An57, 5/2 Be57, Pe61] and we have also assumed this assignment in our discussion of states in T1200 (Chapter IV). Assuming for the moment that this description is correct, our single-particle model cannot account for the very high density of states we observed in T1201 and T1199 above ~l MeV and we are forced to abandon this description in favor of a many-particle shell model. Silverberg [Si6l] has made such a shell model calculation on T1205. He considered the nucleus as three holes moving in a static potential and interacting with each other by some residual force. While this approach could qualitatively describe the behavior of T1205, quantitatively the results were poor. Nevertheless, these calculations did show that the 5/2+ second excited state has as its principal component Isl/2[p1/2f}/2]2;5/2> mixed with a significant amount of Isl/zlpI/zfg/2]3;S/2> and a much smaller mixing of configurations containing the dS/Z proton state. Although we can generalize some of the results obtained by Silverberg to the more neutron-deficient T1 isotopes, the quantitative 295 vulvulnlion of energy levels becomes meaningless, and as these calculations become more complex as one gets further from the closed neutron shell, no significant shell model calculations have been performed below T1205. 6.6.2. Core—coupling Model Nuclei within a few nucleons of a double—magic core can usually be described successfully by nucleons moving in a static shell model potential and nuclei very far from closed shells assume a stable equilibrium deformation and the collective modes can be treated as nuclear rotations with energies which are smaller than the single-particle excitations (Collective Model). For nuclei between these two extremes, such as the neutron-deficient Tl isotopes, which are spherical, but where there is evidence of low lying core excited states, we can use variations of the Core-coupling Model. In this model collective effects are treated as vibrations of the spherical core and are coupled more or less strongly to the single-particle states. I In addition to performing three-particle shell model calculations on T1205 as mentioned in Section 6.6.1., Silverberg [Si6l] calculated this same nucleus as one proton hole coupled to a vibrating core and concluded that this later calculation provided a satisfactory description of this nucleus with considerably less numerical work. Since this work was published in 1961 several additional studies have been made of the odd-mass Tl isotopes using the intermediate—coupling unified model [Cv66, A167, A269] and L'Ull two of these included calculated levels for T1201 and T1199 [Cv66, A167]. These calculations are of particular interest to us as they give us a chance to compare our proposed level scheme with this model. As a complete description of the unified model exists elsewhere [2252], we will restrict ourselves here to a comparison of the assumptions and parameters used in the calculations by Covello and Sartoris [CV66] and Alaga and Ialongo [A167] and end with a comparison of the resulting calculated levels with our experimentally determined levels. In their calculation, Covello and Sartoris [CV66] considered all the proton-hole orbits within the shell ending at Z=82, namely, 381/2, 2d , 3/2 , and 197 h11/2’ 2615/2 /2’ and included all the core states up to three phonons. Alaga and Ialongo [A167] however, used only the 3 , and 2d5 ,shell model states but also included 8 , 2d , 171 1/2 3/2 11/2 /2 vibrator states up to three phonons. The two parameters which specify the collective motion of the core and its interaction with the motion of the hole are the phonon energy hm and the strength of the surface-hole coupling n or a, The phonon energy, am, is deduced for each odd-mass Tl isotope from the 201 this would be the first 2+ state neighboring even isotope (for Pb in Pb202 at 960 keV). The coupling constant 0 or a is treated as a free parameter to be varied within reasonable limits. Although both calculations used the same values for 5w, 960 for T1201 and 1027 for Pblgg, the coupling constant which gave the best fit to the existing experimental data was different as one might expect. 297 Another difference between these calculations has to do with the assumption made concerning the single—hole excitation energies, A, e. -s . J J 1/2 the excited single-particle states in T1207. However, Covello states Alaga assumed these energies were fixed and equal to that these energies can't be taken from the experimental spectrum of T1207 because these experimental energies should be corrected for the interaction with the core, which is also effective in T1207. Further- more, the interaction between the odd hole and the core contains a short-range part, which has not been included in the model Hamiltonian. This short-range part of the force would act in first-order perturbation theory as a renormalization of the single-hole energy spectrum, thus producing effective spacings peculiar to each isotope [8161]. Because of this they considered the quantities Aj as adjustable parameters for all levels except the g7/2 level which is so high in energy that any changes in its position due to these corrections are from the T1207 g level. 7/2 7/2 In Figure VI-9 the energy levels calculated by Alaga [A167] irrelevant and they took A are compared with those calculated by Covello [CV67] and both of these are compared to the T1201 and T1199 levels found experimentally. We have not included the recent high spin levels in T1199 prOposed by Newton and co-workers [Ne70] or the isomeric 972- levels found in both nuclei [Di63]. With the exception of those levels mentioned above, the other experimentally known low-lying levels in T1201 and T1199 are reproduced fairly well in both calculations, although they both fail to account for the high density of states above #1 MeV. The 298 Figure VI-9. A comparison of the levels in T1201 and T1199 from our decay scheme with those calculated by [CV67] and [A167]. The parities for all the leVels are positive except where indicated. . NM) ~\m «\e «\m «\m ~\a ~\m ~\m ”Reece huomnh mmaae III II ~\_ ~\» -\w .mxm mmaae Ill lllll llll ole> oeamae ~\a ~\m ~\m 1.~\a~ Reese“ knoQSH souam l H H II N axe \ ~\m «\w Heo>u_ muoonk scams Ill H «\m ~\a e~\s A :3. As“ ul Kolaug {shag ~.H q.a o.H o.~ 300 order of the states are similar in both calculations for T1201, although they vary considerably in energy. Because of the many states observed in our decay schemes, it is difficult to draw a correspondence between the experimental and theoretical levels. This is especially true for T1199, where most of the In assignments are ambiguous. Because of this we will restrict this discussion largely to the states in T1201. As we mentioned before, the agreement between the calculated and measured energies for the 3/2+ and S/2+ first two excited states are quite reasonable, and we can fairly safely draw a correspondence between them. Both calculations place a 7/2+ level next, 1047 keV according to [Cv67] and‘1065 keV according to [A167] for T1201, while the next observed level is at 1098 keV. However, this 7/2+ calculated level does lend support to our preferred 7/2+ assignment for the observed level at 1135 keV (see Section 5.5.4.) as the only other level we observed in T1201 that could possibly be 7/2+ were at 1920 and 1712 keV. Of course, it is also possible that this state is not populated in Pb201 decay. The next level predicted by both calculations has In-3/2+ and an energy of 1125 keV by [Cv67] and 1200 keV by [A167]. Although the 1098-keV level has been assigned a preferred In of 512+, we couldn't rule out a 3/2+ assignment, so it is possible that this level corresponds to the second 3/2+ state of these calculations. Another possibility for this predicted level is the 1157-keV level, although here too we have assigned a preferred In of 5/2+. Of course the most 301 likely candidates for the predicted level are the 1238- and 1277-keV levels, which we have assigned an unambiguous In of 3/2+, although they lie quite a bit higher in energy than the calculated level. The second 5/2+ level predicted by these calculations lies at 1299 keV [Cv67] and 1335 keV [A167]. The observed l330-keV state falls nicely between these calculated energies, and indeed, a 5/2+ assignment is one of the two In's considered possible for this state. At this point the calculations disagree with each other to some extent. The theoretical calculations of Alaga predict a 9/2+ level at 1405 keV, while Covello does not show a 9/2+ level below ~2 HeV. In support of Alaga, we did observe a state at 1290 keV which we assigned a preferred In of 9/2+ (7/2+ also possible). Although we observed no other states in the decay of Pb201 which could have In: 9/2+, we can't definitely state that the calculated 9/2+ state corresponds to our 1290 keV level as such high spin levels are not very likely to be populated in the decay of Pb201 (g.s.In=5/2-). The second calculated 1/2+ state at 1386 keV [Cv67] or 1530 keV [A167] is also of some interest as we have made a preferred In-1/2+ assignment to only one state, 1550 keV. Based on the closeness Of this energy to that calculated by [A167] for the 1/2+ state, we are tempted to suggest a correspondence here. However, as in the case of the 9/2+ state, l/2+ states are not likely to be populated in the decay 0f Pb201 and we can't rule out the existence of other unobserved 1/2+ States in this region. Both calculations predict another 3/2+ state <60 kev above 302 the l/2+ state, 1412 keV according to [Cv67] and 1583 keV according to [A167]. we observed three levels between these two calculated energies, 1446-, 1480-, and 1550-keV levels. Of these, the 1445-keV level is considered most likely to have a In - 3/2+, although any correspondence drawn here would be mere speculation. The calculations predict another 5/2+ state at 1740 keV [CV67] or 1750 keV [A167]. Although these energies are very close to the observed level at 1755 keV, we can't expect to base a correspondence between the calculated and observed levels for this state on the basis of the energy alone when such accurate level energy determinations are absent for the other levels above ~l MeV. This is especially true in this case as there are six levels from 1617 to 1755 keV which could have In=5/2+. Based on the above observations two conclusions are obvious. 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Concentrate the solution in the test tube by heating in a water bath while directing a stream of air over the solution. Add red-fuming nitric acid to precipitate PbNO3. Centrifuge and draw off the supernatant liquid which contains the Bi fraction. Evaporate the supernatant liquid to dryness as in step 2 and dissolve the Bi residue in a few drops of 6N HCl. Load the B1 activity onto a Dowex 1XB ZOO-mesh anion-exchange column heated with boiling isopropyl alcohol. The Pb fraction can then be removed from the column by elution with 0.1N HCl. The column can now be used as a reservoir for obtaining very clean sources of Pb activity from the Bi parents or the Bi activity can be stripped from the columm.with 1N HZSOu to obtain very pure Bi sources . 312 APPENDIX B Separation of Lead from Thallium.Cyclotron Targets Dissolve T1 target in 6” HNO3, the smallest amount possible. Add 5 mg. Pb++ carrier and 5 mg. Hg+++ as hold-back carrier. Add small amount of 6M’HZSOu to precipitate PbSOu. Centrifuge and wash with small amount of T1+++ and Hg++ carriers in 2M'H280h. Add conc. HZSOu to the PbSOu precipitate and heat until precipitate dissolves. White fumes of 802 should appear just before the precipitate dissolves. Dilute (carefully) the cone. HZSOh solution with 2 volumes of H20 to which 5 mg. of both Hg++ and T1+++'holdrback carriers have been added. PbSOu again precipitates. Wash ppt. twice with 2M H2801“ to which small amounts of Hg“. and T1H+ carriers have been added. Steps 3 and 4 may be repeated, depending on the degree of separation desired; however, the Pb activity is usually sufficiently pure without repeating. {l4 APPENDIX C Separation of Bismuth from Thallium.Cyclotron Targets Dissolve the T1 target in the minimum possible amount of 6N HNOg. Evaporate solution to dryness. Add small amount of cone. HCl. Evaporate to dryness again and add about 10 ml of conc. HCl. Recover Tl by several extractions with diethyl ether saturated with cone. HCl. After the final extraction, concentrate the aqueous phase to about 1 ml and place on a Dowex-1 anion-exchange column. The Pb fraction can then be removed from the column by elution with 0.1” HCl. The column can now be used as a reservoir for obtaining very clean sources of Pb activity from the Bi parents or the Bi activity can be stripped from the column with 1” H280, to obtain very pure Bi sources. filh APPENDIX D Separation of Carrier-free Lead from Thallium Cyclotron Targets Dissolve the T1 target in 1N HNO3. Saturate the solution with $02 gas to reduce the T1 to T1+ and heat the solution to expel excess 802. Add 10 mg of Fe carrier and make the solution basic with NHuOH. Centrifuge the Fe(OH)3 precipitate and discard the supernatant solution. Wash the Fe(OH)3 precipitate twice in dilute NHuOH solution with stirring, centrifuge and discard; wash the solution each time. Dis- solve the precipitate in IN HN03. 4+1- Add 5 mg of T1 carrier and repeat steps 2-4. Repeat steps 2-4 once more without addition of thallium carrier and dissolve the final Fe(OH)3 precipitate in the minimum amount of 6N HCl instead of 1N HN03. Remove the iron by four extractions with equal volumes of ethyl ether. The final aqueous solution contains the carrier-free lead. rill 3” H u S" H E” H " Ell T” H l H 3 1293 030714996 llllllllll