0F MEMBERS 'orvv'maA-sm DECAY 95mm? : -;.-r*1_, {3:5 ThesisfonheDegreeosxm, ? ~- ~ MiCHEGAN STAIEUNIVERSiT-Y . (RICHARD. ROSS TODD 1 _5 f 1971‘ ' ’ mmmnmuuwflmnlmqm GAMMA RAY SPECTROSCOPIC STUDIES OF MEMBERS OF THE A=ll+l DECAY CHAIN presented by RICHARD ROSS TODD has been accepted towards fulfillment of the requirements for PhoDo degreein PhYSiCS N Lilla... H QM Major professor .Date 3991c 29, ’9’)! 0-7639 _,5 ABSTRACT GAMMA RAY SPECTROSCOPIC STUDIES OF MEMBERS OF THE A=1u1 DECAY CHAIN By Richard Ross Todd The techniques of beta and gamma ray spectroscopy have been applied to a study of the decay schemes of 1H1m+gSm 1N1P and m. The goal of this study was to increase our understanding of nuclear structure and to obtain information about energy level systematics in these members of the A=1Hl decay chain below the N=82 closed shell. Ge(Li) singles, Ge(Li)-Ge(Li) two parameter coincidence and Ge(Li)-NaI(T£) (anti) coincidence Spectrometers were used to identify and establish the sequence of gamma ray transitions. The decay schemes were determined and comparisons are made with previous studies as well as the results of recent reaction work. Forty-seven transitions have been identified with the beta decay of lulmSm and 40 of these have been incorporated in a decay scheme involving 16 levels in the daughter lule. These levels are at 0, 196.6, 628.6, 80u.5, 837.1, 97U.O, 1108.1, 1167.2, 1313.2, 1u1u.8, 183u.0, 1983.1, 2063.5, 2091.6, 2119.0, and 2702.u keV. The lifetime of the state at 628.6 keV in lUle was measured to be 700i20 ns and the E3 transition from this level to the ground Richard Ross Todd state was found to exhibit an enhancement of 1.6 spu. The decay of lulgSm was observed to populate levels at “03.9, “38.“, 728.0, 1292.7, 1495.7, 2004.5, and 2037.7 keV in lulpm. Of the 50 transitions observed in the decay of lule 3 “3 of these have been placed in a level scheme containing 23 levels. The levels identified are at 0, 193.8, 1223.3, 13“5.8, l56“.8, 1596.8, 1820.“, 1897.1, 1968.0, 2066.“, 2073.7, 2109.6, 2l“5.2, 22“6.6, 2265.3, 2303.“, 235“.“, 2388.“, 2“30, 2506, 2620, 2805, and 2986 keV. Limits on the possible spin assignments of the levels identified, have been made from calculated log ft values and observation of transitions to levels of known spins. A three- quasiparticle multiplet of high-Spin, odd parity states was identified between 1.“ and 2.7 MeV in lulP m. The decay characteristics of the multiplet suggest the possibility of obtaining information about its structure from shell model calculations using a limited basis. A survey has been made of the energy level systematics and properties of low-lying states in the N=80, 81, and 82 odd mass isotones. These observations have led to some predictions and suggestions for future experiments. GAMMA RAY SPECTROSCOPIC STUDIES OF MEMBERS OF THE A=l“l DECAY CHAIN By Richard Ross Todd A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Physics 1971 ACKNOWLEDGEMENTS I wish to thank Dr. w.H. Kelly for suggesting this area of study. He has always found time, despite a busy schedule, to offer his personal assistance and encouragement in all aspects of this work. His guidance and patience during the writing of this thesis are deeply appreciated. I also wish to thank Dr. Wm.C. McHarris for his considerable help and advice on many phases of these in— vestigations. Dr. H.G. Blosser and Mr. H. Hilbert have assisted with the operation of the Michigan State University sector—focused cyclotron. The variety of available beam energies and the high beam flux have facilitated the production of the isotopes studied in this investigation. Drs. B.H. Wildenthal and F.M. Bernthal have contributed to various phases of this investigation through informal discussions. .Their generous cooperation is gratefully acknowledged. The 1“18m research has involved a Joint effort with Dr. R. Eppley. His significant assistance and advice are gratefully acknowledged. Dr. F.Y. Yap has shared in the lule research and his considerable aid in this project is appreciated. Special thanks is due to Dr. R. Warner for his assistance with these experiments. He and Dr. Eppley have 11 provided me with many satisfying hours of informal dis- cussion pertaining to research and things philosophical. The members and former members of the research group here all contributed in various ways to the successful completion of this thesis work. Dr. D. Beery, Dr. R. Doebler, Dr. G. Giesler, Dr. R. Goles, K. Kosanke, J. Black, L. Samuelson, w. Chaffee, C. Morgan, R. Firestone, and Miss J. Guile have all helped in this work. Dr. D. Beery was of particular assistance in the early stages of these projects. The help of Mr. R.N. Mercer and the staff of the cyclotron machine shOp is appreciated. This thesis has been very capably typed by Mrs. Sandi Bauer. Her assistance and effort in the completion of this project is appreciated. Mrs. Peri-Anne Warstler has assisted with some of the Tables and Figures. Much of the financial assistance for this research has been provided by the National Science Foundation, the U.S. Atomic Energy Commission, and Michigan State University. Finally I thank my wife Jeanne and Chuck and Elizabeth for their patient understanding and continued encouragement during this study. 111 TABLE OF CONTENTS Page ACKNOWLEDGEMENTS..00....IOOOIOOOOOOOOOOOOOOOO0.0.0.0... 11 LIST OF TABLES.................................... ..... 1V LIST OF FIGURES........................................ V Chapter I. INTRODUCTION.................................. 1 II. EXPERIMENTAL APPARATUS AND METHODS............ 8 2.1. Decay Scheme Construction............... 9 2.2. Gamma Ray Spectrometers........... ...... 11 2.2.1. Singles Experiments.. ........... 13 2.2.2. Coincidence Experiments.... ..... 15 2.2.2.A. Multiparameter Ge(Li)-Ge(Li) Spectrometer... ....... 17 2.2.2.8. Split Ring NaI(Tl)- Annulus—Ge(Li) Spectrometer.......... 21 2.3. Data Acquisition and Analysis........... 23 2.3.1. Program MOIRAE....... ........... 2“ 2.3.2. Program SAMPO......... ......... . 26 2.3.3. Programs EVENT and EVENT RECOVERYOOQOOOIOOOO0.0.0000000.o 28 III. IDENTIFICATION OF MEMBERS OF THE A=l“l DECAY CHAINOOIOIOOOIOOOIOI.OIOOOOIOIOOO00.0000 30 3.1. lulNd.............................. ..... 30 3.2. lule................................... 31 3.3. 1&13m............................. ...... 33 1113 Chapter Page Iv. EXPERIMENTAL RESULTS.......................... 36 1“1m “.1. Decay Schemes of Sm and 1”188m”... 36 “.101. IntrOductionOOOO0.0.0.000...O... 36 “.1.2. Source Preparation nd Identification of 1 1mSm and lulgsmOOOOOOOOO...OOOOOOOOOOOOOO 38 “.1.3. Experimental Results of 1ulmSm” “2 “.1.3.A. Half-life determin— ation of l“1g3m and lulmsmoooooooooooooooo “2 “.1.3.B. y-Ray Singles Spectra. _“2 “.1.3.C. Prompt y-y Coincidence Spectra............... ”8 “.1.3.D. Delayed Coincidence Spectra............... 58 “.1.3.E. Energy Se aration of the Two 1 18m Isomers. 66 “.1.“. lulmSm Decay Scheme............. 68 “.l.“.A. The 196.5-keV and Related Levels........ 72 “.1.“.B. The 628.6—keV Level and Spin-Related Levels........... ..... 7“ “.1.5. Discussion............... ....... 75 “.1.5.A. The l”18m Isomers..... 75 “.1.5.B. Sin 1e Particle States in ule.............. 78 “.1.5.C. A Three Quasi article Multiplet in ule.... 85 “.1.5.D. The Remaining States in lule.... ......... . 90 “.1.5.E. EC/B+ Ratios and Cor— rected log ft Values.. 9“ iiib Chapter “.1.6. Discussion and Conclusions...... “.1.7. l”188m Introduction............. 1“1g “.1.8. Experimental Results for Sm. “.1.8.A. 11418Sm Gamma Ray Spectra............... “.1.8.B. lulgSm Coincidence Spectra....... ........ “.l.9. l“lgSm Decay Scheme............. “.1.10. Spins and Parities of Levels in l“le Fed by the Decay of 1 188m Decay............... ..... “.l.ll. Discflssion of Levels Populated by 1 188m Decay................. The Decay of 1nle............... ....... “.2.1. Introduction.................... “.2.2. lule Source Preparation........ “.2.3. Experimental Results lule...... “.2.3.A. lule Gamma-ray Singles Spectra....... “.2.3.B. 1ule y-y Prompt Coin- cidence Studies....... “.2.“. l“le Decay Scheme.............. “.2.5. Spin and Paflities Assignments of Levels in 1 lNd Populated by the Decay of 1 lPm.................. “.2.5.A. 1“le Ground State.... “.2.5.B. The 193.8-keV and Higher States.. ...... . “.2.6. Discussion of the States in lulNdooooooooooooooooocoo ..... o. iiic Page 97 100 102 102 106 11“ 117 120 122 122 12“ 126 126 127 1“0 l“8 1“8 1“9 160 Chapter Page V. DISCUSSION OF RESULTS AND SYSTEMATICS... ...... 165 5.1. Three-Quasiparticle Multiplets in otherNuCIeiOOOOOOOOOOOOIIOOCOOOOOO.I... 165 5.2. Levels in the N=80 and N=82 Odd Proton Nu0181000OOOOOOIOIIOOOOIOOOOOOOI 00000000 167 5.3. Levels in Odd Neutron N=81 Nuclei....... 171 5.“. Characteristics of Similar (Delayed) BIBLIOGRAPHY.. 11/2‘ States in Odd Proton Odd Mass Nuclei in the Region of N=82....... ..... 175 0.0.0000... ooooo oooooooooooo ooooooooooooo 179 iiid Table II. III. IV. VI. VII. VIII. IX. XI. XII. XIII. XIV. XV. LIST OF TABLES Page Characteristics of Ge(Li) detectors used in this studyOOOOOOOOOOOOOOOOOOOIOOOOOOOOOOO. ..... l6 Energies and relative intensities of Y Rays from the decay of 1llllnSm.................. ..... A6 Summary of Y- two-dimensional coincidence results for 1 1mSm............................. 51 l“1m y-ray intensities for Sm coincidence experimentSOOOOOOOOOOOOOOOOOOOOOOOCOIOOOOO00.00 56 Published estimated relations between lLtlmSm and lulgsmOOOI0.0.0...OOOOOOOOOIIIOOOOOIO 000000 67 States in lule populated by lulmSm decay...... 69 Comparison of log ft's assuming either the theoretical EC/B+ ratios or all EC decay....... 95 111138111 summary of singles and coincidence intensity information.......................... 10“ Summary of y-y two-dimensional coincidence results for lulgsmooOOIOOOOOOOOOIOOOOIOOOOO000. 108 Energies and relative intensities of Y rays from the decay Of luleOOOOIIOOOOOOOOOOOOOO000. 128 Y-ray intensities for lule coincidence experiments.................................... 130 Summary of Y-Y two-dimensional coincidence re- sults for lule decay..................... ..... l“l 1ulNd level scheme comparison.................. l“5 The l“le conversion electron results of ChaI‘Vet 813 8.1. (Char 70)....0000000000000000000 151 Characteristics of similar odd proton (delayed) 11/2- states near N=82OIOOOOIOOIOOOOOI..0000... 176 iv Figure l. 10. LIST OF FIGURES Schematic illustration of Ge(Li)-Ge(Li) two- Dimensional coincidence apparatus..... ..... ...... Schematic illustration of Ge(Li)—Ge(Li) multiparameter coincidence apparatus containing Time-to-Pulse-Height-Convertor (TPHC) ............ Shcematic illustration of the anticoincidence apparatus. This same apparatus is used for the integral, anti-Compton, and triple coincidence experiments after the elimination of the 3x3-in. NaI(T£) detector (Epp 70a)................ ....... l“lm+gSm Singles y—ray spectrum for taken with 10.“% Ge(Li) detector. First of six successive 5mSpectra..........OOOOOOOOOOOOOOOOOOO ......... l“lm+g Singles y-ray spectrum for Sm taken with the 10.“% Ge(Li) detector. Last of six successive 5m SpeCtraIOOIOQOOOOOIOOOO0.9.0.000... ........... Results of two-dimensional Ge(Li)—Ge(Li) coin- cidence experiments for 1“ImSm............. ...... Additional gated slices from the two-dimensional Ge(Li)-Ge(Li) coincidence experiments on lulmSm” 511-511-keV-y coincidence spectrum for l“lm+gSm.. lL‘J‘m+gSr11 anticoincidence spectrum........ ........ Delayed-gate integral coincidence spectrum 1“lm SOOOOOOOOOOOOOOOOOOO. ........ .0... 000000 for Page 19 2O ““ “5 “9 50 5“ 55 60 ll. 12. 13. l“a. 1“b. 15. 16. 17. 18. 19a. 19b. Page Delayed signal integral coincidence spectrum forlulsmoooooooooooooooooo ...... coo-0000000009000 61 Spectra resulting from two-dimensional coin- cidence (energy vs. time) measurement of the lifetime of delayed state at 628.6-keV in l“1 63 PmOOOOOOOOOOOOIOOOOOOOOOOOOOOIOOOOOOO 000000000 TPHC Spectra obtained from half—life measure- 1“l ment of the delayed state in Pm... ........ .... 6“ Decay scheme of lulmSm... ......... ...... ......... 70 Decay scheme of 139um.................. ....... .. 71 Upper: M“ transition probabilities for the N=79 and N=81 odd-mass isotones. The 1“lm Sm point is a calculated predicted value. Lower: Values of the squared radial matrix elements for the single-neutron isomeric transitions for the same nuclei............ ...... 76 The positions of known states in the odd mass Pm isotopes............................ .......... 79 The positions of known states in N=80 odd—mass isotones......................................... 80 Symbolic shell-model representations of some important transitions between 1“13m and lule states................................... ..... ... 87 l“lm+gSm 2-dimensional integral coincidence spectrum taken with 10.“% Ge(Li) detector........ 109 1“1m+gSm 2—dimensional integral coincidence spectrum taken with “.51 Ge(Li) detector......... 110 20. 21. 22. 23. 2“. 25. 26a. 26b. 27. 28. Gated slices from two—dimensional coincidence lulgSm...” spectra for ....... ........ . .......... Spectra resulting from bombarding l“Sm with “5 MeV protons.... ........... . ................... Spectra resulting from bombarding 1““8111 with “5 MeV protons for 1 m..................... ...... Decay scheme of lulgSm........ ................... Singles y-ray spectra for lule Upper: First of five successive 10 m Spectra Lower: Fifth of five successive 10 m spectra.... Results of two—dimensional Ge(Li)-Ge(Li) coin— cidence experiment for 1ule...... ............... Additional gated slices from the two-dimensional Ge(Li)-Ge(Li) coincidence experiments on 1“l P0000.00.00.000000000000009.90.0000 oooooooooo Additional gated slices from the two-dimensional Ge(Li)-Ge(Li) coincidence experiments on l“l P000............OOOOOOOOOO0.0.0.000... 0000000 l“le anticoincidence spectrum ...... .. ........... 511-511-keV-Y coincidence spectrum for l“le ..... vii 112 29. 30. 31. 32. 33. 3“. Decay scheme of lule..................... ....... Comparison of experimental and theoretical K-con- version coefficients for some of the y-transitions following the lule decay.......... EXperimental levels in N=82 odd mass isotones.... Systematics of the energy level separations be- tween low-lying 7/2+ and 5/2+ states in odd mass N=80, 82, and 8“ nuclei.......................... Experimental levels in the N=81 odd mass isotones................................. ..... ... Systematics of low-lying 1/2+ and 3/2+ states in the N=81 nuclei demonstrating the apparent quadratic dependence of the energy separation. The value for lu5Gd is a calculated (predicted) valueOOOOOOOOOOOOOOOO......OOOOOOIOOOOOOO ....... 0 viii 150 168 170 172 17“ Chapter I Introduction A measure of our current understanding of nuclear structure is demonstrated by the ability of current nuclear models to explain observed properties of nuclei and to pre- dict other prOperties previously unobserved. The origin and development of these models involves the observation of these same nuclear state properties. In this interplay of theory and experiment, both the quality and quantity of the available experimental data serve to test the existing models and aid in the develOpment of new and improved ones. Radioactive decay studies and nuclear reaction studies complement one another as basic tools of nuclear research. Their ability to determine significant nuclear state pro- perties serves to test and advance our knowledge of nuclear structure. By means of beta and gamma ray Spectroscopy, this investigation seeks to increase our current knowledge, and, where possible, to improve the existing information in the field of radioactive decay. In the past few years there have been many radioactive decay studies of odd mass isotopes in the 50§Z§65, 62§N§82 region of the nuclidic chart (Ti 67)(NDS)(NSA). These studies attempt to characterize the states in the daughter as completely as possible by comparing their results with those of reaction studies and theoretical calculations. Recently there have also been a number of nuclear reaction studies on odd mass nuclei in and near the N=82 closed shell. These involved particle transfer studies such as (d,3He), (3He,d), (p,d), (d,p), (p,t), inelastic scattering experiments such as (d,d'), (d,d'), (p,p') and in-beam gamma experiments such as (n,n'Y), (a,2ny) This region is bounded by the magic numbers of 2:50 protons and N=82 neutrons. Nuclei with these magic numbers have tightly bound or relatively inert spherical cores of protons (Z=50) or neutrons (N=82). As one moves far from these spherical cores, it is expected the nuclei will ac— quire a permanently deformed shape. The transitional region between these extremes is characterized by a change in the nature of the levels, produced by the increasing admixture of various combinations of shell model orbitals and inter- mixed collective effects. As a result, there are a greater number of low-lying levels suggesting a softer nucleus. In the transitional region the line of stability runs approximately through the nuclei 12i8b70, 1:317“, 133C878, and 13$La82. These nuclei, along with the large number of stable nuclei in the Z=50 and N=82 closed shells, are conveniently located for systematically probing part of the transitional region by means of the (3He,xn) and (p,xn) reactions. The beta and gamma ray Spectroscopy group at MSU is currently conducting a systematic study of the region between the 2:50 and N=82 closed shells. In the process of conducting such a study it is useful to begin with a knowledge of levels in the nuclei close to the line of stability. As more in- formation becomes available regarding these levels, it will be convenient to move away from stability in successive steps toward the region of permanent deformation. During the course of earlier investigations in this re- gion, the study of 139mNd resulted in the identification of a three-quasiparticle multiplet of low-lying, high-Spin states in 139Pr (Bee 69b). The existence of such states is rare, and, because they can be characterized in rather simple shell model terms, they yield a significant amount of information about nuclear structure. ,As a result of the information to be gained, it Seemed worthwhile to attempt to identify these three-quasiparticle levels in other nuclei in the vicinity f 139 o Pr. It was suggested by McHarris et al. (McH 69) that l“lm 137m the decays of Sm and Nd would populate Similar multi- plets in the daughters lule and 137Pr. The ready accessibility of 1“18m by means of the reaction 1142Nd(3He,“n)lulSm (Q=-27.3 MeV) suggested it as the next system to be studied. l“1 A satisfactory study of mSm necessitates a knowledge of other members of the A=1“l decay chain as they appear in l“lm+gSm+1“1Pm+1“lm+gNd+l“l the sequence Pr (stable). The decays of 1ulmNd and 11418Nd have been previously reported by Beery et a1. (Bee 68). However, little was known about the “1 l“lm 20.6m 1 Pm and 22.1m Sm activities other than their half-lives and the isomer 1"183m had not been identified at all prior to initiating this investigation. In light of the existing situation, a concurrent study of the decays of 1“1m+gSm and l“le was undertaken. The study of 1“le“ was initiated through a joint effort with R. Eppley. Some preliminary results concerning 1&1 The decay of lule is of interest because the daughter Sm appear in (Epp 70a). 1“Nd is one neutron removed from the N-82 closed shell. As a result,the low-lying states are expected to be of a fairly pure single-particle nature, (i.e. neutron hole states). During the course of this investigation several studies have appeared directed at locating the neutron hole states in the N=81 nuclei by means of pickup reactions such as (p,d) and (d,t) on N=82 nuclei. As a consequence, it has been possible 1“1 to compare the Pm decay study with these data. During the course of the lM‘mSm investigation, a delayed 1“1 state was observed in Pm at 628.6-kev. This state is suggested to decay via M2 and E3 transitions of “31.8- and 628.6-kew,respect1ve1y. Lifetime measurements indicate the E3 transition is enhanced. This observed enhancement, coupled with a study of systematic trends in the N=82 nuclei, suggests a series of interesting experiments to be performed on odd proton odd mass nuclides in the region. At the present time there exists little in the way of theoretical calculations with which to compare the eXperi- mental results presented here. Muthukrishnan and Kromminga have performed a shell model calculation of the three- quasiparticle odd parity states in l39Pr by using several phenomenological potentials and limiting the basis states to those suggested by McHarris gt_§l. (McH 69) for the three— quasiparticle multiplet. The results obtained on the energies of the states were in rough agreement with experiment. How- ever, the theoretical transition probabilities did not agree very well with experiment. Preliminary calculations of energies of states below 2 MeV in l“Nd have been carried out very re- cently by Reehal using the pairing-plus-quadrupole Hamiltonian of Kisslinger and Sorensen (Ree 71), reasonable agreement was obtained. The recent shell model calculations of Wildenthal on the N=82 isotones appear very promising. It is hoped that in the near future these calculations might be extended to the N=81 and N=80 isotones. Moderately successful calculations might be performed using a rather limited basis such as the one proposed for the three quasi- “1 139Pr. particle multiplets in 1 Pm and Chapter II describes in a general way the techniques involved in decay scheme construction and contains a brief description of the experimental apparatus and techniques along with a discussion of the methods of data analysis. Chapter III describes in some detail the steps involved in the systematic identification of the transitions associated with the radioactive decay of members of the A=1“1 chain below the N=82 closed shell. Chapter IV describes the experimental results of this study. The decay of lLumSm involved the identification of “7 gamma rays and the placement of 16 levels in the daughter ll”Pm populated by this decay. A set of Six odd-parity levels between 1.6 and 2.2 MeV were identified as members of a three—quasiparticle multiplet having the configuration [(nd )3(vd )-l(vh )‘11 5/2 3/2 11/2 J“ The properties of these states are compared with the 139Pr. corresponding ones in The half-life of the delayed 11/2‘ state at 628.6-keV in 1ule was measured and the partial half-lives of the 628.6—keV E3 and “31.8-keV M2 transitions were determined. The decay scheme of lL‘lgSm resulted in the placement of an additional 7 levels in the daughter 1“le and the identification of 12 transitions be- longing to this decay. The study of the decay of lule resulted in the place- ment of 23 levels in 1ull‘id and the identification of 50 transitions associated with this decay. A comparison is made between the levels of 1“Nd p0pulated via the beta decay f 1“1 1“1 0 Pm and levels identified in Nd by means of the reaction studies (p,d) and (d,t) on 1H2 Nd (Jol 71), (Cha 71), and (Fos 71). These results are compared with very preliminary theoretical calculations by Reehal (Ree 71) using the pairing-pluS-quadrupole Hamiltonian of Kisslinger and Sorensen. In Chapter V the systematic behavior of some nuclear prOperties in the N=82 region are discussed. The characteristics of the three-quasiparticle multiplet in 1“l Pm are discussed and compared with Similar states in other nuclei in and near the N=82 closed shell. Chapter 11 Experimental Methods and Apparatus AS one moves further away from the region of B—stability the identification of radioactive isotOpes and the construc- tion of their decay schemes becomes increasingly difficult. The difficulties arise principally because the half-lives become shorter and the reactions used to produce the various desired isotopes are not as clean. The limitations on studying a specific activity depend on several factors such as (l) the types of bombarding beams available, (2) the energies of these beams, (3) the Specific nuclear reactions desired, (“) the stable isotopes available for targets, (5) the location of these isotopes with respect to the line of beta—stability, (6) the half-lives of the desired activity and the competing activities, and (7) the data acquisiton facilities available. The isotopes considered in this study were produced by either (p,xn) or (3He,xn) reactions (x$“) on stable isotones in the N=82 closed-shell. The complex decay characteristics of these isotopes required the use of the latest highest quality electronic equipment such as high resolution Ge(Li) detectors (“2.0-keV FWHM for the 1332.“-keV transition in 60 Co), FET preamplifiers, low noise amplifiers with Gaussian Shaping, base-line restoration, dc-coupling, pile- up rejection and Significant (anti-) coincidence circuitry as well as the PDP-9 and XDS Sigma-7 computers and their avail- able data acquisition programs. Section 2.1 describes in‘a general way the pattern fol- lowed in the acquisition of the experimental data and the construction of parts of a nuclear decay scheme. Section 2.2 discusses the two types of gamma ray detectors used in this study and compares the factors influencing their resolution. A description of the various gamma ray spectrometers used in this investigation is also included. In Section 2.3 some of the methods used in the data analysis are broadly outlined. A detailed description of the logical identification of the members of the A=1“1 decay chain considered in this study is deferred to Chapter III. Peculiarities pertaining to a specific experiment are included in the discussion of the experimental results in Chapter IV. 2.1 DecayScheme Construction One of the primary' objectives of this study has been the incorporation of the significant experimental results into a consistent description of the manner in which an iso- tope decays and a comparison of these results with theories and other data such as reaction studies. The important features of such a study involve proper identification of 10 the activity to be studied, determination of the half-life of the parent activity, precise measurement of the energies of the transitions, beta and gamma transition intensities, electron capture disintegration energies, calculation of log ft values for beta decay to each level in the daughter, and Spin and parity assignments. There are a wide variety of experimental techniques available and each isotope has special characteristics requiring variations in the method of approach. The placement of energy levels may be suggested initially by energy sums and gamma-ray intensity balances. Additional evidence for level placement comes from coin— cidence evidence such as 511-511—keV—gamma coincidences, anti—coincidence, prompt and delayed coincidence results, as well as other accessible parameters. AS one moves further from stability,the energy available for decay in— creases generally resulting in an increase in the number of transitions to be placed. In addition to this difficulty, the half-life generally decreases,and, as it becomes shorter, it makes Ge(Li)-Ge(Li) 2—dimensional coincidence experiments more difficult. AS a result such problems make the inter— pretation of the data more difficult. After placing the energy levels, the determination of beta and gamma transition intensities follows. Then, pro- viding the EC decay energies are known, the log ft values for decay to each state in the daughter can be calculated. 11 This involves a rather lengthy but generally routine sequence. A description of the procedure to be followed in doing these calculations appears in (Bee 69a) and (Epp 70a). Computer programs are available which calculate per cent feeding to each level, electron capture to positron ratios and log ft values. The program used in this study was DECAY SCHEME written by D. Beery for use on the Sigma-7 computer and modified by R. Epply for use on the PUP-9 computer (Epp 70a). 2.2. Gamma-Ray Spectrometers In the following study we have used two different photon detectors: NaI(T£) scintillation Spectrometers and Ge(Li) solid state Spectrometers. The processes involved in the detection of the energy deposited in the NaI(T2) crystal and the Ge(Li) crystal are quite different. As a result the energy resolution of the two types of Spectro— menters differs markedly. As a means of comparison the energy resolution of a very good NaI(T2) system is m60 keV full-width-half—maximum (FWHM) for the l332—keV y-rays of 6000. The energy resolution of a typical Ge(Li) system is 2.0- to 3.0-keV FWHM for these same 6OCo y-rays. The reason for the difference in resolution can be understood from the following brief description of the processes in— volved. 12 Each spectrometer relies on the fact that incident radiation deposits some portion of its energy in the crystal. In the NaI(T2) Spectrometer the deposited energy appears as visible and ultraviolet light. This light is detected by a photomultiplier tube optically coupled to the NaI(T£) crystal. This produces a voltage pulse proportional to the intensity of the incident light that is in turn proportional to the energy deposited in the crystal. In the Ge(Li) Spectrometer the incident radiation creates hole-electron pairs that are collected by an electric field across the crystal. The result is a current pulse proportional to the energy deposited by the radiation; this is converted to a voltage pulse by the preamplifier for purposes of pulse Shaping etc. The basic reason for the disparity in the energy resolution of the two systems is essentially statistical. The Signal resolution in NaI(T2) results from the large statistical fluctuations in the relatively small number of resultant photoelectrons emitted by the photocathode per given amount of energy deposited in the scintillating crystal. Ge(Li) solid state detectors employ direct col- lection of the ionization produced by the radiation. In Ge(Li) a hole-electron pair is produced for every 3-eV of energy (on the average) absorbed from the radiation. Correspondingly in NaI(T£) on the order of 300 eV are required for every photoelectron produced. 13 Hence in the NaI(T2) system the relatively large statistical fluctuation is due to the small number of electrons ejected from the photocathode of the photo- multiplier, whereas in the solid state detector the primary energy is distributed over a far greater number of carriers and the relative statistical fluctuation is considerably reduced. 2.2.1 Singles Experiments In the present study, singles experiments refer specifically to pulse height Spectra taken with the aid of a Single Ge(Li) spectrometer. In point of fact the Ge(Li) Spectrometer is an electronic system composed of the following components: a) A lithium-drifted germanium crystal enclosed in a cryostat and cooled in liquid nitrogen,temperature (770K), b) a high voltage bias supply and a charge-sensitive pre- amplifier with an associated field—effect transistor (FET)—— the FET may be either cooled to ml20°K or operated at room temperature, c) a spectroscopy amplifier (for purposes of amplification and pulse Shaping) with pole-zero can- cellation, base-line restoration, and dc level adjustment, d) an analog-to-digital converter (ADC) and computer or multichannel analyzer (MCA), and e) a storage unit of some type with associated spectrum readout. The performance characteristics of a particular Ge(Li) Spectrometer are primarily determined by the Ge(Li) crystal 14 and the associated FET preamplifier. Such features as the volume of the Ge(Li) crystal, the manner in which the crystal is drifted (i.e. true-coaxial, trapezoidal or wrap-around, or planar), the capacitance of the crystal, and the operating bias all play interrelated parts in determining the final resolution of the system. The cooled FET will usually re- sult in a lower noise system and as a result yields somewhat better resolution as compared to a room temperature FET preamplifier. The volume and Shape of the crystal will play important parts in determining the efficiency and peak-to-Compton ratio for a particular detector. In general a larger volume crystal results in increased count— ing efficiency (related to the size) but this increase in volume usually results in a decrease in the resolution pro— duced by an increase in crystal capacitance and poorer charge collection. Other experimental features which are important in achieving satisfactory resolution are amplifier Shaping constants, proper pole—zero cancellation, base—line re- storation and correct dc-level adjustment. These can be determined rather quickly by using an oscilloscope and ob- serving pulse-shapes and dc-levels at appropriate places in the system. The use of base-line restoration is strongly dependent on the counting rate and usually is best determined empirically. 15 The characteristics of several different Ge(Li) spectrometers used in the current gamma-ray studies are listed in Table I. 2.2.2. Coincidence Experiments In the course of this study the character of the overwhelming majority of nuclear states observed suggested they had lifetimes <<100 nanoseconds (ns). Consequently it proved beneficial to use two or three singles spectro- meters in various coincidence configurations to perform anticoincidence, prompt coincidence and delayed coincidence experiments. The standard procedure employed in a typical decay scheme study is to determine a) a singles spectrum, b) an anticoincidence spectrum, and c) coincidence spectra. l6 Table I Peak-to Res. ngg Eff.* Shape Compton l332—keV Co Manufacturer 0.4% Trapezoidal 4.5:1 3.4 keV MSU Cyclotron Lab 2.5% Trapezoidal 12:1 2.2 keV Nuclear Diodes 3.6%# True Coaxial 18:1 2.0 keV Ortec 4.5% True Coaxial 24:1 1.9 keV Ortec 10.4% True Coaxial 36:1 2.2 keV Nuclear Diodes *With respect to the 1332 keV peak of 60Co at 25 cm from a 7.6 x 7.6—cm NaI(Tl) detector. #This detector was damaged by an electronic malfunction and replaced by the manufacturer with the 4.5% detector. 17 The techniques involved are routine and are discussed in greater detail in (Bee 69a) and (Epp 70a). A "classical" pattern of experiments was used in the study of the l”le decay described in Section 3.2, whereas the study of the Sm isotopes 1”188m and 1ulmSm required the use of a wider variety of methods to determine the decay schemes and their characteristics. The study of 1141Sm involved isotopes with similar enough half—lives, 11.3m in the case of 1“188m and 22.1m in the case of lulmSm, that the counting methods had to be modified. As an added feature, l“lm the decay of Sm involved the population of an 11/2- isomeric level in lule which was observed to be a delayed state. The measurement of this half-life is discussed in Section 3.1.3.D. By using two Ge(Li) detectors placed in 1800 geometry we were able to perform several useful prompt—coincidence experiments as described below in Section 2.2.2.A. Other prompt and delayed coincidence experiments performed with Ge(Li) detectors inside a Split-ring NaI(T£) annulus are outlined in Section 2.2.2.B. 2.2.2.A. Multiparameter Ge(Li)-Ge(Li) Spectrometer With the availability of larger volume and more efficient Ge(Li) detectors it has become practical to use them in obtaining 2 parameter coincidence information. 18 The goal of such an experiment is to Simultaneously pro— vide coincidence information for any and all peaks or regions of the Spectrum in a Single run. Despite the relative inefficiency of Ge(Li) detectors, as opposed to NaI(T£) detectors, there are decided advantages associated with their use in a multiparameter experiment. The most important features are the excellent energy re— solution and the greater peak-to-Compton background ratio. AS a result, satisfactory coincidence information can be obtained with fewer counts per channel using a Ge(Li)—Ge(Li) system than are normally required when using Ge(Li)-NaI(Tt). The electronics arrangement used in this lab is a relatively standard technique and is discussed in more detail in several theses and journals (Epp 70a). A diagram of the arrangement can be seen in Fig. 1. With minor modifications, it is possible to obtain more information than described above. This information refers to a third parameter, namely the time between the two coincident events. It is with this in mind that I describe an alternate Ge(Li)-Ge(Li) coincidence spectrometer as seen in Fig. 2. This is still a double coincidence experiment where we wish to record the linear Signal from each of the de- tectors. The added feature in this configuration is the presence of a time to pulse height convertor (TPHC) in place of the fast coincidence unit. This provides us with 19' I I i 69 LIIEAR TIMING AM’UFIER AM’UFIER FAST SCA COINC. SEALAR F—J LINEAR GATE L__1,___1 X’SIDE 8l92 CHANNEL ADC O-BV 8|92 CHANNEL Y-SIDE ADC TIMING LINEAR AMPLIFIER AMPLIFIER SC A LINEAR GATE O-BV SYW CHANNEL NO, ‘ 0F EVENT FROM SIDE X lNTERFAgEOTO SIGMA 7 CHANNEL NO. OF EVENT FROM SIDE Y BUFFER IN SIGMA 7 uwpga 'JANUS' CHANNEL NOT CHANNEL No ONE WOR D MAGNETIC TAPE CHANNEL NO OF EVENT IIZ WORD CHANNEL M3 GP EVENT IIZ WORD Figure 1. Block diagram of the apparatus used to collect two-dimensional "megachannel" y-y coincidence Spectra, using the Sigma—7 computer. O uaom IDRE AMP[Ge(Li) Ge(Li)‘lPRE AMP mmHhudmx<_ mzoo HZOHQI mmqam OH WZHH 1 .nHHV 848 ILV Dad OH T4 $2392 .23 , has 338 I1 (Juo ¢¢u83 . m enema [am 1 and Sfl'IflNNV 0 30m -V L 3:0 9!: :53: con 202.08 23 experiments are first discussed in (Aub 67) and additional discussions appear in (Bee 69a) and (Epp 70a). 2.3 Data Acquisition and Analysis The lifetimes of the nuclides considered in this study are relatively short, ranging from m20m down to mlm. Be- cuase of this time limitation and the high resolution of our Ge(Li) detectors (2.0 to 2.5 keV) the Spectrum Size has varied from 102“ channels for some x-ray spectra to 8192 channels from some y-ray spectra, with “096 channels being the normal spectrum Size. These spectra have been taken using several different data acquisition modes. The vast majority were taken using either the Sigma-7 or the PDP-9 computers with the particular choice being determined by the acquisition time and convenience. There are various data acquisition programs available to users of the Sigma-7 and some of the important programs used in this study were GEORGE (GEO), HYDRA (HYD), TOOTSIE (TOO), EVENT (EVE) and POLYPHEMUS (POL).1 Choice of a particular program is governed by the requirements of the particular experiment. The data analyses required for this study were performed using the Sigma—7 and PDP-9 and programs written or adapted for these computers. Brief descriptions of the programs used to determine gamma-ray energies and intensities, MOIRAE and SAMPO, are given below. A brief description of EVENT RECOVERY, the program used in the recovery of two-parameter information for magnetic tape, also follows in Section 2.3.1.C. 1Brief descriptions of these programs will be found in the Bibliography. 24 Discussions pertaining to peculiarities encountered in specific experiments are mentioned in Chapter IV. 2-3-1E_ Program MOIRAE The areas and centroids of many of the photo peaks appearing in this study were analysed with the aid of the Sigma-7 computer and program MOIRAE. MOIRAE is a machine language program Operated on—line with the Sigma-7 computer. It incorporates the major functions of the program MIKIMAUS, originally written by G. Berzins and outlined in Appendix D of (Ber 67). The program provides a live display of the Spectrum to be analyzed and provides the operator with the ability to execute instructions on the computer by means of sense switches arranged below the display oscilloscope. The Spectrum may be expanded both horizontally and vertically and, if desired, the entire Spectrum may be walked across the screen horizontally. In addition to the live spectrum display, the oscilloscope provides a movable channel marker, a display of the fitted background curve, the centroid position of the analysed peak, and the option of displaying the net Spectrum resulting from the subtraction of all points below the fitted background. By means of the channel marker, the operator is allowed to select a set of points in the Spectrum for determination of the background. 25 A polynomial curve of preselected order (up to degree 10) is fit to the data by means of an internal least squares fitting routine and displayed on the oscilloscope. If the curve is not satisfactory it may be rejected. If the curve is satisfactory the channel markers can then be released to define the limits of a particular peak. The computer then determines the center of gravity of the peak and its net area after subtracting the area below the background curve. This information (i.e. centroid, net peak area) is then output in the form of punched cards and/or via the line printer. In order to determine the energies and relative intensities of the various gamma-ray peaks from such an analysis, it is necessary to use the data with an external program on the Sigma-7. The primary program for such an analysis is MOIRAE E(I). It determines the energies by per- forming a least squares quadratic fit to preselected peak centroids and interpolating for the others. Internal pro- visions allow for the calculation of the relative intensities of the peaks with an efficiency curve for the particular detector used in obtaining the Spectrum. There are some distinct disadvantagestx) the use of MOIRAE. Among them are the need for auxiliary programs and the inability of MOIRAE to strip out the individual peaks in an unresolved multiplet. This must either be performed by hand or by another program, such as SAMPO (a program which does many of the same operations as IMIUUUSand is described in the following section). 26 2.3.2. Program SAMPO This program was developed for the express purpose of performing automatic peak search and analysis of y-ray spectra from semiconductor systems. It is described by Routti and Prussin (Rou 69) and, with modifications, has been adapted by C. Morgan for use on the Michigan State Sigma-7 computer. During the course of this study the pro- gram has been modified to allow the user to interact with the program via a storage oscilloscope. However, the majority of spectra in this study were analyzed without this feature. The intent of the program is a development of a mathematical formalism for the representation of photopeaks and the continua in their vicinity. This formalism will then be applicable to the analysis of such Spectra taken under a wide variety of experimental conditions. The pro- gram and its use on the Sigma-7 computer are described in greater detail in (Rou 69) and (Epp 70a) and in the MSU Nuclear Chemistry Annual Report for 1970. An abbreviated description follows. The important features of the program are: a) the fitting of photopeaks to an analytical function (Gaussian- plus-exponential tails) in order to determine the precise centroid of each peak, b) a routine that searches the spectra and finds peaks, c) fitting and subtraction of background, 27 d) determination of energies and their statistical errors from an internally determined calibration curve, e) cal- culation of the net area of peaks, including the statistical error in the area, with conversion of these areas into re- lative intensities by means of an internally prepared cali- bration curve, and f) the stripping of multiplets using para- meters determined from other well—resolved peaks. The program determines peak fitting parameters from preselected, intense, well-isolated lines designated by the user. These peaks are fit with a Gaussian-plus-exponential shape function using a linear approximation for the back— ground continua under the peak. Parameters for the other peaks in the spectrum are calculated by linearly interpolating between the selected fiducial lines. Energies are determined by doing a piecewise linear fit between successive standard points selected by the user. If it is desired, higher order fits may be calculated but caution should be used in these cases to be sure the curve is smooth and does not oscillate. Relative intensities are calculated by means of an internal efficiency curve. This curve may be calculated for the program or determined by supplying external parameters. Other features of the program include an option which instructs the program to automatically search for peaks in the spectrum. This option is useful in the initial inspection of the data, but is not to be relied on for the final analysis. 28 Proper use of the program involves considerable attention on the part of the user but the overall power of the program certainly warrants this. 2.3.3. Programs EVENT and EVENT RECOVERY With the availability of large volume Ge(Li) detectors, the capability of doing high resolution Ge(Li)-Ge(Li) 2— dimensional coincidence experiments is easily realized. The immediate goal of such an experiment is to obtain coincidence information on any and all peaks or energy re— gions of a spectrum in a single run. A system capable of realizing this goal is diagrammed in Fig. l. Coincidence infomation is recorded using the task EVENT which runs under the JANUS monitor system of the Sigma-7 computer. Basically the program does the following. The addresses of the two coincidence events are each written in two halves of a word in one of two buffers. When the buffer is filled the 256 words are then stored sequentially on magnetic tape. Further details concerning this can be found in (Doe 69) and (Epp 70a). The magnetic tape contains all the information concern- ing pairs of coincident events. All that remains is to retrieve the information from the tape. This is accomplished by selecting a peak or energy region of interest and searching the other half of the tape for events in coincidence with those of interest. These gated spectra are recovered 29 off-line on the Sigma-7 computer by means of the program EVENT RECOVERY (EV RE). This code allows the user to search the tape for a maximum of 10 gated regions per single pass through the tape. If desired, background subtraction may be affected for any and all of the gates included in a single pass. In the current study all the coincidence runs used a MO96xu096 array. Chapter ITT Identification of Members of the A=lul Decay Chain The identification of a radioactive species to be studied relies on several interrelated techniques, such as a knowledge of the excitation function of the reaction used in the production of the isotope, identification of a known activity in the decay of the daughter, production of the isotope by other reactions (i.e. cross bombardments), a knowledge of the systematics of other nuclei in the region being studied, information obtained from charged particle reaction data, chemical separation, isotopic mass separation and identification of the x-rays associated with the decay. What follows is a general description of the methods used in identifying the radioactive decays of members of the A=lul chain below the N=82 closed-shell. These include lulm+gNd 1&1 lulm+gsm , o Pm, and m+iu1Nd+1u1 The sequence of decay is 1u18m+1u1 P Pr (stable), but the identification process starts with the stable isotope 1“Pr and proceeds away from stability in successive steps. lhlm+gNd constitutes the first link in the chain and has been studied by Beery (Bee 68). The following is a brief discussion of his results. 3.1. lulNd 1141Nd is produced by bombarding Pr203(which is 99.97% pure) with 9-MeV protons to produce the reaction 1“1Pr(p,n)lulNd. The energy at which this reaction occurs 30 31 is low enough that it is relatively free from the pre— sence of unwanted competing reaction products. As an additional check on the identification, chemical separa- tion was also used. The half-life of 1ulgNd was deter- mined to be 2.5-hrs. and 10 transitions were identified with the decay. The energies of the more intense gamma-rays listed in order of increasing relative intensities in the gamma-ray singles spectrum are 1126.8-, 1292.8—, 1A5.A-, and 1298.7-keV. The decay of lulmNd was found to involve a single transition of 756.7-keV from an ll/2- isomeric level with a 603 half—life to the 3/2+ ground state. Additional information about the levels in 1ulPr comes from their observation in reaction studies such as (3He,d), (d,3He) (W11 71) and (n,n'y) (Dav 70). 3.2. lulpm Making use of these l“lNd results we can then pro- 1’41P ceed to identify the decay of m. The principal means of producing l“le was via the reaction 1“ l“le. 142 2Nd(p,2n) Targets of Nd2 bombarded with 25—MeV protons from the Michigan State 03 (enriched to 390% Nd) were usually University cyclotron. An excitation curve was obtained using absorbers of different thickness to vary the in- cident beam energy from below threshold for the (p,2n) (Q=-1U.2 MeV) reaction to well above the threshold for 32 the (p,3n) reaction (Q=-2A.7 MeV). Results indicated the (p,2n) reaction to be relatively free from unwanted competing activity. Confirmation of the production of lule was established by observing transitions aSSOCiated lulNd 1&1 with the decay of the daughter . The Nd transitions appeared in the spectra with the proper time dependence and in the correct relative intensity ratio. An additional check on the identification of lule was made by producing l'41Pr(3He,3n)lu1P lAle it via the m. These results comfirmed our identification of . Several strong transitions associated with this decay were identified at 1223.3-, 886.3—, and 193.8-keV. Measure- ment of their intensities as a function of time corroborated previous results which had assigned a half-life of 22m to this isotope (Fis 52). Weaker transitions associated with the l“le decay were identified by comparing their intensity to that of the 1223.3-keV line as a function of time. Beginning 2m after each bombardment five successive 10 minute spectra were accumulated with the process re- peated several times to improve statistics. Those gamma- rays which maintained a constant intensity relative to the 1223.3-keV line in the five successive spectra were 1ule. Other transitions could identified with the decay of be ruled out because their intensities varied erratically when compared to that of the 1223.3-keV transition. 33 Additional confirmation of the l”le decay scheme resulted from the identification of several levels in lulNd 1U 1141Nd populated in the charged particle reactions 2Nd(p,d) IUZ 1“] (J01 71) (Cha 71) and (d,t) Nd (Fos 71). 3.3. lulSm Continuing up the A=lul chain we arrive at 1“18m. From studies of systematics in the N=79 isotones it was expected that l“18m would possess two decay modes. A first mode associated with the decay of the 1ulSm ground state and the other mode would involve direct beta-decay from an 11/2- isomeric level. This latter decay would be characterized by the population of three quasi-particle states in the daughter lule. lulSm was via the lUZN The primary method of production of 1“ 1U1 reaction 2Nd(3He,Un) Sm (Nd2O enriched to 290% d) 3 with a Q value of —27.3 MeV. This method proved to be relatively free from unwanted interfering activities. A careful study of the energy systematics of the reaction was performed by varying the incident beam energy from below the (3He,un) threshold to well beyond the (3He,5n) threshold (Q=-36.2 MeV) in 1 MeV steps. It was observed that the maximum yield of 1&1 Sm occurred at Mo MeV, slightly beyond the (3He,5n) threshold. These results were found to compare favorably with theoretical predictions made with the computer code CSBN. This is a Berkeley code used for the calculation 34 of heavy-ion cross sections. It has been modified for use on the Sigma-7 computer by Clare Morgan. Assurance of the production of 1A1m+g8m came from the excitation function, the time dependence of the decay and finally from the growth and decay of the daughter 1141P 1H1 activity m. The strong transitions in the Pm de- cay, such as the 886.3- and the 1223.3-keV lines, were readily identified in the proper intensity ratio. In addition, the x-ray spectra prominently displayed the Pm x-rays, which would arise from the beta-decay of a Sm iso- tope. Results of the excitation function work indicated the presence of two distinct activities with half-lives of 11.3m and 22.1m. The longer lived 22.1m activity was 1ulmSm. This identified as originating from the isomer identification was made after determining that the 22.1m activity populated a delayed state in the daughter at 628.6-keV, which we concluded had a spin of 11/2-. The shorter lived activity was then identified with the decay of 1“IgSm. The similarity of the resultant lulgSm decay scheme with that of l39gNd is additional evidence which tends to support this identification. In order to identify the transitions belonging to the decay of 1“188m and lulm Sm, we took 6 successive 5 minute spectra, with the first such beginning 2 minutes after the end of the bombardment. The process was repeated 35 several times to enhance the statistics. Those transitions associated with the decay of 1ulgSm were identified as maintaining a constant intensity when normalized to the 403.9-kev transition (Tl/2-11.3m) in each of the six spectra. The transitions arising from the lulmSm decay were identified as exhibiting a constant intensity when compared to the 431.8-keV transition (1 -22.1m) in each spectrum. On this 1/2 basis we could then rule out transitions in the spectra which might arise from the decay of 1&0 1&2 Sm (Tl/2'6.0m) or Sm (11/2-72m). To further check the identification of the 11.3m activity as arising from the 1“183m decay we produced l“Eu via the 1M3m(p,un)lu1Eu. We were then able to observe reaction the presence of 1“138m peaks in the correct intensity ratio 1u1 + lulg as they were fed by the decay Eu é% Sm. In addition to this we have been able to identify the vast majority of contaminant peaks. Chapter [V Experimental Results 3:1 Decay Schemes of lulmSm and lulgSm Section u.1.1. Introduction These results of our study of the decay of lué28m79 complement the similar work done on the decay of 132$Nd 79 by Beery, Kelly, and McHarris (Bee 69b). The decay of 11/2- 139mNd selectively populates six high-spin states at 139Pr. relatively high energies in These states were characterized as three—quasiparticle states having the configuration, (nd5/2)(vd3/2)-l(vhll/2)-l, and the preferred 139m 2 —2 EC decay of Nd could be written as (nd5/2) (vd3/2) -1 -1 —l (Vhll/2) +(nd5/2)(vd3/2) (vhll/2) The work on 139mNd decay led to the prediction by McHarris, Beery, and Kelly (McH 69) that other N=79 or N=77 nuclides might possess the requisite configurations for similar EC or B+ decay into three-quasiparticle multiplets. In addition to the configuration,such a nuclide must also have sufficient decay energy to populate states above the pairing gap in its daughter, and it must be "hung up" with respect to other modes of decay——for example, if it is a metastable state the energy for the isomeric transition must be low enough to make that transition quite slow. Working ones way out from B stability, he finds that lulmSm and 137mNd are the next likely candidates. For 36 37 example, the addition of two protons to the 139mNd configuration produces the very similar configuration for lulmSm, (ads/2)"(vd3/2)'2(vh11/2)'1. The calculated QEC is more than 5 MeV, allowing it to populate high-lying states in lule. Finally, as in the other N=79 odd-mass isotones, the h11/2* d3/2 (metastable + ground state) separation is small (2171 keV) making the MN isomeric transition very slow. The results that we present in this study do indeed confirm these predictions-—a multiplet of six, possibly seven, states lying 1U1 between lulu.8 and 2702.“ keV in Pm receives some 67% of the decay, and they appear to be three-quasiparticle states 139mNd. similar to those populated by Prior to the current investigation very little work had been done on 1”18m, and, in fact, it is doubtful if it had been observed at all prior to 1967. In 1957 an activity reported (Pav 57) as l“Sm was assigned a half-life of 17.5 to 22 days, in 1966 another report (Lad 66) concluded that 1“Sm must have a shorter half-life, probably less than three days. The first correct identification of lulmSm seems to have been made at about the same time by Bleyl, Mflnzel, and Pfinning (Ble 67) and by Arl't gt_al. (Arl 67). The results from these studies, primarily Just half-life assignments are essentially in agreement with our results——we obtain a half-life of 22.1:O.3 lhlm min for Sm. In 1969 Hesse (Hes 69) published a more complete decay scheme and identified 32 y rays as belonging 38 to the decay of this nuclide. However, his decay scheme was incomplete and there was some confusion in his not recognizing the presence of ll.3t0.3-min 1ulgSm. The latter species was first identified by Eppley and Todd (Epp 70a) and has a decay scheme paralleling that of 139gNd, (Section u.1.u). Most recently, we presented (McH 70) a preliminary decay scheme of lulmSm in a talk at the Leysin Conference on Properties of Nuclei Far from the Region of Beta-Stability, and Arl't g£_al. (Arl 70b) presented an abstract at the same conference in which they recognized the three-quasiparticle nature of the three states in lule that receive the strongest population and in which they report a half-life of 9.5:O.S min for l“158m. Section 9.1.2. Source Preparation and Identification of llllmSm and lUlgSm The principal means of producing lulgSm and lulmSm was via the reaction lu2Nd(3He,Nn)lulSm (Q=—27.3 MeV). We have also produced 1“Sm indirectly by the EC 141 Nn)lulEu 1N1 Sm(p, + Sm reaction (Q=-36.8 MeV). The second reaction was used primarily to enhance the pro- duction of 1“138m with the hope of being able to distinguish lhlm it better from Sm which is (too) abundantly produced in the first reaction. The targets used in the 3He bombardments consisted of 1H2 1&2 10-25 mg of separated Nd) obtained from Nd203 (>90% 39 Oak Ridge National Laboratory). These targets were bombarded with a HO-MeV 3He beam from the MSU Sector-Focused Cyclotron. Beam currents were typically 0.1-1.0—uA, and bombardments were usually for periods of 1 to 5 minutes. For the proton bombardments, the targets consisted of :25 mg 1““8m 0 2 3 lM‘Sm, also from ORNL), the beam current (enriched to 95.1% was 20.5-uA at “5 MeV, for a period of l-min. No chemical separation was attempted because of the short half-lives 141m of 1“155an (11.3 min) and Sm (22.1 min). In order to be certain of the identification of the two isotopes 1“188m and lulmSm, several methods of identification have been employed in this study. The principal reaction (3He,4n) takes place rather far from the line of stability and at an energy of “0 MeV, and other reactions such as (3He,axn) might offer serious competition to the desired production mechanism making identification of the products a problem. For this reason a systematic study was made of the excitation function for the (3He,un) reaction from below threshold (Q=-27.3 MeV), to well beyond the threshold for the (3He,5n) reaction (Q=—36.l MeV). The study involved increasing the incident beam energy in 1 MeV steps from 27 MeV to 50 MeV. Several spectra were taken at each energy to aid in identification of the primary activity produced. These results demonstrated that the (3He,un) reaction was the dominant reaction between 3A- and NO-MeV and competing 40 reactions were of little signifcance. We were able to identify several strong transitions which exhibited ex— citation curves consistent with the energy dependence of this reaction. The energies of these transitions were 196.6-, “03.8-, “31.8-, A38.H-, and 538.5-keV. After the initial identification of these transitions 1“18m we measured the half-lives as belonging to the decay of of their decay. The results of this half-life measurement indicated the 196.6-, h31.8-, and 538.5-keV transitions were associated with an activity whose half-life was 22.110.3m and the u03.8-keV and A38.u-keV were associated with an activity whose half-life was 11.310.3-min. The longer lived 22.1-min activity-was identified lulmS 1Hlmo om as belonging to m. This identification of the activity was made by observing that the 538.5—keV transition lUl fed a delayed 11/2‘ isomeric level in Pm (half-life of 700120 ns). This would be the case if the state in lulmSm was a high spin state, i.e. an 11/2- isomeric level. This result was anticipated from a previous knowledge of other odd mass N=79 isotones which possessed 11/2- isomeric levels. The 403.9—keV transition (T =ll.3-min) was then identified 1/2 as belonging to the decay of 1”183m. Further evidence that these five transitions are associated with the decay of a Sm isotope comes from x-ray—gamma 2-dimensional coincidence studies. This ‘1 experiment is identical to a Ge(Li)-Ge(Li) 2-dimensional experiment except one of the Ge(Li) detectors is replaced by a Si(Li) x-ray detector. Each of these 5 transitions was found to be in coincidence with the x—rays of Pm, confirming that their production arises from the beta decay of a Sm isotope. 42 H.1.3. EXperimental Results h.l.3.A. Half-Life Determinations of 1“188m and lulmSm l“1!“an and 1“133m were determined The half-lives of simultaneously with the aid of the MSU Cyclotron Laboratory Sigma-7 computer and a computer code called GEORGE (GEOR). This allowed the accumulation of A0 successive N096-channe1 spectra, each of 2-min duration. A pulser peak (60 Hz) was included in each spectrum to allow determination of the proper dead-time correction. The half-life of 1“ms": was determined independently from the 196.6-, u31.9-, and 538.5-keV peaks (cf. next section). The results were then averaged to yield a value of 22.110.3-min. (The spectra and half-life curves can be found in (£99 70). The half-life of 1“133m was deter~ mined from the “03.9-keV peak and the result obtained was 11 o 310 o 3"m1n s fl.l.3.B. y-RaySingles Spectra Two Ge(Li) detectors were used to cross-check each other in making energy and intensity measurements on the 1“ImSm y rays. These were a five-sided coaxial detector 43 having 2.5% efficiency (at 1332 keV compared with a 3x3-in. NaI(T£) detector, source distance 25 cm) and a true coaxial detector having 10.A% efficiency. Under typical operating conditions, using a cooled FET preamplifier, an RC linear amplifier with near-Gaussian shaping and DC—coupled base-line restoration, and 12— or l3—bit ADC's coupled to a PDP-9 or Sigma-7 computer, we achieved a resolution of :2.2 keV FWHM for the 6000 1332-keV peak. The energies of the prominent 1ulmSm y rays were determined by cmnufidxugthe l”18m source simultaneously with several well—known calibration standards. Above 800 keV, 5600 was the principal standard, (Cam 71) while at lower energies SuMn, 57Co, l“Ce, l370s, 207Bi, and 2L‘3Cm (cf. (Epp 70) were used. A number of spectra were taken, and the energies of the stronger 1ulmSm y rays were obtained from the average values. These peaks were then used to 1ulmSm y rays, which determine the energies of the weaker were obscured by the standards. The relative intensities were determined by correcting the net peak areas by using a previously prepared efficiency curve. For more details on the methods of data reduction used, cf. (Epp 70) or (Epp 71). The first and sixth of the successive 5-min singles spectra that were used to determine the energies and intensities and to assign Y rays on the basis of half-life are shown in Figs. A and 5. 44 O'I'G 9‘96! new .-. ( N.)fll.ll —/ —Ms'oou ms» (”mu 6'99” ‘- «Mass (“6'“) “all II I‘D 9'1!" v‘sou / I‘SOOI -.«. “as I . In: l." 26 - O‘GLO - I758 'IMG‘GOlQm 239‘ --:--—_.:':_:- » ;— ; 9'91. TOW-[gag 3 Z‘VOJ. --—-—- 9'.”- 9319/ £029 T " 6109 911C 9“; ' FIE. elf—a - (Oistov ms ‘ 461w» . (0)6199 ( )v‘tZfi 1 2500 Io° - o‘— i "o 0‘ IO3 4 "IBNNVHO 83c] SiNflOC) 4000 3500 3000 l 1 I0001 I500 L 500 CHANNEL NUMBER Singles y-ray spectrum for 1“IM+98m taken with 10.42 Ge(Li) detector. First of six successive 5 m spectra. Figure 4. 45 V l l l r P y.— U'Wl' €069! _ “Nd".K‘ZZI "9H t— . 690 n 44 3.9.4.3!- .... ‘1’"; p— 010, i 9129 ‘ 9K9 0"8 --— _ ‘/ ‘ MSCAD P 3— - (Murat 6'02? 996! fi. 13.0.3 m3 l N O WBNNVHC) 83d SiNflOC) 0 K) [p :03: IO' e 4000 3500 3000 2000 2500 NUMBER 600 CHANNEL Singles y-ray spectrum.for 1“IM+98m taken with the 10.42 Ge(Li) IOOO detector. Last of six successive 5 m spectra. Figure 5 . 46 Table 11- Energies and Relative Intensities of y Rays From the Decay of ll‘1'"an ... _..———- .—_-.'.-. -..- . .. .-_..-—-_.__. -.- This Work Hesse (Hes69) Energy Energy (keV) Intensity (keV) Intensity 108.510.3 0.5 10.1 --- --- 149.110.3 0.7 10.2 --- --- 196.610.3 184 118 196.510.S 260 130 247.910.2 1.9110.35 --- --- 431.810.1 100 15 431.710.5 100 110 538.510.3 20.9 11.4 538.010.5 18 15 577.810.3 2.2 10.6 --- --- 583.410.3 0.7 10.2 --- --- 607.910.2 2.5 10.3 -—- --- 628.710.1 6.6 10.20 628.310.5 6.8i1.0 648.710.3 0.9 10.2 --- --- 676.810.3 3.4 10.5 --- --- 684.610.2 19.6 11.5 684.210.5 21.812.6 704.210.3 1.1 10.2 --- --- 725.710.5 3.6 10.6 726.310.7 9.9+l.7 750.310.3 3.9 10.60 749.510.8 4.611.0 764.310.3 0.4 10.1 —-- —-- 768.210.5 0.4 10.1 --- --- 777.410.3 50.3 12.0 777.110.S 58.217.0 785.910.1 16.9 11.0 785.810.5 20.712.5 805.910.1 8.8 11.6 806.010.6 10.8il.6 820.710.3 0.4 10.1 --- --- 837.110.2 8.8710.3O 836.710.7 11.5i3.0 875.010.1 3.1 10.1 874.61O.7 4.611.0 882.010.3 0.4 10.1 --- --- 896.510.1 3.6 10.4 896.210.7 5.6tl.5 47 Table II. - Continued. --.- _.-.-.-_— - -— This Wbrk Hesse (Hes69) Energy Energy (keV) Intensity (keV) Intensity 911.310.3 22.8 10.6 911.110.s 26.5.3.5 924.710.l 5.7 10.8 924.410.7 6.9tl.3 952.110.2 2.2 10.1 --- —-- 955.4tO.S 1.7 10.1 --- --- 974.1t0.5 0.5 10.1 --- --- 983.3t0.3 18.0 t0.8 982.9:0.5 21.6:3.5 995.8t0.5 0.9 10.2 -—- --- 1009.1to.a 7.2 :0.6 1008.3t0.5 10.512.0 1029.6t0.6 1.3 10.3 --- --- 1108.410.2 3.1 10.3 -—- --- 1117.6i0.2 8.0 10.6 1117.2t0.6 11.1t1.8 1145.1i0.2 21.6 $0.8 114a.9:0.5 27.5ta.o 1287.6t0.4 0.7 10.3 --- --— 1380.9i0.6 0.5 10.2 --- --- 1434.9to.a 0.9 10.3 --- --- 1463.4to.6 4.5 10.8 1462.1to.8 5.4t2.0 1490.3to.1 22.9 11.5 1490.2+0.5 27.4t3.5 1786.410.4 27.1 11.1 1785.9:0.6 33.6t4.0 1898.Otl.0 0.94:1.15 1898.5t1.2 2.511.0 1979.6t0.2 0.99:0.12 1979.210.8 1.610.7 2073.7t0.2 3.53:1.2 2072.9t0.6 11.8t2.0 --- --- 1136.6t0.8a 5.211.5 --- --- 1530.7:1.oa 2 +1 --— --- 1879.9:0.8a 6.311.S --- --- 1966.9t0.8a 2.511.o --- --- 2281.1tl.0a 1.510.s --- --- 2302.6tl.0a l.1#0.4 --- --- 2582.3tl.0” 2.9+o.8 "‘ ...... ...,L-.. a These y rays could not be identified in the present investigatior. 48 On the basis of half-lives and relative intensities we have assigned at least “7 y rays to lulmSm decay. The energies and intensities of these are listed in Table 11 where they are compared with the results obtained by Hesse (Hes 69). The two sets of data are in fair agree— ment, but we see many weak transitions that he did not ob— serve, and we do not include seven y rays that he associated with lulmSm decay. 4.1.3.C. Prompt y-y Coincidence Spectra Several types of prompt coincidence experiments were performed using Ge(Li)-Ge(Li) and Ge(Li)-NaI(T£) spectro— meters. These included two-dimensional coincidence experiments, pair spectra experiments, and anticoincidence experiments. Details of the two-dimensional coincidence experiment can be found in (Epp 70a), (Epp 71), and (Gies 71) so they will not be repeated. Suffice it to say that the 2.5% efficient Ge(Li) detector and another one having 3.6% efficiency (also 2.0 keV resolution FWHM at 1332 keV) were placed in 180° geometry, with a graded Pb shield placed between them to help prevent scattering from one detector into the other. Coincident events (resolving time, 21=l60 nsec) from both sides were processed, and their addresses were listed in pairs on magnetic tape. This yielded a 4096xH096-channe1 array of prompt coincidence ca... .5... ~1\I‘u i - 3i. f I 1 D'”l > m . ’_ the 009- < 0 s on. 1.19%» .J 4 g towns - E ham: S x 22- '> I "DI I C 06" 43I8 - keV GATE Hill mm 5385 - keV GATE 2000 L500 000 I500 IOOO 500 i"“ - Able 99$ _ 91¢ oufi (9)650! 6192 V96 '1 l 41 l l n u _ n N - _ 9 9 e 0 9 9 9 9 [ T w—T‘ I T I 0‘31» — 1 LAJ F— - 1 s a. 1 if < l g 3 win“ 0 l a > ~ 1 I r— . Z ‘3 m ' P4 x ‘9- v 8 w - N __ a; two] _ hils ‘ur \ 026 ink €75 ’- I 'In ‘ :13?“ WW“ .. mm: _4 1 . em 5‘99 '1‘1 C094 9x1 r771 9m _. up." 61,9 rm . 701 ' c. ;S§.,j‘ ‘ t. __ C. H\‘. HT. 1 ' 23"» 8"? —- . l 75"”- * ‘ ... 7 MI’OZS rr ‘ _ Give 1 996 1%, —‘ '____1 L .1 n1 1 J l l u — n N —. _ Q 9 Q Q Q Q o "‘o o WBNNVHC) NUNBER CHAN\EL Results of two-dimensional Ge(Li)-Ge (Li) coincidence experiments for ”WSm. Figure 6. .85.; do 383.3270 consequawou 3.58-358 sausageuofi 2: Sum .83. 638 18333 .a «as»: mg 12.45 comm own. owe. : a m I e. 8.3 , cm». W 33 >._.-o.£.o _J _ _.== _ - = _= .5 _ r 33 >2. 32 m 2. ~20 {8-3.3 n. - o _+ if _ u. . 4 m a - a 1 E8 1,2738.»st F _. _J=l: : AF n A v n. A 2.5 >3 - mNOo ~43 — Hm mm 1 W ubg >3. ..vckL. “:6 >3- 0 N13 b. . .-.. -...m 1.0. 9'98! '13WVHQ 83d SJNWOO 51 Table III. Summary of y-Y Two-Dimensional Coincidencc Results for H1mSm Gate Energy y-Rays Enhanced (keV) (keV) Integral 196.6, 247.9, 538.5, 577.8, 607.5, 684.6, 750.3, 777.4, 805.9, 837.1, 875.0, 896.5, 911.3, 924.7, 983.3 196.6 431.8, 607 9, 777 4, 875.0, 911.3, 983.3, 1009.1, 1117.6, 1145.1, 1490.3, 1786.4 247.9 538.5 431.8 196.6, (1490.3)3 538.5 247.9, 896.5, 924.7 577.8 837.1 (weak) 607.9 196.6 676.7+684.6 (196.6)8, 750.3, 805.9 750.3 (196.6)8, 684.6, 858.5b 777.4 196.6, 1117.6, 1145.1 785.9 No coincidence in evidence 805.9 684.6 837.1 No coincidence in evidence 875.0 196.6, 911.3 911.3 196.6, 875.0, 983.3 924.7 (196.6)3, 538.5, (777.4)8 335:5 i382. 911.3 1009.1 196.6, 777 4 52 Table III. - Continued 1117.6 196.6, 777.4 1145.1 196.6, 777.4 1490.3 (196.6)8 1786.4 196.6 aThese intensities are less than expected for these transitions to he in coincidence. They are considered to arise from chance coincidences. b 141m This is not a Sm transition. 53 events, which could later be sorted off-line in gated slices. The integrated coincidence spectra from each detector are shown at the top of Fig.(5. Some representative gated spectra are shown in the remainder of Fig.63 and in Fig. 7; the remaining gated spectra can be found in (Epp 70). In these spectra gates were set on the X side (2.5% detector) and the displays were taken from the Y side (3.6% detector). A summary of the two-dimensional coin- cidence results is given in Table III. The pair (Sll-keV-Sll—keV-y) coincidence spectrum shown in Fig. 8 was obtained with total annihilation absorbers around the sources and the 2.5% Ge(Li) detector placed inside an 8x8-in. NaI(T£) split annulus (Aub 67). The single-channel analyzer associated with each half of the annulus had its window adjusted to accept only the Sll-keV region. A triple coincidence (21=100 nsec) was required before the fast coincidence unit would generate a gate signal and allow pulses from the Ge(Li) detector to be stored. Thus, only double—escape peaks and peaks due to transitions from levels fed by 8+ decay appear in the spectrum in Fig. 8. The yi peak is a measure of the chance contributions to the Spectrum. Finally, to complement the various coincidence spectra, an anticoincidence spectrum was taken, again employing the 2.5% Ge(Li) detector and the 8x8-in. NaI(T£) split annulus. 54 ...—mute;H you asuuooam moaovfloaaoo >I>oxtaamnaam .m muswwh 592:2 ..mzzqzo 88 88 com. 80. com _ a 1 _ ._ Doc -0 c - .0 00". '00. ‘0'! '0... -l!-. . .....- " - k .. .... __ “mm... _ 6 2 . _. a n r W .I . O 9&9! 6‘981. m ‘BNNVHO 83d SlNflOO .Euuoonu «0:030:3ng Emmott: .o 93”: mum—2:2 szzqro OSn OOOn OOnN OOON 000. 08. 8n 4 4. d. d H — . _ m $3.... I 4. ...» h m ,. .... 1 u .... u u 4 I . .. o n m m mmuim a. m h}? 5 m .... O . l I O .. mw '/ . n. m H M m . .. l , m m I. m I _ _ . LI _ . . @- ‘IBNNVHO 83d SlNflOO 56 Table IV. x-Rax Intensities for 1“1m8m Coincidence Experiments Relative Intensities 59er3y Anti- Integral 196.6-keV Delayed Integral \aeV)a Singlesa Coinc. Coinc. Gate Coinc. 196.6 100 77.1 60.4 --- 5.74 247.9 1.04 1.67 0.98 --- 1.67 431.8 54.6 2.32 22.3 53.0 2.32 538.5 11.3 11.3 8.81 --- 11.3 577.8 1.22 --- 1.30 --- --- 607.9 1.38 --- 1.79 1.90 --- 628.7 3.60 --- --- --- --- 684.6 10.6 8.32 7.30 --- 8.32 725.7 1.95 --- --- --- --- 750.3 2.14 1.55 1.70 --- 1.55 777.4 27.4 --- 27.4 27.4 --- 785.9 9.20 10.2 4.99 --- 10.2 805.9 4.78 4.50 3.45 --- 4.50 837.1 4.82 --- 3.99 --- --- 875.0 1.68 --- 1.64 1.97 --- 896.5 1.95 --- 1.33 --- --- 911.3 12.4 --- 12.6 12.3 --- 924.7 3.08 1.67 2.06 --- 1.67 952.1 1.20 --- 0.82 —_- --- 955.4 0.91 -—- --- --- --- 974.1 0.27 --- --- —-- --— 983.3 9.80 --- 8.03 7.96 --- 1009.1 3.93 --- 4.04 4.46 --- 1029.6 0.68 --- ——- --- --_ 1108.4 1.67 --- 1.64 --- --- 1117.6 4.37 --- 3.99 3.97 --- 1145.1 11.8 —-- 12.5 14.1 -- 1463.4 2.43 --- —-- —-— —- 3490.3 12.5 17.9 3.3. --- 1 « 57 Table IV. - Continued. Relative Intensities Energy * ~-_1...__“_ (keV) Anti Integral 196.6-keV Delayed Integral Singles Coinc. Coinc. Gate Coinc. 1786.4 14.6 --- 9.78 17.0 --~ 1898.0 0.51 --- --- --- --- 1979.6 0.54 —-— --- --- --- 2073.7 1.92 --- -—- --- --- aThe errors placed on these values are given in Table I. 58 This spectrum is shown in Fig. 5% The 1ulmSm source was placed at the center of the annulus tunnel, with the Ge(Li) detector in one end and a 3x3—in. NaI(T2) detector at the other end. The Ge(Li) detector was operated in anticoincidence with Y rays above 100 keV in the NaI(T£) detectors (2T=100 nsec). The true—to-chance ratio was 3100/1. This spectrum enhances those transitions that are not in prompt coincidence with the other y rays or with 8+ emission. It is particularly useful in placing transitions to the ground state or to a metastable state, providing those transitions are primarily EC fed and not 8+ fed. A summary of the relative intensities of lulmSm y rays in some of the more important coincidence spectra is given in Table IV. U.1.3.D. Delayed Coincidence §pectra The systematic behavior of states in this nuclear- region suggested the presence of a metastable nh state 1111P ll/2 m. In the earlier work on 139mNd de- in the daughter, cay (Bee 69b) a delayed coincidence spectrum was essential in elucidating the decay to the higher-lying states, so we felt compelled to perform similar experiments here. We have identified this state as the state at 628.6-kev in lule and we have determined its half—life to be 59 700220 ns. This half—life is long enough to obtain useful integral coincidence information using a resolving time of 100 ns. Three types of delayed coincidence spectra were ob- tained. The first two spectra, shown in Figs. 10 and 11 are delayed integral coincidence spectra, the third set of coincidence spectra pertain to the measurement of the life- time of the 628.6 keV state and are shown in Fig. 12 and 13. In the first of the delayed integral coincidence spectra, shown in Fig. 10, the gate signal is delayed 250 ns with respect to the linear signal. The result of this is to enhance those transitions that depopulate the state having an appreciable half-life. In the second, shown in Fig. 13, the linear signal was delayed 250 nsec with respect to the gate. Here the result is to enhance those transitions feeding the state having an appreciable half— life. The latter primarily are the transitions that de- populate what we propose as three-quasiparticle states, so this spectrum is crucial in identifying these high-lying, high-spin states. In order to measure the half-life of the 628.6 keV level we used a modified version of a fast-slow coincidence system, with a 10.4% Ge(Li) detector in the start leg of the system and one-half of the 8x8—in NaI(T£) split annulus in the stop leg. Any pulse above 30-keV in the Ge(Li) 60 gamer: mums—Dz 1522410 OOON 000. 000. How asuuoomm ooewvfioawoo Heywousa eumwuvommamn .OH unawwm - 11d I ' I I- I. .--.i'.’ -..--i00I I- .0. 0! g I ...--. -.. ' I 0 CI -.. ..-! Ole. II...‘. 0" ---O.!-!|! ! I! ' t.-- '00. "‘i.e'... i 0' ' OI- - II 0 0| 00"-.-00' .0!....0. - e. 00 0 0.0009. - '0. ....e‘. I. 000--....‘0000000'0 I... e ' 0'. .-- 0' C I. e 0 0.00.0... 'IIO"0 .- I e. on '00:- o -6 I. '0 00. 00 '0' .0 .00 e O.-eoooeoolae '0'”. a ' .0 O a a e. O S. .. O 0 § 0 N 1 o 6199 ~- .. - -. O. 'IBNNVHO 83d SanOO 61 OOON .353 How aauuoeem oucovaunaou Heuweunfi Hmnwwmuvuhmaen 00m. 1 1 mums—Dz 1522410 000. 1 .HH unawam own W1 _ 1 - -11- .111 .1_ ‘ . M .1 1 1; , 1 1 1 ‘1 O’IIQ u O. O. O. .0. 'IBNNVHC) 83d SanOO 62 detector started the TPHC (time-to-pulse-height-convertor) and any pulse arising in the NaI(T2) detector above the 511.0 keV annihilation peak stopped the TPHC. Two-parameter data were accumulated (time vs. Ge(Li) energy) using TOOTSIE (Bay 71). The data were stored on magnetic tape using the auxiliary program EVENT and recovered off-line. These experimental results are shown in Figs. 12 and 13. The top spectrum in Fig. 12 is the Ge(Li) integral coincidence spectrum, the center spectrum in Fig. 12 represents pulses generated by the TPHC, while the spectrum at the bottom of Fig. 12 is the calibration spectrum used to test the TPHC linearity. In the TPHC spectrum in Fig. 12 the pulse height is proportional to the time interval between the Ge(Li) pulses and the electronically delayed NaI(T£) pulses—— the prompt coincidence peak appears on the right in the TPHC spectra. The three spectra seen in Fig. 13 were obtained by gating on the peaks at 196.6-, 431.8-, and 628.6-keV, respectively, in the Ge(Li) integral coincidence spectrum and displaying COUNTS PER CHRNNEL 63 V led CEILI) INTEGRRL COINCIDENCE NITH TPHC 2 un4 «ms 433 «ms sac saw 628.7 ems 7859Z4 and NDCXD .I‘ H H O O 0 HI I l L T_ T T V 10“ TPHC sthTED av RNchEILI) PULSE '25 10“.. 1031 2 1o #- T 1 It) ‘ 19.21876 HHZ CRLIBRFTION FOR 98191" TPHC 10° 17131111 12 . 1000 i CHRNNEL NUHBER 2000 Spectre reeultin. from two-Mimi coincidence (energy ve. tine) ”aunt of the lifetime of deleyed etete at 628.6- kev in “uh. COUNTS PER CHRNNEL 64 TPHC STRRTED BY 198.6—KEV TRRNSITION 55 1° 51 _L TPHC STRRTED BY'H31.8-KEV TRRNSITION D 1 1 1° TPHC STRRTED BY'628.7-KEV TRRNSITION I 2 10 4 I00 200 CHRNNEL NUMBER Figure 13. TPHC spectra obtained from half-life measurement of the delayed state in 1“Pm. 65 the TPHC coincidence pulses. From the decay scheme it can be observed that the 628.6-keV transition represents decay from the isomeric level directly to ground while the “31.8- and 196.6-keV transitions are in cascade. The “31.8 keV transition is from the 11/2- isomeric level to the first excited state. It should be noted that these three spectra are not plotted to the same scale. However, the slope of the three time spectra are essentially the same. The resolution of the system is 10.0 ns FWHM, and the timing calibration was obtained using RF pulses from the cyclotron. Calculation of the half-life followed a least- squares fit to the logarithms of the data in Fig. 13 (after subtraction for chance coincidences). The value of the half-life was determined to be 700i20 ns. These results indicate the E3 transition from the 11/2- isomeric level to ground is enhanced over the single particle estimate by a factor of 1.6. Correspondingly, the 431.8 keV M2 transition is retarded by a factor of 58. A similar 11/2‘ state identified in luSEu at 625.3-keV has been observed to have a lifetime of 800 ns. It should be noted that the 196.6 keV transition from the 7/2+, first excited state to the 5/2+ ground state involves (vg7/2)+(vd5/2), and hence is an l-forbidden Ml. We were unable to determine the lifetime of the 196.6 keV state but we are able to place a limit of, Tl/2<2 ns, for its half—life from these data. 66 1L1.3.E. Energy Separation of the Two l“Sm Isomers Because the 1“18m isomers so admirably meet the re- quirements of essentially separate decays, it is difficult to determine their energy separation with much precision. Based on a least-squares fit of the previously measured Mu transitions in the other N=79 nuclei, we arrive at an energy of 171.6 keV between the two. An Mu transition of this energy, compared with the EC/B+ half-life, would be expected to be quite weak. The vicinity of 172 keV is a particularly unfortunate region for the observation of a photon of low intensity, since the backscatter peak from annihilation quanta is 170 keV. We have looked for the isomeric transitions both in y-ray and electron spectra (using a Si(Li) detector) without success. An MN transition of this energy should be somewhat easier to see in the electron spectra than in the y-ray spectra, as the conversion coefficient should be large (:u) (Hag 68); however, there is no evidence of any transition peaks above the 8+ continuum. Based on the statistics of our y-ray spectra, we have placed an upper limit of 0.2% of the total 1ulmSm decay for this transition. The results of the different investigating groups who have considered the relations be- lulmSm tween and 1“188m are summarized in Table V. Table V. Published BStlmated Relations Between 67 lulmSm and 1"198111 ‘ E 7. .111. Authors t1/2(1‘*1’"sm) t1/2(”’lgSm) (tor MA) in de:ay Arl't et al. 21.5 min <10 min 215 keV -—- (Ar167) Bleyl, Mfinzel, 23.5 min :2 min :200 keV .12 and Pfenning (Bley67) Arl't et al. 22.5 min 9.5'0.S min --- —-- (Arl70) This work 22.1:0.3 min 11.3'0.3 min ~l7l.6 keV 0.2% 68 u.1.fl. 1ulmSm Decay Scheme We have constructed a decay scheme for lulmSm from a combination of the foregoing prompt and delayed coin- cidence spectra, aided only incidentally by energy sums and intensities. This decay scheme is presented in Fig. an along with the decay scheme (Bee 69b) of 139mNd, Fig- lub, With which it should be compared. Of the N? y rays that we con- cluded belonged to 1MlmSm decay, only six very weak ones could not be placed. All energies are given in keV, and since the presence of 1ulgSm in our sources prevented a precise measurement of the lulmSm B+ spectra, the QEC of 35015 keV is a calculated value (Myr 65). The (total) y—transition intensities are given in percent of the disintegrations of lulmSm. The B feedings are also given in percent of 1ulmSm disintegrations, and they include both the 3+ and EC decay——the surprisingly small 8+ feedings will be discussed below. The energy assigned to each level is a weighted average based on our confidence in the energy precision of the transitions out of that level. In Table VI we compare our level scheme with those obtained by Hesse (Hes 69) and Arl't et_al. (Arl 67) and (Arl 70b). The discussion of the construction of this decay scheme will be broken into two parts: discussion of 1) those levels ‘thse mode of decay bypasses the (metastable) 628.6-keV level anci 2) those levels that decay primarily through this metastable level. 69 Table VI. States in 1“Pm Populated by 11+“"8111 Decay w- “-..—... -.‘- —..~-—.-.. ...-....-- - Arl't at al-. This Work Hesse (Hes69) (Ar167,Ar170)a State Energy Jfl State Energy 6’7T State Energy (keV) (keV) (keV) u 5/2+ 0 (5/2+) 0 (571*1 196.6 7/2+ 196.5 (5/2+) 198 47/"» 628.6 11/2' 628.3 (11/2') 629.9 (11/2’) 804.5 11/2+,9/2+ -—— --- -—- --— (837.1) 9/2+ --— --- 850 (1572') 974.0 9/2+ 973.6 --- 979 (9/2+) --- --- 1033.2 --- --- -—- 1108.1 7/2*,9/2+, 1107.6 —-- --- —-~ (5/2’) 1167.2 13/2’(+).11/2’(+), 1166.3 --- --— —-» 9/2‘(+) 1313.2 13/2'(+),11/2’(+), 1312.5 ——- »—- --. 9/2'(+) 1414.8 11/2',9/2' -—- -—— --- --- --- --- 1759.4 --- -—- --_ (1834.0) 11/2',9/2‘ ——- -—— —_~ --- 1983.1 9/2‘ 1982.2 (9/2’) 1982.6 h 2063.5 11/2‘,9/2' --- —-- --- --- 2091.6 11/2',9/2" 2090.6 --- 2091.6 h 2119.0 11/2‘,9/2" 2118.5 —-- 2119.0 (9/2’.11/2‘) 2702.4 13/2',11/2',9/2' -—- --— —-- aHes 69 did not include the 1982.6- or 2091.6—keV states. b . . Arl 70 includes these plus the revised energy for the 2119.0-keV state and also recognizes that these three States are low—spin members of the rhrvv quasiparticle multiplet. 70 l4! Pm GI 80 Flam-e 14.. Decay ech-ee of “Wu. 2.3‘ 6.3 2|“ 5.7 4.3% 6.! ”$5.9 LOSS) m 05 2.7! 7.0 3“ 0.9 ”I M 2.3! 7.2 CI! 0.6 M‘ 6.7 7“ I 7.0 71 5.5 h IZJ’loIT 30m -3 (DA%¢,5.ID $306.3 1.6-16’po 7.3 (exams?) m 138 59 P' so Figure 14b. Decay scheme of 139mm. 72 u.l.U.A. The 196.6-keV and Related Levels The 196.6—keV transition has been placed as proceeding from the first excited state of lule on the basis of its being the most intense transition in the 1ulmSm spectrum. A first excited state at 196.6 keV is completely consistent with the systematics of this region (cf. Section lLl.6.). Placement of the 628.6—keV level itself is also straightforward. It decays to ground via the 628.7-keV transition and to the 196.6-keV level via the 431.8—keV transition, as indicated in the prompt coincidence spectra. Expected to be a metastable state on the basis of systematics, this was proven by the delayed coincidence spectra. Placement of most other levels is not so straight- forward and depends on considerable tortuous logic involving the prompt and delayed coincidence spectra. For example, of the eleven y rays in coincidence with the 196.6-keV y, only five feed the 196.6-keV level directly. We present only a few crucial points here. The 800.5-keV level is placed on the basis of a 196.6- and 607.9-keV coincidence and the fact that no y ray as intense as the 607.9-keV y was found in the 607.9-keV gated spectrum other than the 196.6—keV y. Purely on the basis of sums, y rays at 1029.6 and 1898.0 keV are placed as feeding this level. These are weak transitions that would not show 73 up in the 607.9-keV gated spectrum. Note that the 805.9—keV Y, which feeds the 628.7-keV level, has no relationship to the 80u.5-keV level. Very similar logic, aided by the presence of a 97u.l-keV ground-state transition, places the 97M.0-keV level. The two levels at 1108.1 and 1983.1 keV are also placed on the basis of the prompt coincidence data, and the placements are corroborated by multiple energy sums and are consistent with the intensities of the interconnecting Y transitions. The level placed at 837.1 keV is somewhat of a puzzle. The 837.1—keV Y is enhanced in the integral coincidence spectrum and attenuated in the anticoincidence spectrum. There is weak evidence for coincidence with the 196.6—keV from inspection of the 837.1-keV gated spectrum. However, there is no enhancement of the 837.1-keV peak in the 196.6- keV gated spectrum. Possibly, Compton events from other peaks (e.g., 875.0—keV) could account for the 196.6-keV peak appearing in the 837.1-keV gate spectrum. With this some- what conflicting evidence, the 837.1-keV transition has been placed as originating from a level of the same energy. This is substantiated to some extent by the placement of the 577.8-keV Y, which we shall see in the next section fits nicely between the 837.1—keV level and a level at 1Ulu.8 keV that will be placed by coincidence data. 74 u.l.N.B. The 628.6—keV Level and Spin-Related Levels The 628.6-keV level is almost certainly a "hll/2 state. This is demonstrated both by the systematics of the region and by the fact that it has a half—life long compared with the typical resolving times (213100 nsec) of the prompt coin- cident spectra. That this is in fact the high-spin state is demonstrated by the results of the "delayed gate" coincidence spectrum (Fig. 10). In this spectrum only the peaks at 196.6 and “31.9 keV are present, and that these two transitions are in coincidence is substantiated by the coincidence spectra gated on each one. And further support for the assignment of an isomeric state at 628.6 keV comes from the enhancement of the relatively weak 628.7—keV Y in the anticoincidence spectrum. That lL'ImSm is also an 11/2’ state means that its primary B+/EC decay will populate relatively high—spin lule and these states should (9/2, 11/2, 13/2) states in decay by cascades that go through the 628.6-keV state. The "delayed spectrum" integral coincidence spectrum (Fig. 11) almost immediately places six of the high-lying, lulmSm. [This assumes high-spin states fed directly by that the six Y rays enhanced in this spectrum feed the 528.6—keV level directly. The absence of sum transitions 01‘ observed coincidences and the fact that the resulting Ckacay scheme is the simplest consistent with the experimental datza lead us to accept this assumption of direct feeding.) 75 These six levels lie at 1167.2, 1313.2, lulu.8, 2091.6, 2119.0, and 2702.“ keV. In addition to these six levels, we place a level at 2063.5 keV on the basis of sums——the weak 1&3u.9-kev y connects this level with the 628.6-keV level. The 2063.5-keV level is confirmed by the coincidence relations between the 750.3- and 68h.6— and between the 896.5— and 538.5-keV y's. The remaining level, at l83u.0 keV, is placed only on the basis of energy sums and should thus be considered only tentative. Numerous corroborations of the placements of most of the states can be found in the coincidence summary of Table Ill and the coincidence spectra themselves, Figs. 6 and 7. 8.1.5. Discussion U.1.5.A. The l“18m Isomers Ths odd-mass N=79 isotones actually consist of three- hole states, but to a reasonable first approximation their low—lying states can be treated as a single-hole state much in the manner of the N=81 isotones. Among the latter there are now seven known isomeric pairs having (\1d3/2)—l ground States and (vhll/2)-l metastable states connected by MD iso- meric transitions (Epp 700). In Fig. 15 we show the energy )‘1 '1 Spacings between the (Vd and (vh states and 3/2 11/2) also show the squares of the radial matrix elements, IMI2, 7’ ENERGY (keV) IMI2 6 05 O O .b 8 N 8 ("I A . '3906 m'fid ”’3'“ Mng “37E363 "' C '35Xe _ e 133 Te 1 x x x u . .. x mic 3580 13703 '39Nd HIE. '3'Te m + ...; ting." " x ‘ l33xe 135 ,. l " __ 03376 30 '37Ce M 135% , 137 Ba "59.06 141;“ 9 145' I439“ Gd 2= 52 Figure 15. 76 54 56 58 60 62 64 PROTON NUMBER Upper: .Mfi transition probabilities for the Ni79 and N-81 odd-mass isotones. The 1“1m8m.point is calculated (predicted value. Lower: Values of the squared radial matrix elements for the single-neutron isomeric transitions for the same nuclei. 77 for the MU transitions as calculated using Moszkowski's approximations for single-neutron transitions (Mos 65). Both the energies and matrix elements follow a gentle- enough trend that one has reasonable confidence in extrap- olated values, providing the extrapolation is not carried too far. (The fact that the values of |M|2 are not constant is discussed in some detail in (Bee 69b), (Epp 70b), and (Jan 69).) The IMI2 values for the five known isomeric transitions in the N=79 isotones are somewhat larger than for those in the N=81 isotones. This would indicate that they are less good "single-particle" transitions than the N=81 transitions. However, they are all indubitably Mu transitions and can be described with considerable accuracy as (vd3/2)-2(vhll/2)_l+ (vd3/2)-3. As mentioned in Section u.l.3.E., we were unable to detect the presence of an isomeric transition between lulmSm and 1ulgSm, but its predicted intensity should be so low that one would not expect to observe it. More to the point, the decay properties, Section “.1.U., of lHgSm make it reasonably certain that it has a 3/2+ ground state. (The vs1/2 state crosses the vd state in this vicinity, 3/2 1M5g + but a Gd ground state of 1/2 has quite different decay 1 properties (Epp 71) from 141gSm and, in fact, populates three quasiparticle states somewhat analogous to those populated lulm IUIm by 139mNd and Sm.) Also, the analogy of Sm decay 78 to 139mNd, where the MA transition was easily measured, gives us reasonable confidence that the primary configuration of lulmSm is indeed [(ng7/2)8(nd )“]0[(0a )-2(vh )'1 5/2 11/2 311/2- and that of 1ulgSm is th7/2)8(nd 4 3/2 3 5/2) ]o[(vd3/2) 33/2+° U.l.5.B. Single Particle States in lUle For convenience of discussion the states in lAle populated by 1ulmSm decay can be divided into three classes: 1) single-particle states or states exhibiting appreciable single-particle character, 2) three—particle states, viz., the high-lying multiplet receiving most of the direct B+/EC population, and 3) the remaining states, most of which appear to contain core-coupled vibrational components. As relatively little has been done in the way of calculations on states in rare-earth nuclei on the neutron-deficient side of N=82, one has to rely rather heavily on parallels with 139mNd de- cay and on general systematics of states in this region. As an aid for following the systematics and trends, in Figs. 16 and 17, respectively, we plot the known states in neutron- deficient odd-mass Pm isotopes and in the odd-mass N=80 isotones. Because few reactions studies have been made on nuclei in this region, the number of states reported is very much a function of QEC' Between the closed shells at 50 and 82 the available single—particle orbits are g7/2 and (15/2 lying relatively 79 3000 l l I 1 LOW - LYI N G STATES 2800 - _ - '4 "/2 ,9/2 30? IN Pm ISOTOPES 2600 - “ 2400 - “ 2200 " 'l/2.,9/2- 30p ll/2-,9/2- 3GP A II/Z',9/2'3QP ._ 2000 7 9/2' 3013 1800 -11ié‘.'9'/'2-(301=1_ " E, I600 - ‘5/2+) " v ’ ' + >_ 1400 -"’2—'9-|:2-g 3°? 342— - o 33 1200 ~8m9/2f1/2 - 02.1 7/2, 9/2 .(5/2) IOOO P 242; 5325‘ ‘ amt... /'"’2 \\ 30° ”unis/27’ 1‘ 25/2‘1 " 114.2; 7’ \ 600 F- \ 53+ -' \SL’zl‘U/i) ‘7C)<> 7' ‘~~‘.EE£EL"' -q ”SILL 200 __ 7/2‘. 'a‘ \\\ A \ (m m‘) SVGE' O 5721 572*. (5121‘— L ;..- 1.7/21 |4| I43 I45 I47 A Figure 16. The positions of known states in the odd mass Pm isotopes. 80 LOW LEVELS IN N=80 ISOTONES 2800 .— _ — WEB/2" 2600 '- 2400 1- " 55g I2“. 72" :2". 2200 9 ll )—-(;’2__:_"/n ' ( '5’?) (9/2'11/2‘ 1 2000 1. “agile/2 19/2 11/21 1972') 800 19/2_,11_/2’1 (11(_2_,_9/21 | F-Qggsm IGOO __ 113/31 3'“ 1912 __1_.1/2‘1 ‘E? (112+,3m All (31372 1 (112’ *,5/2*)_.: - " I400 --55/2* 19122717313721 “VI-MI % —“‘”’2"3’2" 112' 15:32.2;- (132.122.9121 x WEI-5’2.) ‘ M ' ... (SET/21 ; '200 ""' .1431 1137211759721 3 _ 1__1/2- (77:12+ 2'1 ”may?“ til 'OOO _ — 3’2 5’2 ‘ {'12 +mfi lla- m 2+) + 242: 2’1 Age-:35, .....(3/ . 92’ ) 800 157271 ("£1 (sag—772.1,: "’2 ‘ 97211-— 11172. 972*) _ 117211 \112'5721 (3’2 5’2) 600 *— _" MI 1172 «mg/3.7m — 157255721 “/3122” (34211123 400 t- 512* “‘~-L__5/22 (712’) 200 7123’”,— O '- 142-1- --—?-{-2-’— “fix—1:: -323“ 4572*) 53 55 57 59 5. ' 7 £22 141 Figure 17° The Positions of known states in ”-80 odd-mass isotones. 81 close together and then, after a gap of 2500—1000 keV, 31/2’ and d3/2 also relatively close together hll/2’ (W11 71). Systematics as exhibited in Figs.lfi$ andlY and extrapolations of the calculations of Kisslinger and Sorensen (Kis 60) suggest 5/2+ and 7/2+ for the two lowest states in l“1P m. As previously pointed out (Bee 69a) six- teen nuclei in this region have well-characterized ground states and first excited states with 5/2+ and 7/2+ or 7/2+ and 5/2+ assignments. In the Pm isotopes there is a change of ground state spin between 1u7Pm and luSPm, with l“7Pm and the heavier isotopes having 7/2+ ground states and l“SPm and lu3Pm having 5/2+ ground states. Similarly, in the N=80 isotones there is a reversal of these spins between 137La and 139Pr. In addition to these general arguments, we have three specific arguments for assigning the ground state 5/2+ and the 196.6-keV first excited state 7/2+. First, the decay 141 properties of Pm itself (Section 4.3.) are consistent with a ground state assignment of 5/2+ and not 7/2+——the decay of lAle strongly populates the ground state of 1M1Nd, which has been quite unambiguously characterized as a (vd )"1 state (Bee 68)(Jol 7l)(Fos 7l)(Cha 71). 3/2 Second, since 141m Sm is undoubtedly an 11/2- state, a 8 transition between it and a 5/2+ ground state of 1”le would have to be at least third forbidden and hence negligibly 82 small. This is consistent with our findings. On the other hand, 8 decay to a 7/2+ first excited state would be first forbidden unique. Although only a small branch would be expected, from the ybray intensity balance (including corrections for internal conversion) for this state it appears that as much as 7.6% of the decay might populate directly, resulting in a log ft 27.0. This compares with 33% decay (Bee 69b) from 139Nd to the first excited state in 139Pr with a log ft of 37.6. Actually, it would seem reasonable to attribute much of this apparent feeding to errors in the y-ray intensities and/or to undetected transitions into the state. However, the pair coincidence spectrum (Fig. 8), after correction for chance events and feeding via higher energy states, shows that there is in fact some 8+ decay to the 196.6-keV state. (In both 141m 139m Sm and Nd decays this first forbidden unique transition would involve ng7/2+vh11/2, which would be one of the more favorable cases for such a transitions, but we would expect the log ft to be greater than 7.0.) Also, in our measurement (Epp 70a) of the half-life of the 196.6—keV y we found no evidence for an 11.3-min component, which means that lulgSm does not populate the 196.6-keV state either directly or indirectly. All of the B feedings are thus consistent with a 7/2+ first excited state. 83 Third, the respective 5/2+ and 7/2+ assignments are consistent with the branching ratios from the 628.6-keV state, which will be discussed below. We conclude that the lAle is a 5/2+ state and the primary com- ground state of ponent of its wave function is [(“87/2)8(nd5/2)3(vd3/2)—2]5/2+, taken with reference to the closed shells at Z=50 and N=82. Similarly, the primary component of the 7/2... 196.6-keV state is eXpected to be [(n87/2)7x(nd5/2)u(0d3/2)-2]7/2+. That the 628.6-keV state is the "hll/Z state {complete configuration of primary component [(ng7/2)8(nd5/2)2 (nh11/2)(0d3/2)-2]11/2-} has already been discussed in Section 11.1.3.0. The log ft of 6.7 for the 3+/EC decay from lulmSm is on the high side for an allowed transition, but this is entirely reasonable when one examines the particle rearrangements necessary. These are included in the stylized diagram presented in Fig. 18. To first approximation the transition would appear to involve the conversion of a d proton into an h neutron, either 3/2 11/2 directly or perhaps through an intermediate state, plus the simultaneous promotion of the 05/2 proton remaining from the pair up into the h orbit. Thus the transition un- 11/2 doubtedly goes by smaller components in the wave functions. That the log ft of 6.7 is smaller than that for analogous transition from 139mNd (there it is 7.0) can be attributed to the "hll/2 orbit's dropping in energy with increasing Z, 84 thereby providing larger occupations of hll/2 proton pairs. Later we shall show that this is consistent not only with 1“18m and lulpm behavior, but also with the general trends of single-particle states throughout this region. Since no conversion-electron data are available for transitions following the decay of 1“18m, we show no multipolarity assignments on the decay scheme. However, the 431.8- and 628.7-keV transitions are expected to be M2 and E3, respectively. We measured the half-life of the 628.6-keV state to be 700 ns. This leads to a partial half-life for the E3 transition of 10.6 Us. Assuming little or no mixing in the transition, the Moskowski single-particle estimate for its half-life (neglecting the statistical factor) is 17 us. The M2 retardation (a factor of 58) determined here is quite reasonable, since M2's are customarily retarded. However, the E3 is enhanced over the single—particle estimate, and this enhancement appears to be at least by a factor of 1.6. Although most E3's are retarded, this is now the fifth known E3 in this particular nuclear region that is enhanced. The others are in 139Pr’ 137La, lu7Eu, and lugEu. These all involve similar 85 hll/2 states. The simplest explanation is for an octupole core-coupled component of the 5/2+ ground state to be admixed into the 11/2- states. Unfortunately, the positions of octupole states in neighboring even-even nuclei are not known, and a more quantitative persual of the problem must await such information. We do not see any states populated by lulmSm decay that we can identify with the nd3/2 or "51/2 single~particle orbits, nor could we expect to do so. Even from the decay of l4lgSm, Section 4.1.7.,it is difficult to associate these orbits 141P with particular states in m. Both appear to be fractionated and have their strength spread out over many states. . 1111, 4.115.01. A Three-Quaulparticle Multiplet in Im + 141m Some 65% of the 8 /EC decay from Sm populates the multiplet of six states at 1414.8, 1983.1, 2063.5, 2091.6, 2119.0, and 2702.4 keV. (There are seven states if we in— clude the one at 1834.0 keV, which receives about 2% of the population; however, this state is only tentatively placed and thus omitted from the present discussion.) The log ft values range from 5.7 to 6.6 implying that these are all allowed transitions; in fact, they are faster transitions than the 11/2-+1l/2- transition populating the 628.6-keV state. 86 The explanation for this superficially peculiar be- havior of lulmSm is actually quite straightforward and has been anticipated in the introduction Section 4.1.1. It is given in stylized form in Fig. 18. This multiplet of high-spin, high-lying states is a three—quasiparticle multiplet that the rather unique structure of l“lmSm forces it to populate. ll‘lmSm, three neutron holes below the N=82 closed shell, undoubtedly has as its primary structure, [("87/2)8(Wd5/2)u(vd3/2)-2(Vhll/2)-l]11/2-, as depicted in Fig. 18 and discussed in Section 3.1.5.A. Because Of its large QEC it can easily populate states in lule above the pairing gap(s). Now, the most probable form of 8 transition, given by the above structure, is "dB/2+vd3/2, resulting in the structure, [(ng7/2)8(nd5/2)3(vd3/2)'1(vh11/2)-1]J-, or more simply, [(nd5/2)(vd3/2)-l(vhll/2)-1]J_. Here J‘ can be anything from 3/2- to l9/2—, but note that allowed transition are limited to 9/2’, 11/2', or 13/2-. As can be seen from 139m Fig, 14, the analogy with Nd's populating six similar quasiparticle states is striking. Some examples of narrowing down the spin assignments follow. The 1983.1-keV state is the only one that populates anything lower-lying than the 11/2' 628.6—keV state, and, in fact its 1786.4-keV transition to the 7/2+ 196.6-keV state is its strongest mode of deexcitation. The only J1r assign— ment consistent with this is 9/2-. We shall see later .moumum and: was 5m:fl demands mdowuamnmuu “33.395 caom mo muoaumuaomounou Hovoalaaonm once—Rm .3 cud—mam om m Ska thmNm .3 2: II... III“. I... 11. A 111111111 In). A team A1 111111111 "a.“ lusting ¥N>.£ 1 . what .. 3 a $1 22... om .0 2. «m >9. mwNm E 00 .m E 0.: 622... don 8a.: we: flusm A at...” A” team 51-11-11.118.” AH UHWA 1111111111 Mn“ is: . 4:7:“=£ 48::Q:£ 88 (after assigning the 974.0- and 1108.1-keV states) that all three y transitions depopulating this state are most likely El, which could be in agreement with the branching ratios. However, we shall not dwell too much on y-ray branching ratios when speaking to the transitions involving the three- quasiparticle states. By their very nature these states are quite different in structure from the lower-lying states; thus, most 7 transitions will proceed primarily via small admixtures in the wave functions, and consequently the branching ratios can be very misleading. For a rather striking demonstration of this the reader is referred to 139Pr (Bee 690), where many of the multipolarities could be assigned on the basis of conversion coefficients. There it was almost the rule rather than the exception for Y transitions between and out of the three-quasiparticle states to differ from single—particle predictions by orders of magnitude. Assignments for four of the remaining five states, at 1414.8, 2063.5, 2091.6, and 2119.0 keV, can be narrowed down tO 9/2- or 11/2-. All except the 2063.5-keV state decay to the1628.6-keV state, indicating some similarities in their WEi‘Ve functions with it, and all four of them also decay tC’ at least one other state that is in the next section asstigned 9/2+ as its highest possible spin. Assignments or 3l3/2- wouldfknmme these branches to be M2 transitions, not likkely to compete even with retarded Ml's to the 11/2- states. 89 After our finding the three-quasiparticle multiplet in 139Pr, it seemed worthwhile to perform a shell—model cal- culation for the states in the multiplet, using as simple and truncated a basis set as possible. This was carried out by Muthukrishnan and Kromminga (Muth 71) who calculated negative-parity states in 139Pr using only the two con- figurations, [(ads/2)(vd3/2)-l(vhll/2)-l] (the basic three- quasiparticle configuration), and [(nhll/2)(vd3/2_2)] (the basic configuration of the 821.9-keV metastable state in 139Pr). Using an exchange mixture postulated by True, they obtained excellent results for energy predictions agreeing with the experimental positions. There was still some problem, however, with transition probabilities, and it appears that configurations of the sort, [(nd5/2)2(ng7/2)—1(vd3/2)-l(vh11/2)-l], will have to be added to the basis set. This can be seen empirically by the gradations in the properties of the states of the three- 139Pr and lulpm, e.g., the quasiparticle multiplets both in 7 transitions directly to the 7/2+ first-excited states. The main point is that calculations that intentionally used a truncated basis set give approximately correct answers for the three-particle states in l39Pr. 90 4.1.5.D. The Remaining States in lule The structure of the remaining six states, at 804.5, (837.1), 974.0, 1108.1, 1167.2, and 1313.2 keV, are not so straightforward and appear to lie somewhere between the single-particle and the three-particle states. They can perhaps be described best as weakly core-coupled states. Their wave functions would thus contain many components, which partly explains why the three-particle states decay down through many of them. It would also be consistent with the y—ray branching ratios depopulating them and with the fairly large log ft values for the B+/EC transitions feeding them. Although not completely conclusive, there is evidence that the 2+ one-phonon quadrupole vibrational state lies at 2770 keV in both of the adjacent even-even nuclides, lqum and ll‘ONd (Mal 66) and (Rem 63), quite in line with those in the lighter N=80 even—even isotones. Thus, the energies of the six 141 Pm states are in the right range for core-coupled states, the lower ones probably involving the (jS/Z or g.”2 single-particle states and the upper ones the 1311/2 state. The 804.5-keV state (log ft =8.6) appears to be fed by a first-forbidden transition, implying positive-parity assignments from 9/2 to 13/2. (A first-forbidden unique transition would also allow 7/2+ and 15/2+, but we hesitate 91 to include these because all the log ft's are somewhat high, and it is difficult, if not impossible, to concoct a structure for the state that would allow it to be populated by a relatively fast first—forbidden unique transition.) The 13/2+ possibility can be eliminated on the basis of the 607.9-keV y to the 196.6-keV state, for this would force that transition to be an M3. However, neither of the other possibilities can be eliminated. With the 11/2+ assignment the 607.9-keV transition would be a pure E2 and with the 9/2+ assignment it would be a mixed Ml/E2. Either way the E2 component should be collectively enhanced. In fact, an intelligent guess at the nature of the 804.5—keV state would be that it consists Specifically of the g7/2 single-particle state coupled to a 2+ quadrupole core vibration-—hence the depopulation solely to the 196.6-keV state. 11/2+ or 9/2+ is also consistent with the feedings to the 804.5-keV state. We have been rather cautious about using Y-ray branching ratios to predict JTr assignments except when the structures of the states could be corroborated in other ways, viz., the "hll/2 state at 628.6—keV. It is instructive here to give an example of how such branching ratios could well be misleading. Of the three possible y transitions depopulating the 804.5-keV state (ignoring the presence of some low—spin states, which lie below it-—known from the decay (Section 4.1.7.) of lulgSm), one is present. The missing 804.5— keV transition does not tell us much, and it could be 92 consistent with single-particle estimates. However, the missing 175.9-keV transition to the 628.6-keV state must be an El. And single-particle estimates (Mos 65) for this El, a 607.9-keV M1, and a 607.9-keV E2 yield respective 1”, 1.1x10‘13, and 1.5x10"10 sec. half-lives of 4.6x10- Obviously the E1 is severely retarded over such estimates. With our inferred structures for the 804.5- and 628.6-keV states this retardation is quite understandable, even ex- pected. However, wrong conclusions could easily have come out of blindly applying these "external" selection rules. The 837.1-keV state, if correctly placed (Section 4.1.4.A) must be a 9/2... state, probably a coupling of a d single- 5/2 particle state with the 2+ core. The log ft implies a first-forbidden transition, and the Y transition to the ground state eliminates the 13/2+ and 11/2+ possibilities. ' Similar logic, involving feedings both into and out of the 974.0-keV state allows us to assign it 9/2+. Within the limits of our y-ray intensities there is no 8 feeding to the 1108.1-keV state. The y transitions into it from above, most specifically the 875.0-keV y from the 9/2‘ 1983.1-keV state, limit its J" to 5/2‘, 7/21, 9/21, 11/2i, 13/2' (assuming El, M1, or E2 multipolarities). Its depopulating y rays, especially the ground-state transition, further limit J" to 5/2', 7/2’, or 9/2+. The fact that this State is not populated by l“188m decay (Section 4.1.7.) is a weak argument against the 5/2- assignment, but we cannot Justify eliminating it completely. Also, the internal 93 structure of this state is probably the least certain in the decay scheme. The states at 1167.2 and 1313.2 keV exhibit similar properties. The log ft‘s of 6.9 and 7.0 could mean either allowed or first-forbidden (non-unique) decay. The implied J: assignments would be 13/2i, ll/2i, or 9/2i. The y-ray feedings into them from the three-quasiparticle states do not allow any narrowing down of these values. And each state decays by a single y ray to the 628.5-keV state, again allowing no reduction in the possibilities. It is tempting to think of these states as couplings of the hll/Z single-particle state to a 2+ core. They lie at approximately the expected energies and such a configuration would explain why even with a 9/2- assignment there would be no Y branching to the 196.6-keV or ground state. Also, with such a structure, there should be considerable con- figuration mixing with the three-particle states (cf., e.g., (Muth 71)), which would explain why the log ft's are only slightly higher than the log ft for B+/EC decay to the 628.6-keV state itself. Thus, we prefer the negative parity options. 94 4.1.5.E. EC/s+ Ratios and Corrected log ft Values Straightforward EC/B+ ratios as calculated by the methods of Zweifel (Zwe 57) are not in good agreement with the ratios obtained from the pair coincidence spectrum (Fig.8 ). This spectrum shows that the 196.6-, 403.9-, and 438.2-keV transitions (the latter two resulting from lulgSm decay) are the only ones to receive significant B+ decay. Now, as the calculated ratios are for "simple" allowed transitions, it is expected that the B+ components will be much smaller for the first—forbidden transitions. However, it is now be- coming apparent that hindrances even to an allowed transition usually reduce the 8+ component much more rapidly than the EC component (Epp 71). Thus, the rearrangement depicted in Fig. 18 for the decay to the uh 628.6-keV state would 11/2 be expected to result in the EC/B+ ratio being larger than the calculated value. (Note, however, that because of the half-life of this state the pair coincidence spectrum would pick up only a small component of its 8+ feeding.) The log ft values presented on the decay scheme itself are so-called "normal" values, obtained using the calculated EC/B+ ratios. We used these for ease of later improvements and corrections and also because most people are more familiar with decay scheme logic based on such values. In Table VII we compare these with the log ft values we obtained assuming only EC Table-VII. 95 Comparison of log ft's Assuming Either the Theoretical EC/B+ Ratios of all EC Decay Level energy Normal EC-decay only (keV) 196.6 27.0 6-4 628.6 6.7 6.1 804.5 8.6 3-2 837.1 7.2 6-3 974.0 6.8 6.4 1167.2 6.9 6.5 1313.2 7.0 6.6 1414.8 6.6 5-2 1834.0 6.9 6-7 1983.1 5.9 5.7 2063.5 6.4 6.2 2091.6 5.8 5.6 2119.0 5.7 5.5 2702.4 6.5 6.3 96 decay. The latter are expected to be more realistic. Un- fortunately, very little has been done on the quantitative theory of EC/8+ ratios. 97 3;}.6. Discussion and Conclusions The decay schemes of 1ulmSm and 139m Nd, presented in Fig. 14, are remarkably similar. However, there are some differences, and these differences allow us to glean some important information about the behavior of shell—model orbits in this region of the nuclidic chart below N=82. The most striking difference between the two is that whereas in l39Pr there are many low-energy apparently en- hanced Y transitions between members of the three-quasiparticle multiplet, in l4le such transitions are fewer and weaker. Also, the transitions out of the multiplet to lower- lying states appear to be much more retarded in 139Pr than in 141P m. Actually, this is more or less what one would expect from the known behavior of shell-model orbits in this region, The retardation of transitions down to the 628.6-keV state, for example, is represented in Fig. 18. With only g7/2 and (15/2 proton states occupied, these transitions would be formally two-particle transitions. A d5/2 or g7/2 proton would have to be converted into an hll/2 neutron, either directly or perhaps through an intermediate state, and the remaining member of the pair would have to be promoted up to the hll/2 orbit. Thus the transitions must proceed instead via small admixtures in the wave functions. Such happens in 139Pr. It is 98 known, however, both from reactions studies (W11 71) and from the B+/EC decay of some N=81 nuclei that the "hll/2 orbit drops rapidly in energy with increasing Z in this region. For example, the log ft values for the 11/2-+ 11/2— transitions in 1ulmNd->1u1Pr, 1l‘BmSm-vllGPm, and l45mGd+145Eu are >7.0, 6.7, and 6.2, respectively (Fir 71) (Fel 70)(Epp 70b) and the speed of these transitions should reflect sensitively the occupation of the "hll/Z orbit by proton pairs. At Sm, specifically 1“Sm, it has acquired enough population by proton pairs to allow the Y transitions to proceed via this component and be less retarded than those in 139Pr. The B+/EC decay of 139mNd and 1ulmSm to the nh11/2 state in their daughters is another indicator of occupancy of the flh11/2 orbit by pairs (cf. the transition in Fig. 18). Thus, we would expect the transition to be faster in 141m 141 Sm+ Pm than in l3gmNd-tl39Pr, quite consistent with our experimental findings. The decay of lulmSm thus is important in two respects. 1) Taken by itself it provides information on a whole multiplet of high-lying states in lule. Simple shell—model calculations should provide information on the major com- ponents of their wave functions, and a closer scrutiny of the transition probabilities should provide much in- formation about the smaller components. 2) In the context 99 of this region below N-82, it provides corroboration of the systematic population of low-lying three-quasiparticle multiplets by the B+/EC decay of some of the 11/2' isomers of the N=79 and 77 isotones. One should next look for this 137(m) 143(m)Gd. behavior from the decay of Nd and perhaps 100 4.1.7. l“188m Introduction As mentioned in Section 4.1.1, the first positive lulmSm was made at about the same time identification of by two different groups, Arl't e§_§l. (Arl 67) and Bleyl Mdnzel, and Pfinning (Ble 67). At that time, 114188111 was not identified, however, its half-life was estimated to be less than 10m (Arl 67). The first positive identification of 1algSm was made by Eppley and Todd and reported in (McH 70). They determined the half-life to be ll.3i0.3-m and constructed a preliminary decay scheme. Further confirmation of these half-life results was presented by a Russian group (Arl 70) who reported the half-life to be llil-m and identified transitions at 403.8- and 438.4-keV as belonging to the decay of 1“188m. The study of 1“138m (11.3 m) is a natural outgrowth of the study of 1“ms m. These results also complement the previous work by Beery, on the N=79 isotones (Bee 69a), who studied the decay schemes of 137m+gCe and 139m+gNd. Additional interest in 1MgSm concerns the spin of the ground state. A glance at the systematics in the region of 50§z§65 and 72§N§82 indicates the odd mass, odd neutron isotopes possess ground state spins of 1/2+ or 3/2+. These ground states can be characterized in shell-model terms as (VSl/Q) or (vd3/2). The spin of the ground state changes from 1/2+ to 3/2+ somewhere in the vicinity of 1“Sm. The 14 ground state spin of 1398Nd is 3/2+ while that of 5899 101 appears to be l/2+. Indications are that the 1/2+ and 3/2+ 1"18111 with either level being equally levels are very close in probable for the ground state. The character of the 1“188m decay scheme could be expected to exhibit quite different features depending on whether its ground state spin is 1/2+ or 3/2+. If the spin of luIESm is 3/2... its decay scheme should have features in common with that of 1398Nd (Bee 69a). If the spin is 1/2+ it might be expected to have features analogous to those of 1u580d (Epp 71). 102 4.1.8. Experimental Results for l“188m 4.1.8.A. l“138m Gamma Ray Spectra The identification of those gamma-rays associated with the decay of 1“188111 is a rather difficult task, and the results lulmSm. The are not as definitive as those from the study of difficulties arise from the relatively short half-life of 1“133m (11.3m) and the character of the lulgSm decay (~90% of the EC/B+ decay is directly to the ground state of 1I'H’Pm). As a consequence of its half-life, the transitions of 11418Sm must always be observed in the presence of the longer lived and more intense ll‘lmS m. This makes identifi- cation of the weak lines in the 1“138m spectra somewhat less than certain. The transition at 403.9-keV has been identified as the most intense transition belonging to the decay of ll‘lgSm. This placement is made from the observation that it exhibits approximately the same excitation threshold as the peaks identified with the 1ulmSm decay and from consideration of its half-life. In the excitation studies it was deter- mined that two sets of transitions with different lifetimes (11.3m and 22.1m) exhibited approximately the same excitation threshold and they could be assigned to the decay of 1”18m. The longer lived (22.1m) component could be identified with 1ulmSm. This was because it was observed to 141 the decay of feed a delayed state in Pm at 628.7-keV which was identified 103 as an 11/2- isomeric level. As a consequence of this identification, the shorter lived (11.3m) component could then be identified with the decay of the 1“18m ground state. After this identification the other transitions of 1“188m were determined to be those which maintained a constant intensity relative to the 403.9-keV peak intensity in six successive 5 min. spectra. In order to maximize the presence of 1“$8111 relative to that of 141m Sm it is necessary to observe the activity as soon as possible after production. To gain some idea of the problem faced in identifying the 1“gain activity it is only necessary to compare the relative intensities of the 403. 9- keV peak (lulgSm) and the 431. 8-keV peak (lulmS m) in the two spectra in Figs. 4 and 5. In the first spectrum counting began 2 min. after the end of the bombardment and continued for 5 min. The ratio of the intensity of the 403.9-keV line to that of the 431.8-keV transition is 100/347. In the second spectrum counting began 27 min. after the end of the bombardment and continued for 5 min. In this instance the relative intensity of the two transitions is now 100/746. The intensity difference points up the difficulty of observ- ing the weaker 1”188m transitions in the presence of the more intense 1ulmSm activity. 'With very few exceptions, all of the transitions observed in these spectra have been identified. These exceptions are transitions with such short half-lives that they appear in only one or two of the earliest spectra, or 104 Table VIII. 1"198111 Summary of Singles and Coincidence Intensity Information Singles Anti Energy Intensity Coincidence Coincidence 324.4io.2‘ 51 403.9io.l 100 100 100 438.420.1 82.6:3 84.7 82.1 728.0:O.2 7.9:O.5 --- --- 854.410.2 3.720.3 --- 10.4 1057.4i0.1 5.510.4 -- --- 1091.9io.2 6.1io.4 --- 13.2 1292.7io.l 16.9tO.S 15.9 19.0 1495.710.2 12.3:0.4 --- --- 1600.910.3 10.310.5 --- 20.7 2005. t(0.5) 1.2:0.2 --- --- 2037.9t(0.5) S.410.4 2.9 --- 8This transition is observed to have a small intensity in the 1'”98111 decay. It is only possible to estimate its magnitude. 105 transitions which display such erratic behavior as to pre- clude any positive identification. 4 In addition to the results of the 1“2N’d(3He,4n)l 1Sm reaction previously discussed we have observed 1“Sm and its subsequent decay products as it is produced via the B—decay of 1ulEu It was anticipated that lulhu would not contain a B-emitting isomeric-level and the resulting lulEu decay would enhance 141mS the ratio of lulgSm to that of m. The results tend to confirm this assumption. The production of l“Eu was via the reaction ll‘uSm(p,4n)lu1Eu 144 5 (Q=-36.8 MeV). Targets of enriched Sm(95.l%) were bombarded for periods of l- to 1.5-m with “1.5uA of 45 MeV protons from the Michigan State University cyclotron. Counting began within 2m of the end of the bombardment, and continued for approximately 2h. During this time several spectra were accumulated and the counting intervals were varied from 5m for the earliest spectrum to l h for the final spectrum. The resultant enhancement of 1ulgSm over that of 1ulmSm can readily be seen in the spectra in Figs. 21 and 22. In the second of these spectra the ratio of the relative intensity of the 403.9-keV transition (lulgSm) to that of the 431.8-keV 1ulmSm) is loo/39.5. This same ratio of relative transition ( intensities in the 1“2Nd(3He,4n)lulSm reaction is 100/347.3 for the corresponding time after bombardment and the same duration. In order to reduce the effects of very intense, 106 high energy positron decay, a block of paraffin 5 cm. thick was placed between the source and the detector. The intense peaks in the first spectrum at 556.8—, 768.3-, and 1023.6-keV have a half-life of 2.010.5m and are tentatively identified as belonging to the decay of lquu. 4.1.8.B 1”188m Coincidence Spectra As pointed out in the previous section, the (11.3m) l”188m activity can only be observed in the midst of the more intense (22.1m) 1ulmSm activity. This limitation becomes more serious when trying to obtain meaningful coincidence in- formation relating to l“lgSm. To obtain the best yield of 1”133m activity relative to that of lulmSm in a source, it is necessary to begin counting as quickly as possible after the end of a bombardment. In addition,the counting interval must be kept short since the longer lived lulmSm activity will eventually overwhelm the shorter-lived l“J‘gSm activity. With these limitations in mind,a 2-dimensional Ge(Li)—Ge(Li) coincidence experiment was performed. The detectors employed were of 10.4% and 4.5% efficiency with 2.2-keV and 1.9-keV resolution (at 1332- keV) respectively. Counting of the l4lm+gSm sample began within 2m of the end of the bombardment and was continued for a period of not more than 30m, with the sample replenished every 5 to 10m. At the end of each 30m period the source was completely replaced with a freshly prepared source, and 107 the procedure was continued. Statistics were accumulated in this manner for a period of 12 hrs. The results of this effort can be observed in the spectra.iri Figs. 19 and 20- Though the results of this coincidence experiment seem somewhat meagre they were obtained with some difficulty, and, in con- Junction with other evidence such as energy sums and intensity balances, were of significant value in the construction of the 1ulgSm decay scheme. A glance at these spectra will emphasize the difficulty encountered in constructing the lulgSm decay scheme, namely the lack of significant detected coincidences between transitions. This result was somewhat anticipated since the decay scheme of lL‘lgSm could be expected to exhibit similar overall features with that of 139gNd. The results of the 1398Nd study (Bee 70) also exhibited few coincidences. In addition to the above mentioned coincidence data, we were able to use the same 511-511-keV-gamma triple coin- cidence and anticoincidence spectra for both lulmSm and l“lgSm. These spectra assisted us in placing those transitions which decay directly to ground and those exhibiting strong positron-feeding. The anticoincidence spectrum is seen in Fig. 9 and the 511-511-keV—gamma triple coincidence spectrum is seen in Fig. 8. A summary of the various coincidence data for l”133m is displayed in Table IX. 108 Table Ix. Summary of y-y Two-Dimensional Coincidence Results for 1”198m Gate Energy (keV) Y—ray Enhanced (keV) 324.4 196.6, 403.8, 511.0, 777.4 403.9 324.4, 511.0, 1091.9, 1600.9 438.4_ 854.4, 1057.7 (very weak) 854.4 511.0 1091.9 403.9, 511.0 1292.7 511.0 1600.9 403.9 109 l 1 2000 m 8 o. B: H 3' H LU g t—' «3 vau1 CE H CD * -saBL 1929 __J -1ssm com» ‘ (1: arm» 0: V9001 -Lzsm-—————_ III zanvr’ wswnttan F—- 1sn1 . 2: 9oi‘1fl1.111 V usmm/ “‘4 snza- euro 01867600 -- 9‘55 ”was _. 2196 >< E'Il6~ ' 9969 99 —— ‘ “'“’ 193.998 —— meg“ 6‘908 . —1'ozs ”'LLL ($209], IE'BZDVGZI. . 9191 g 1mm. 2 -——-ses wuwaoam one see L l 1 1 1 ml' a,1 m1 “fr H1 c5 c3 c: CD CD I—H v—Il 0-4 e—d v—-Q WBNNHHJ 83d SINHOJ 10 4000 3000 1000 CHRNNEL NUMBER 144m+gSm,2-dimensional integral coincidence spectrum taken with 10.41 Ge(Li) detector. Figure 19a. 110 a) E? D- E U] a: H I H U3 o u LU “ o | :3 1798“ a CI: H « -6amu (wszumem IL'SGM —] 2 06M \ CI: v29"- CI: (newnecvo 4 (L3 urzea LJJ (wmumtza I Icw1 :2: 9;“: ‘ ”'90" ' 6 IGOI “—4 1‘ 962m‘\ ‘ 91:01 A Et86 vces I. >_ mquh 9%9 153898- 091-9 69085 - 1 - 1029 v11; 698L, 9 09" - menu 92; sues— 9929 o”; -vmv 919v '6209 vvac “ 61va sea 1 1 1 1 1 tn I :f T m I N I —. C) C) O O C) 1—0 v—d H H F4 1 0 4000 3000 2000 1 0(30 CHRNNEL NUMBER 1“1m*9Sm 2-dimensional integral coincidence spectrum taken with 4.52 Ge(Li) detector. Figure 19b. 111 .ammH:H new muuooam mucovauaaoo HmcoamuoaavIN aoum mooHHm voumu .o~ unawwm mwmzsz szzqzu «mats: qwzzczu E3 1 1 ificw 21|1 CE: 11| _18+o_ .2 amp: 1 88 .1 5mm u 82 . g h m Av—Hu— m u_xq-»1rcnmm_t m_xqux zo wcco >mx-n.mmm_ o_ M_xc-».»cnmm_u m.xcux zo memo >ux-o.oomm o.:_:m 1 _ _ d : ;___ a _, .J _ m ..o. N _ I. S d H mm 1v~O~ 3 O W m N .I m4 0 .vnO— Al m. #10 >wx1:.mmw1o_:_flm O. m—x¢1> >¢4Lm_o m—xc1x 20 w co >ux1m._mo— u_r%:m . 4 0 1:0— 9 :«O. u m m. m w l\ W m ”M if n0- _ 0 .lt. W S vw% 9 m m w o w. m_xq1> 3.3mm; m_xc1x zo mhco >mx1o.:m Baum m_xc1x >¢._Lm_o m—x¢1> zo whco >mx-m.mo: ”:72?“ o— ..o_ 11 o— 13NNUH3 83d SlNflOJ 112 7 MINUTES AFTER BOMBARDMENT 22 MINUTES AFTER BOMBARDMENT . :dB 0 o O '1. . _ A B 5.. .mn/ #23 _ 981.5f.3831 1 331.33!!! .Fffiflfl vaw. 8.de 1 .irzfimwfl . Ea... “afouvfinx. .501. So. _ E59. . Ken: 1 29x:§91ms9 . 389 _ Sagan No. 1 3:. 339' [5.80. 3MLm99 .Sdfl _ ayxzfio a «8 . A I. (a . 38.0 :6 2L. “mg. 32.3%.: 5.3V? “room: .fi!.3fl~23E:£1 Ll E0600 1 .a. Jaw. 3uezefim aquLW1lu AIM] 0.20 33 an» Em .nvlulll 3.8-0, 4' . 3.: on: 8px 2 ST 11 at!» :3! i! I Q1+ 111-ll“?! 24. I* o 3* Q“ I.“ 2” Lu 0 0 0 0 0 0 0 0 0 0 0 NU 0 Amzzcxc xwm mezzou 4wzquu mum mhzzou ’HJTJO 3000 2000 1000 NUMBER Spectra resulting from bombarding 1““Sn with 65 MeV protons. CHHNNEL Figure 21. \‘Ik7.\~‘\\ 113 -... - -.c— —. 37 MINUTES AFTER BOMBARDMENT 9 ing an“... 5‘8! 6311.11 13.39. 3.3.33. 3.89 .51. 3%. C. ‘ took-.1013 gwava £38 I 30. 3.35 name 52.3 “we... E 0.8 .51 3.0 1 .89. «as... - 5.89.. .32. 132:5. . 33. 1.538. - 1—.| 2 0 ‘1‘ z. .3 18.1. .stZQIu mum m. 10° .5135 31.33 1 “111111111 ‘11 1 * it In? 2:23.”! Eat! . . . . 52 MINUTES AFTER 301mm _ . .. I n1! £95 98 . 3.8: $.39. 21!. one». .58. 5.3.... ... . 31.3... (can... angina... 5.3 . ‘ I... .n 123 :32 13.2.1] 1 30100.00! D. . .{nooo :38 in» was. 3133 :8. ...? 043 60.8. + _ 1 2d 1. O 0 0 0 0 0 l 1 1 -. wzzmiu mum mpznou 0 M11; 0 3 L I HM)“ 2000 1000 CHRNNEL NUMBER Spectra resulting from bombarding ”"91: with 45 Rev protons for 1111. Figure 22. 114 “.1.9. 1“188111 Decay Scheme The decay scheme of 1“ISSm we have been able to construct from our measurements is shown in Fig. 23. The transition energies are given in keV, and the QEC is a calculated value (Myr 65). The (total) y-transition in- tensities and the B feeding, both 8+ and EC, are given in percent disintegrations of 1“88111. As can be seen from the singles spectra Figs. u and 5 the U03.9- and “38.4-kev transitions are the most intense lines arising from the decay of lulgSm. Together these two transitions account for 71% of the 1“185m gamma-ray intensity. This evidence seems to support the placement lule at “03.9 and “38.“ keV. Further evidence of levels in supporting these placements comes from the observed enhancement of transitions at “03.9 and D38.“ keV in both the anticoincidence and Sll-Sll-keV-gamma triple coin- cidence spectra Figs. 8 and 9, indicating strong EC/B+ feeding to these levels. The placement of levels at 11195.7 and 2005.0 keV are suggested by the observation of peaks at 1091.9 and 1600.9 keV in coincidence with the “03.9-keV transition. Con- firmation of these placements comes from observation of transitions of 1u95.7— and 2005.0-keV. The level at 1292.7 keV is placed on the basis of the enhancement of a peak at this energy in the anticoincidence 115 3/2' I l.3 m_ 141 $111 62 79 $32.73: Q: " 5.0 MeV _. 1“ 999 2 10.3%)72 "1 ‘ ‘ mnmwas 1...] 588 812.2 8.3775 ‘____!L__ 5215352 P49E5 ‘IJSVQ 6MB . 1 “—71“ @3/215/2 = 9 12 11.4%) 7.0 4:33: ID"- V m . $2333 0 0 a3 ' 1/2- 3/2- 9' a. 728.0 102 as) 8.0 0.371... . . 1 99113119“? 4/ V2,3/2,5 274110939 43L (4.4%) 6.9 M3253. - 1. 1 401 {15.27. 6.9 1 to. 1 .22. j , vv _ 5721; L L 4 (86. 27.15.13 141 Pm 61 so Figure 23. Decay scheme of 1“193m. 116 spectrum and the observed coincidences between the “38.4- and 854.4-keV gamma-rays. The observation of a transition at 2037.7-keV in the anticoincidence spectrum suggests a level at this 1’41Pm energy in Placement of the level at 728.0—keV comes about in the following manner. A transition of this energy has been ob- served to have the proper half-life and in addition, we have observed an apparent coincidence between the 403.9—keV line and the 324.4-keV peak in the 324.4-keV gated spectrum. The evidence is somewhat hard to reconcile since the 324.4-kev line exhibits decay characteristics closely approximating the 22.1m half-life of 1“HISm. One possible explanation of these coincidence data is that the 320.4- keV peak is in the energy region of the intense compton edge from the 511.0-keV annihilation photons. The possibility exists that these coincidences arise from the very strong underlying compton background. However, this does not completely explain the enhancement of the 324.4-kev line in the 403.9-keV gate. An alternate possibility is that there are two transitions at this energy with the stronger of these exhibiting a 22.1m half-life. As a result we have dashed-in the transitions arising from the 728.0-keV level indicating that the evidence is not conclusive, although it slightly favors placement of a level at this energy in lule. ' 117 u.1.1o Spins and Parities of Levels in lule Fed by the Decaygof 1“188m The log ft values shown on the decay scheme are cal- culated assuming the total EC plus 8+ feeding to the lhle ground state by the decay of 1“138111 is 86.2%. Be- 141m cause of the presence of Sm and the decay of the daughter l“Pm, the positron feeding by 1ulgSm to the ground state of lule is difficult to measure directly. We have estimated 86.2% to be an upper limit for the direct decay of the ground state of 1“Sm to the ground state of lule. This value of 86.2% was arrived at in the following manner. The total 8+ feeding arising from the decay of 1ulmSm was estimated from the EC/B+ ratios (it was assumed 1ulmSm lule ground state directly). The EC/B+ 1N1 did not feed the ratios to the states in Pm fed via the 1“188m decay were determined and their resultant (total) 8+ feeding was determined. In addition the Sll-keV peak intensity was corrected for the intensity arising from the decay of the 141 daughter Pm. This latter correction was made by normalizing the Sll-keV peak intensity to the 1223.3-keV peak and subtracting the contribution due to the lhle decay as determined in Section 4.2. The intensity of the Sll-keV peak after these corrections was assumed to arise solely from the beta-decay of 1“188m to the ground state of 1“Pm. This assumes that there is no B+ intensity 118 which might arise from.contaminants. This seems reasonable in light of the cleanliness of the target material, and of our observation of the reaction products. The log ft values shown on the decay scheme are calculated assuming the total beta-decay (i.e. EC+B+) to the 1N1 Pm ground state is 86.2%. These log ft values are not significantly affected by this number, however. An error of 20% in the estimated in- tensity of the 81+ decay results in a change in the log ft values of 0.1, at the most. The spin and parity assignments which appear on the decay scheme are arrived at by assuming the ground state 1”18111 is 3/2+. This assumption is based on the following argument. The ground state spin of lule spin of is 5/2+ (arguments substantiating this are presented in Section 1.1.5.3). The log ft value for the beta-decay between 1”188m and lul Pm is 5.8. This value is somewhat higher than the log ft of 5.1 for the decay between 1398Nd and the 139Pr ground state, however it is certainly an allowed transition. This limits the possible spin of 114185111 to 3/2+, 5/2+, or 7/2+. The values of 5/2+ and 7/2+ seem unlikely, from the systematic behavior of other nuclei in this region. The odd neutron, odd mass nuclei with 5632;63 are characterized by ground state spins of 1/2+ or 3/2+fl The 1/2+ assignment is ruled out because the log ft value of 5.8 indicates an allowed transition between lulgSm and the lflle ground state. 119 In addition to the preceding argument, further evidence tending to favor the 3/2+ assignment for the ground state of 1“188111 is the overall character of the decay scheme. The 1ulgSm decay is similar to that of 1398Nd and 103 141 Sm, whereas, if the ground state spin of Sm were l/2t the resultant decay scheme might resemble that of 145ng (Epp 71). lUSng was observed to populate several high lying low-spin states but no direct decay to the 1“51m ground state was observed. Apart from the ground state,the remaining seven levels are all populated by 3+/EC decay from 3/2+ 1”133m with log ft values ranging from 6.9 to 8.0. Because of the total absence of any direct population of the 7/2+ first excited state from these levels, it is difficult to narrow down the spin as- signments . The two levels at 403.9- and “38.4-keV both appear to be populated strongly in the 511-511-keV-gamma triple coin— cidence spectrum, and the log ft value of 6.9 for both levels indicates they are fed by allowed beta decay. This limits each of their possible spin assignments to l/2+, 3/2+, or 5/2+. It is tempting to limit these assignments to 3/2+ or 5/2... because of the absence of an observed transition to the 7/2+ first excited state. However, the 1/2+ can not be ruled out, without more positive evidence. The level at 728.0-keV is populated with a log ft of 8.0. This is high for an allowed transition, and, as a result, the 120 possibility of a first forbidden decay can not be ruled out. Consequently we have assigned spin possibilities to the 728.0-keV level of 1/2* or 3/2i or 5/2i. The remaining levels at 1292.7—, 1495.7-, 200N.5-, and 2037.5-keV all have log ft values between 6.8 and 7.2. These are probably fed by allowed beta decay. In fact,these log ft values compare favorably with those arising from de- 1U1Nd cay to the high lying levels in . As a result,we can assign these states spins of l/2+, 3/2+ or 5/2+. £41.11. Discussion of Levels Populated by lulgSm Decay It is interesting to attempt a brief comparison between the decay schemes of lulgSm and its nearest odd neutron neighbors, i.e. 1398Nd and 11438m. All three decays populate the ground state strongly via beta decay 88%, 86%, and 96% for 13993M, 1“133111, and 1u3 Sm,in that order. The corresponding log ft values are 5.1, 5.8, and 4.8. None of these B+/EC decays exhibits any measureable feeding to the 7/2+ first excited state in the daughter. The decay of 139%Nd populates two low—lying states at “05.0 and 589.2 keV both with log ft values of 6.4. The state at 405.9 keV in 139Pr[}pin (3/2+, l/2+i]decays directly to ground, bypassing the 7/2+ first excited state at 113.8 keV. The 139Pr state at 589.2 keV is assigned (5/2+) and is observed to populate the 5/2+ ground state, the 7/2+ first excited state and the level at “05.0 keV. 121 In the case of 1ulgSm the decay populates two levels at 403.9 and N38.“-keV,both with log ft values of 6.9. These states both decay to the 5/2+ ground state bypassing the 7/2+ first excited state at 196.6-keV in 1“P m. Their energy separation is 34.5 keV, too low to observe possible feeding from the 438.4 keV to the level at 403.9 keV. The decay of 1“38m populates three low-lying states at 1056.5-, 1173.1-, and luo3.2-kev with spins of 3/2+, l/2+, and 3/2+, respectively, (Wil 71) and (DeF 70). The corresponding log ft value for beta decay to these levels are 5.9, 6.“ and 6.“, respectively. It is interesting to note that these three levels in lLl3Pm all decay directly to ground bypassing the 7/2+ first excited state at 271.8-keV. From this comparison, the similarities seem to imply that the levels fed in the decay of lulgSm have more in common with the levels populated by the decay of 1”38m than those populated in the decay of 1398Nd. However, without more evidence it is difficult to reach any definite conclusions. 122 141 4.2. The Decay of Pm “.2.l. Introduction lule is twice removed from stability on the neutron deficient side of the N=82 closed shell and decays with a 20.9m half-life to 2.5h lulNd. The character of lgéNdal is such that the decay of l“Pm might be expected to populate low-lying neutron hole states in the daughter, which are of a fairly pure single-quasiparticle nature. This is indeed the case, and it is possible to compare the beta decay results with those of recent particle transfer 142 141 studies. These include Nd(p,d) Nd by Jolly and Kashy (Jol 71) and a similar (p,d) study by Chaumeau et al. (Cha 71). In addition, Foster et al. (Fos 71) have reported 14 141 on their studies of the 2Nd(d,t) Nd reaction. Our interest in the decay of lule arises from several directions. The decay of 1“Nd has been studied previously by the gamma ray spectroscopy group at MSU (Bee 68), and this study constitutes the next link in the A=l4l decay chain. In addition we have been pursuing the identification of three-quasiparticle multiplets in the odd mass N-80 isotones. The first such multiplet was identified in the 139m decay of Nd and the next decay expected to exhibit these 141ms characteristics is that of m. We report on the results of the l4lm+gSm decay in Section 4.1. 123 141m Because the half-life of Sm (22.1m) decay is very similar to that of the daughter decay 141 essential to the lulmSm study that we be able to dis— Pm (20.9), it was tinguish between the transitions associated with the decay of each isotope. In order to avoid confusion it is very advantageous to produce the lule activity separately from that associated with l4lm+gSm. of the decay of lule we have been able to obtain more In carrying out this study information than has been available heretofore. The first positive identification was made in 1952 l4le was produced 142 by V. Kistiakowsky Fischer (Fis 52). by bombarding isotopically enriched samples of ( Nd2)O3 with 20-30 MeV protons. The half-life was determined to be 22t2m, in good agreement with our results of 20.910.2m. Prior to 1967 little else had been reported about the de- 141 cay of Pm. At that time Bleyl et al. (Ble 67) reported the half-life to be 20.90:0.05m and identified several of the strong transitions and compared their results with a Russian study (Ben 66); no attempt was made to construct a decay scheme. More recently Hesse (Hes 69) observed 11 transitions that he identified with the decay of lule These transitions were identified as a by-product of his 141mS study of the decay of m. To date the most complete study has been performed by Charvet et al. (Char 70). This study identified 24 transitions arising from the decay of 1anm. Of these 24 transitions 19 were placed in a decay scheme containing ll levels. 124 As a result of this current study we have identified 50 transitions belonging to the lule decay. Our proposed decay scheme (Fig.;fi9) places 43 of these transitions and contains 23 levels. 4.2.2. lule Source Preparation The 21 min. lule activity was produced primarily by the very clean 1L‘2Nd(p,2n)l“1Pm reaction on targets of l“2Nd203 (enriched to 290% lu2Nd). The bombarding times were typically l-2m with leA beams of 24-MeV protons from the MSU cyclotron. Counting began within 2m of the end of the bombardment and a particular source was not counted longer than 50m. A study of the excitation function was made by varying the incident beam energy in N3 MeV steps from below 14 MeV, the -Q value of the (p,2n) reaction, to 30 MeV, which is 5 MeV above the -Q value for the (p,3n) reaction. This allowed us to determine that our targets were free of the 6m 1L‘OPm activity and other possible contaminants. 1“le sources 141Pm In addition to the above experiments, 14 were also produced via the reaction lPr(3He,3n) (Q=-l4.7 MeV) at 25 MeV as a check on the identification. The 1“le peaks were observed in the same intensity ratio and to have the same decay rates, but this reaction was not as clean as the (p,2n) reaction. The l4le activity has 125 also been observed in the InISm experiments where it is 141 the daughter of Sm. Comparison of the strong gamma-rays in the spectra with the results of reaction studies, has allowed us to confirm the identity of several levels in 1u1Nd. 126 1&1 u.2.3. Experimental Results Pm u.2.3.A. 1“Pm Gamma-Ray Singles Spectra Gamma-ray energies and intensities observed in the decay of lule were determined with the aid of a Ge(Li) detector having a counting efficiency of 2.51 with a cooled FET, and a typical resolution of 2.2-keV FWHM for the 1332.u8— keV line of 6000. With the exception of the 193.8-keV transition,the 1N1 energies of the stronger lines in the Pm spectrum were determined by observing them in the 1“ms“: spectra. These lines were in turn used to determine the energies of the weaker transitions. The previously measured 1ulmSm energies were used as standards for the determination of 131 the Pm energies. The results of this method were checked by subsequently observing the strong lines in the 1H1 1&1 Nd decay (Bee 68), and using Pm to determine these energies. These results proved accurate to within the quoted error limits. The energy of the 193.8—keV transition 1u1 was determined by counting the Pm activity in the 192 presence of 57Co, Ir, and 21‘3Cm (Epp 70a). The results from several different spectra were then averaged to arrive 127 at the value of 193.8—keV for the energy of the first 1&1 excited state in Nd. Transitions belonging to the decay of lule were identified by their intensities in successive spectra relative to the 1223.3-keV line. Starting 2m after bombardment, 5 successive 10m spectra were accumulated. The process was repeated several times with the spectra in the corresponding time intervals added. Those gamma- rays which maintained a constant intensity relative to the 1223.3-keV in each of the five 10m spectra were identified as belonging to lule. The first and fifth of these spectra are displayed in Fig. 2“. We have identified 50 gamma-rays associated with the decay of lul Pm and incorporated “3 of these into the current decay scheme. Table X contains a complete list of the energies and intensities of the transitions observed in this study. 4L2.3.B. IMle y-y Prompt Coincidence Studies Several prompt coincidence experiments were performed using two germanium detectors. These included two- dimensional megachannel coincidence experiments using Ge(Li)-Ge(Li) spectrometers, and anticoincidence and elec- tron-positron annihilation triple coincidence experiments utilizing Ge(Li)-NaI(T£)-annulus spectrometers. Results of the different two-dimensional coincidence combinations are summarized in Table X. The results of the other coin- cidence experiments appear in Table XII. 128 Table X. Energies and Relative Intensities of y Rays From the Decay of 1“1Pm. This Work Charvet et.a1 (Char70) Hesse (Hes69) Energy Intensity Energy Intensity Energy Intensity (keV) (keV) (keV) 180.2:o.2 0.6:o.2 193.8:o.1 34.9:1.7 193.7:o.3 32:3 289.4:O.l 3.6:o.6 538.2:o.1 1.7:o.4 544.710.2 1.4:0.4 597.2:o.1 1.2:o.3 622.2:o.2 18.9:1.o 622.0:o.1 20:2 621.2:O.5 19.8,2.3 647.2:0.1a 1.3:o.3 706.0:o.6 1.0:o.3 756.7:0.1 1.7:o.4 886.3:0.1 52.5:2.6 886.1:o.2 52:5 886.0:0.5 56.7,7.o 901.2:o.2 1.1:o.s 958.7:o.1a 1.3:o.4 966.3:o.1a 2.0:o.5 1022.8:o.1 2.8:o.5 1029.910.1 7.8:0.5 1029.6i0.5 7:0.7 1028.510.8 7.8t3.1 1080.7iO.4 1.0iO.2 1223.3:0.2 100. 1223.3:0.1 100. 1222.9:0.5 100. 1345.8i0.1 27.6i1.4 1345.4i0.2 3013 1345.0iO.7 49.4111 1371.0iO.3 2.210.? 1370 $1.0 310-5 1403.2t0.1 16.011.0 1403.210.3 16i2 1403.1:0.7 17.7i4 1564.8:0.1 17.9i1.0 1564.7i0.2 17i2 1563.7:0.5 21.3i4.7 1596.810.1 16.1i1.0 1596.9i0.2 13.611.5 1597.7i0.7 20.614 1626.7i0.1 5.8i0.6 1626.5i0.7 6i1 1703.810.2 1.2i0.2 1703.3i1 110.2 1820.410.3 1.630.4 1820.211 1.710.3 Table X - continued 129 1872.610.1 1879.910.1 1897.1i0.1 1967.610.1 2052.810.1 2066.210.2 2073.710.1 2109.610.1 2145.210.7 2246.410.2 2265.3i0.3 2303.610.2 2311.6i0.2 2354.4t0.2 2388.4t0.1 2419.110.5a 2429.810.5 2505.010.5 2602.1:o.5a 2612.3i0.5 2619.6t0.5 2690.3i0.5a 2804.910.5 2985.5i0.5 0.5i0.2 6.910.7 1.1i0.3 3.6i0.4 2.6i0.4 1.3i0.2 13.4i1.0 1.8i0.2 0.330.1 1.5i0.4 0.710.1 2.4i0.2 0.510.1 1.010.2 1.330.2 0.2i0.1 0.610.1 0.610.2 0.1i0.05 0.310.1 0.4i0.1 0.2t0.1 0.510.1 0.910.2 1879.830.5 1897.011 2053.331 2073.510.4 2247 11.5 2266 11.5 2305 11.5 2355 12 2389 12 2505 12 2804 12 8&2 0.9t0.2 2.4i0.5 12i2 1.410.3 1i0.2 1.5i0.5 0.8i0.2 1 10.2 005:0.1 0.410.1 1879.%0.8 9.812.3 2072.910.6 18.413.1 aThese transitions belong to 1“1Pm but we were unable to place them in the decay scheme. 302.9b 788.9 1037.7 1043.2 O‘O'U‘ bThese transitions were observed to have a half-life slightly longer than 141 ~ that of Pm. (11,2 est. 30 min.) 130 Table XI. yfRay Intensities for l4le Coincidence Experiments Relative Intensities Energy Anti- Integral 193.8-keV Sll-5ll—keV (keV) Singles Coinc. Coinc. Gate Coinc. 180.2 0.6 193.8 34.9 26.4 574.1 7.5 289.4 3.6 39.6 1.9 511.0 2385 65.5 538.2 1.7 7.4 544.7 1.4 597.2 1.2 4.7 622.2 18.9 7 3 58.7 4 6 647.2 .3 706.0 .0 886.3 52.5 20.5 114.3 3.3 4.2 901.2 1.1 958.7 1.3 .1 966.3 2.0 .4 1022.8 2.8 4.7 1029.9 7.8 1.9 17.3 6.9 5.3 1051.9 2.0 12.6 1080.7 1.0 1223.3 100 56.1 100 100 1282.1 0.9 3.3 1345.8 27.6 9.0 39.3 8.6 1371.0 2.2 2.7 2. 1403.2 16.0 5.8 17.5 16.0 6.6 1564.8 17.9 13.9 6.5 16.9 1596.8 16.1 16.1 3.2 19.4 1626.7 5.8 7.1 5.8 1703.8 1.2 0.7 0.6 1820.4 .6 2.0 1.0 1872.6 0.8 1.3 131 Table XI Con't. Energy Anti- Integral 193.8—keV 511-511—keV (keV) Singles Coinc. Coinc. Gate Coinc. 1879.9 6.9 3.8 4.2 6.0 1897.1 1.1 1967.6 3.6 3.4 2052.8 2.6 2.5 2066.2 1.3 2073.7 13.4 16.0 1.8 2109.6 1.8 2. 2145.2 0.3 2246.4 1.5 2.5 2265.3 0.7 2303.6 2.4 2 5 2311.6 0.5 2354.4 1.0 1.1 2388.4 1.3 1.5 2429.8 0.6 0.3 2505.0 0.6 0.5 2602.1 0.1 2619.6 0.4 0.3 2690.3 0.2 0.1 2804.9 0.5 0. 2985.5 0.9 132 (A) "'Pm SINGLES ”(M ' as reset“ 5 g ix. -. v we‘ll Q i 1 a . , .‘ V- . 1 0 ° ; s .n . .' fig _ a N ..f‘ g I 7; a ‘ | .‘ ,. ‘ - 1 J i g T! ‘9 T j ‘ 1 1 1 : :4 —+— -~—-~-- . ' COUNTS PER CHRNNEL :3.- (B) '4'Pm SINGLES .i' v 4‘ ' 1 1 1 1 " "III I . “r“f“. l U fi '“‘I_\,‘ 1.1 -\_L"\_.| ZOOC ~-‘ \- . CHQNNEL \VV333 Figure 24. Singles y-ray spectra for “’15. Upper: First of five successive 1m spectra. Lover: Fifth of five successive 10m spectra. 133 Details of the two-dimensional megachannel coincidence experimental set-up are outlined in Section 2.1.2.A. The detectors used were the 4.5% efficient and 10.4% efficient Ge(Li) detectors with 2.0—keV and 2.2-keV resolution FWHM, respectively, at 1332.48-keV. The detectors were placed in 180° geometry with a graded Pb absorber between them to help prevent scattering from one detector into the other. The source was placed off-center as shown in Fig. 3 in order to reduce the 180° coincidences between the 511-511-keV annihilation photons. Coincidence events were recorded on magnetic tape using the data taking program EVENT and the Sigma-7 computer. This yielded a 4096x4096 channel array of prompt coincidences with a resolving time 21:120 ns, typically. The integrated (total) coincidence spectrum is shown in Fig. 25, and the various gated spectra are shown 141 in Figs. 25 and 26. Because the Pm EC/B+ decay is 390% to the ground state of 1“Nd and the half-life is 21m, the 2-dimensional Ge(Li)—Ge(Li) coincidence experi— ment is difficult and information is hard to obtain. This two-dimensional experiment represents a counting time of 12 hrs with very frequent replenishment of the sample and replacement of the old source by a new source every 40- 50m in order to achieve higher counting efficiency. Despite the low statistics involved, these spectra were invaluable in construction of the decay scheme and their sax? \ 134 L J_‘ experiment )f,.'.'. 1000 CHANG. NUMBER Results of 2-dinensionsl Ge(Li)-Ge(Li) coincidence for lule. H 93; l I N n meow \.|..|I E 1 rt .15.. ,P T A ll. 1' U m.5m. 1 R l R H H G H C . . , C N ”Meson“ . V |.. V I : 037.. V E V 3.9:: -.Illl, E m r. K m l K 932.! 3 H11 7 Y. . 1.. Us. N 5 MN! ; ,5 “a 1 H” r.m~m~ -. M“ R o mom. ... ./ . a 5m. ... ... H .H .II.) .. «do: I . al..» l r“... 0.:nT‘IW o my? \ . 1 m.m~o_ diwmo. . ndum u m.moo - M.Uoo I mice 1 a w.~.~u I «Nam \ o N. I o :m 2m 9.2m - L m.mom / 36mm \ 1 . 08m. 1 . : 060. I Odfl “.2: i 3.1.“ v r - - Jrl > 4 4 0 s 4. 4L 0 IT -.mzzqro En. £238 - Figure 25. 135 .33.: no mason—Hummus 02830—38 Agavmonfiengo Huaouaauaaeau «as scum «ocean sous» Hauonufiee< .sou «usage mmmzsz muzzcxu mwmtaz JwZZIIU oobm con: . . - opow coo“ fi.4l. 4. 2 H W 1 _ 90— 7+ — . 1 1 , oo~ _ 1 . . mv—O~ m h. . _ ir—Od 7v 3 m r u u m. . m ... .. r m In 6 z a m E 1(0— . . menu >mx m.wmm : r - .f . . u I... ...... m E u... :2: . i2 > x . x . x 1 A “Em— mZM—i Assn: x 1 _: m: o E N a m We .Jo~ 1r_O~ _ . _ +NO—- m 1.. I m n. 6 6 0 e w. . . 8 1T O~_ I. 9 n M ..~o_ 1-!4.f--mwamlfiw.-Ela was >3. ~.m~m WBNNUHD 83c! SiNflOJ 136 opom 358-368 Haoeaofiee 2: 39a .83» .83 18333 ammcaz smzzqzo opo_ .amH:H do nunsawuugxs ounovauaaou 17.: 8'86! mmmznz muzzaxu opom . opoi spam >mx w.mom_ w_1o >ux 3.0mm— -liluT L: mequ >wx m.mmm_ 8'E6! - .1: when >mx m.m:m_ £3 «33...— Z'ZZS ~ o~ O -..- ...... -..... 0 cu- weau >mx o.Hum1 b 8‘8 mpco >mx m.~mom 8°66! - + J whco >mx N.mo:~ 8'56! - c3 33d SanOO é: WBNNVHO .10. ..o_ 137 results were confirmed by other independent evidences (Char 70) and charged particle reactions (Jo 71), (Cha 71), and (Fos 71). Additional coincidence data were obtained using the NaI(T2) annulus and a Ge(Li) detector. The anticoincidence Spectrum shown in Fig. 27 was obtained under the experimental conditions described in Section 2.1.2.B. This spectrum was taken with a 0.4% Ge(Li) detector (having a 3.0-keV resolution FWHM at 1332-keV) placed in the tunnel of the 8x8 in NaI(T£) annulus with the 141 Pm source in the center of the annulus. The Ge(Li) detector was operated in anticoincidence with y-rays above lOO-keV in the NaI(T£) detectors (2I=100 ns), and a true-to-chance ratio of 2100/1, resulting in the enhancement of those transitions not in prompt coincidence with other y—rays, x-rays or with 8+ emission. This was particularly useful in placing transitions to the ground state or to a metastable state providing these transitions are fed primarily by EC and not 8+. The pair (Sll-keV-Sll—keV-y) coincidence spectrum shown in Fig. 28 was obtained using absorbers to enclose the sample to assure total annihilation of the positrons. The experimental details are described in Section 2.1.2.B. In this experiment the single channel analyzers associated with each half of the annulus had their windows adjusted ago: I _ 138 .asuuoonn soaovaoaaoowuam an?H .nm «Human mmmZDZ IQZZQIQ Doom OQON - opoi r A ;::: _ l A ...“ fl T1 , ., . _, v .3. _1 ”:2 . m 5285260 .. :24 Sn. .1 di- 015 etc: WBNNUHJ 83d 8141003 139 coo: .amH:H How asuuoonn conuvaonwoo >I>oxIHHnIHHn mwmznz 4mzz¢1u .mN ounwam Doom Doom ooo~ _ 1 _ .....o— . 1, hh.w W _ we com me .... a 8 m _ “D “C I ) . . Wm H m m :22 a II c s u o .9 wuzmofiuz_ou meHmH >mx -mI~—mlctz¢o u 0 13NNUH3 83d SiNflOD 140 to accept only the 511-keV region. A triple coincidence (resolution 2100 ns) was required in order for the fast coincidence unit to generate a gate signal and allow signals from the Ge(Li) detector to be analyzed by the ADC and stored in the analyzer. The result is that only double-escape peaks and transitions from levels fed by 8+ decay appear in the spectrum. The various coincidence results are summarized in Table XII. 4 2.4. 1ule Decay Scheme The l“le decay scheme we have constructed from our current measurements is illustrated in Fig. 29. All energies are given in keV and QEC=3.73 MeV is the value determined by Charvet g£_al. (Char 70). This compares with the theoretical value of 3.622 MeV (Myr 65). The (total) y—transition intensities, as well as the total beta feeding, are given in percent disintegrations of 1“IF m. The log ft values were calculated using 20.9-m for the half—life of l4le. Placement of the level at 193.8—keV is straight—forward and is based on a number of facts. It is the third strongest gamma-ray in the spectrum and is not observed in coincidence with the two strongest lines at 1223.3- and 886.3—keV. Other evidences include the observation of a large number of transitions in coincidence with it, Figs. 25 and 26, complemented by the observation of 7 transitions whose Table XII. 141 Summary of Y-Y Two-Dimensional Coincidence 141 Results for Pm Decay Gated Energy Energies of y-rays in the Gated Spectra (keV) Strong Medium to Weak 193 8 289.4, 1029.8, 1371.4, 886.3 143:3; 23221 289.4 193.8, 597.2, 1223.3, 1626.7, 1820.3 538.2 622.1, 1345.5 597.2 1223.4 622.2 1345.8 886.3 193.8, 1223.3 1022.8 1223.3 1029.9 193.8, 886.3 1223.3 886.3 1345.8 622.2 1371.0 193.8 1403.2 193.8 1626.7 289.4 193.8 1703.8 193.8 1820.4 289.4 1879.9 193.8 2052.8 193.8 142 '0‘ =373MeV 7.2 (1.. $1 0». (I Diflfll-l I. I4! Figure 29. Decay scheme of ll”I’ll. 143 energy is equal to the sum of 193.8-keV and the energy of a corresponding transition observed in the 193.8-keV gated spectrum. In addition, this level has been ob- served in (p,d) and (d,t) reaction studies (Jol 71), (Cha 71), and (Fos 71). The placement of levels at l223.3-, 1345.8-, 1564.8-, and 1596.8-keV is suggested by the enhancement of gamma— rays at these energies in both the anticoincidence Fig. 27 and 511-511—keV—gamma triple coincidence spectra (Fig. 28). The levels at 1820.4-, 1968.0-, 2073.7-, 2109.6-, 2246.6-, 2303.4—, 2354.4- and 2388.4-keV can be placed from the fact that peaks at these energies are enhanced in the anticoincidence spectrum. Levels at 1820.4-, 1897.1-, 2066.4-, and 2265.3-keV are placed by observing that transitions from these states are in coincidence with the 193.8-keV transition and transitions from these levels to the ground state are enhanced in the anticoincidence spectrum. By relying on energy sums we can postulate levels at 2506- and 2805—keV. Transitions of 2505.4- and 2804.9-keV have been observed,ahd corresponding transitions of 2311.6- and 2612.3-keV are observed to add with the 193.8-keV transition to give 2506 and 2805. The low efficiency of the Ge(Li) detectors for high energies, prevented these coincidences from being readily observed. 144 The remaining levels at 2145.2-, 2620-, and 2986-keV are placed solely on the observation of transitions of these energies in the singles experiments and our inability to observe possible sums with other transitions. These transitions are of sufficiently low intensity, that it would be very hard to observe them in either the anticoinci- dence or triple coincidence experiments. In addition to these levels, we Show the level at 756.7— keV which is the well known 60 sec 11/2‘ isomeric state in l“Nd. Referring to the first spectrum in Fig. 10, which began 6m after bombardment and was accumulated for 10m, the 756.7-keV transition appears quite intense. This is attributed mostly to the direct production of lL‘lmNd via the reaction 142 141 Nd (p,p'n) Nd (Q=-7.1 MeV). The 756.7- keV intensity drops quite rapidly in subsequent Spectra, in agreement with this assumption. However, the second Spectrum in Fig. 10, which was taken for an elapsed time of 10m beginning 54m after bombardment, shows that the 756.7-keV transition still has a measureable intensity. One would normally expect that the intensity of this peak 5“ in this amount of would have decreased by a factor of :2 time. In fact the relative intensity of this line in the last four of the five successive ten minute spectra is re- latively constant, with an average value of 1.7 (normalized to the 1223.3-keV line). This would seem to indicate that there is some indirect feeding of this level in the decax 145 ... ... -\HH omam ... ... ... ... ... ... -\~.-\m.-\m ~.m=H~ ... ... ... +\efi+\mv e.mo«~ +\~.+\m.+\m m.moa~ ... +\m omom A+\mv omom A+\mv+\m m.muom +\m.+\m s.m~om ... ... A+\mv omom ... «\m.«\m e.mmom A+\s.+\mv ... ... ... A+\~v a.~mma .I\m.u\> o.owma ... ... ... o.>mma “\m.«\m H.pmma +\H smma ... +\H osma ... ... A+\mv News +\m cams A+\mv some ~.ommH Asxm.u\mv.\+m =.o~ma . ... ... ... A+\mc+\m m.eama .+\m.+\m m.emms A+\mv Hmma +\m coma A+\mv coma ~.=mmd A+\mv+\m m.amma A+\~V mama opma A+\sv ommH Au\mvn\~ =.mam~ A«\m.l\~v+\~ o.m=mH A+\mv Hmma +\m ONNH +\m ooma +\m m.m~mH A\+mv+\m m.m~ma n\aa mm» I\HH mm» +\HH ow» HI\HH um.mm~ o\aa o>.wm>u +\H mes +\H mum +\H ems +\H e.mma +\H m.mma +\m o +\m o +\m o +\m o +\m o acoecmamm< Mwwwwm unmecwamm< “wwwwm newscwamm< ”wwuwm usescmamm< Mwmwwm acoacwaun< ”wwwwm 0 .ad um pmpmom o.Hm um xsmemno . 1Nnmmx cam haaon n m.Hm no um>umno cowameEoo mEonomuHo>mq oz HHHX mama“. Hza xnoz pgoamam gas nomve Ade «cove ads Hence Ace segues Ame convamm oomv .ohoaoed :2 Has: one no «so no nonwheuosusno Hus: soon no: cases uo house on» :« voosasnon won on uZBAaHo Edna ... 32 Km 33 . .. . : so: +:. gem ... . .. sss see so. see H\FQ\«MQ«\M mam“ ... +\m onmw cnmw ... ... ... ... oom~ .omm «\m.«\m mom~ ... ... ommm ... nxh..\m.-\m ommm ... ... ... ... +\m.+\m comm ... ... ... ... e omam ... . ... ... ... e a.wmm~ ... . ... ... ... e 3.2mmw ... onmm A+\~v comm ... - n.momm ... ... ... ... s m.mwmw . {teams seam ... +\> oamw . ... ... ... ucoacmaoo< Aboxv unoscwuee< A>oxv unoacwanm< A>oxv causewamm< A>oxv acoacwwen< A>mxv museum awpocm awaocm hwnosm hwsocm U.Hm um powwow o.am pm «soaono panama new maaoh m.Hm no vo>haco mmo: ucomonm .u.coo HHHN manmh 147 of lule, or another y-ray of this same energy. By adding a level at 1403.9-keV we could place the 607.9- and 706.0- keV transitions between the level at 2109.6—keV and the level at 756.7-keV. The intensity of 607.9-keV transition does not balance with that of the 756.7-keV transition, and we have placed it elsewhere on the basis of other energy sums. However, these transitions have intensities low enough to have an error of 10-20% associated with them. Consequently it is difficult to determine whether these are the only transitions which might possibly feed the level at 756.7-keV. Others may have gone unobserved. This decay scheme (Fig. 29) is in good agreement with that reported by Charvet gt_al. (Char 70). However, we have added ten levels to the 13 initially proposed by Charvet and placed an additional 25 transitions for a total of 43. The only significant disagreement between our results and those of Charvet concern the spin of the levels at 1345.8- and 1968.0—keV (discussed in Section 3.2.4.). In Table XIII we compare our results with those of Charvet gt_al. and include the results of the (p,d) and (d,t) reaction studies (J01 71), (Cha 71), and (Fos 71). 148 4.2.5. Spin and Parity Assignments of Levels in lL'lNd Populated by the Decay of lulpm 4.2.5.A. 1M'Pm Ground State The ground state spin of 1“1le has been determined by the atomic beam method to be 3/2 (Alp 62), and in shell-model terms it is predicted to be a (d )-1 neutron state. Re- 3/2 sults of (p,d) and (d,t) reaction studies have confirmed a 3/2+ assignment for this state (Jol 71), (Cha 71), and (Fos 71). This evidence, coupled with our data, indicates the ground-state spin of l“le is 5/2+, as described below. Our results show that 89% of the EC/B+ decay of lale 141N populates the 3/2+ ground state of d. The log ft value for the transition is 5.5, and this certainly suggests an allowed beta decay. This implies the spin assignments of + lule. 1/2 , 3/2+, or 5/2+ for the ground-state spin of We can narrow down the possible spin by observing the systematics of odd-proton odd-mass nuclei in the 51§Z$63 region. These nuclei all exhibit ground-state spins of 5/2+ or 7/2+. In the Pm isotopes the ground-state spins of l”3Pm and lurD-Pm are 3/2+ while the heavier isotopes, beginning with 1u7Pm, exhibit 7/2+ ground state spins. The possibility of a 7/2+ spin for the lule ground state can be eliminated because this would require a second-forbidden non-unique B-transition. This clearly is in conflict with the observed value of 5.5 for the log ft.‘ Hence we will assume in our following argu- ments that the ground state spin of 1ule is 5/2+. 149 4.2.5.B. The 193.8-keV and Higher States We observed no direct population (50.26%) of the 141Nd. This figure could certainly 193.8-keV state in be much less since it is based on intensity balances of the gamma rays feeding this level, and these could be in error by as much as 110%. As a result of this situation, we have calculated a minimum value of the log ft for decay to this level of 7.9. We feel the actual value is certainly no lower than this and quite probably much higher. These observations are consistent with this state being assigned 31/2 from the (p,d) and (d,t) reaction studies. This state has also received careful study from Charvet g£_§l, (Char 70). The lifetime of this state was found to be 1.1710.15 ns and 2 the E2/Ml mixing ratio was determined to be: 6 =0.1510.04. 141Nd .The log ft values for the remaining states in range from 6.3 to 7.7. This immediately suggests either allowed or, at most, first forbidden non—unique transitions and limits their possible spins to 3/2, 5/2, or 7/2. In the following we try to narrow down the spin assignments by making comparisons with the (p,d) and (d,t) studies (Jol 71), (Cha 71), and (Fos 71) and the conversion electron work of Charvet et al. (Char 70). For reference, we include the electron work of Charvet et a1. (Char 70) in Table XIV. In Fig. 30 we graphically compare these experimental results with the theoretical calculations of Hager and Seltzer (Hag 68). 150 .hmoov am~:~ ecu wafisoaaom maowufioasuul> osu mo oaom How muaowoammsoo nowmuo>aooIM Hmuwuuuoosu one Houaoawuonxo mo domwummaoo .om unease ENERGY (keV) IOOO O TRANSITION 200 ._ 4. m ...Zmfiimmoo zofimw>zoo v. 151 Table XIV. The l4le Conversion Electron Results of Charvet et a1. (Char 70). Photon K-Conversion Energy Relative Relative Experimgntal Muigipoiggity (keV) Intensity Intensity “K g a b 193.7:0.3 32 :3 100 180 M1+E2 +0.5 +1.2 622.010.1 20 12 1.2 _0.3 2 8 -0.6 El 886.110.2 52 :5 1.7 10.2 1.9 10.5 E2(+Ml) 1223.310.l 100 2.1 10.6 1 2510 4 E2,M1 1345.410.2 30 13 1.6 10.3 3.1 11.0 M2 1403.210.3 16 12 0.4 10.2 1.5 11 (E2,Ml) or E3 1564.710.2 17 12 1.4 10.3 4.4 11.0 M3 1596.910.2 13.611.5 0.3610.2 1.5 10.8 (E2,Ml) 1879.8:0.5 8 :2 <0.2 <1.4 (E2,M1) or E1 2073.510.4 12 12 <0.25 <1.l (E2,Ml) or E1 3The value of IK(193.7)=100 serves as the fiducial value for the determination of electron intensities. bThe value of aK=180 serves as the fiducial value for the determination of conversion coefficients. 152 In Table XIII we compare the levels identified and our Spin and parity assignments with those of Charvet and the (p,d) and (d,t) studies. The 1223.3-keV State The level we have placed at 1223.3-keV has a log ft of 6.3. This implies an allowed transition and suggests spins of 3/2+, 5/2+, or 7/2+. Because this state feeds both the 1/2+ state at 193.8-keV and the 3/2+ ground state, this would tend to rule out the possibility of 7/2... as a spin assignment. The intensity ratio of the 1223.3-keV Y to the 1029.9-keV from the 1223.3-keV state is 11. If the spin of this level were 7/2t the multipolarity of the 1223.3-keV transition could be expected to be E2 and that of the 1029.9-keV transition would be M3. The single-particle estimate (Mos 65) predicts a ratio (E2/M3) of 4lxlo5, and for a Spin of 5/2+ we would have a predicted ratio (Ml/E2) of 840. For a possible spin or 3/2+ (both Ml's) we obtain a ratio of 1.7. It is clear from the comparison of the results that the 7/2+ assignment is unlikely. Experimentally it is common to find that E2's are enhanced, (i.e. their transition probability is greater than that predicted theoretically) and Ml's are re- tarded (their transition probability is less than theoretically predicted). It would not require much E2 enhancement or Ml retardation to be consistent with the 5/2+ assignment. 153 The conversion electron work of Charvet et_al. indicates the multipolarity of the 1223.3-keV gamma is E2+M1 (Table XIV), suggesting a 3/2+ or a 5/2+ assignment. The Spin and parity of this level have been assigned 5/2+ because of the observation of i=2 momentum transfer in the reaction studies. The inter- pretation of these data is straightforward and relatively un— ambiguous. The results of Chameaux gt_al. include the use of a polarized proton beam to study the 3/2+ ground state and the lowest 5/2+ state in l“38m (Cha 71). Comparing our results with with the reaction data, it seems probable that this is the same 5/2+ level that is observed at this excitation in the (p,d) and (d,t) reactions. The 1345.8-keV State The level at 1345.8-keV is observed to decay via a Single transition to the 3/2+ ground state. The fact that it fails to populate the l/2+ first excited state suggests this level possesses a possible spin of 5/2 or 7/2. However, it should be recognized that this is a weak argument and that 3/2 could also be possible. If this were the case there would probably be certain internal selection rules that would be operative-—and we have no information available about these selection rules. It is difficult to draw any conclusions concerning the parity of this level because the log ft value of 7.2 does not allow a definite distinction between an allowed 154 or first forbidden beta decay. As a result of this evidence, the spin assignments could be 5/2i or 7/21 (with 3/2i being less likely). The conversion electron results of Charvet e£_§g, indicate the experimental K conversion coefficient of this transition is aK=(3.l 1.0)x10-3. Theoretical conversion coefficients for M2, E2, or E3 transitions of this energy are aK(M2)=3.4x10_3, 01K(E2)=1.05x10-3 and aK(E3)=2.04x10-3, and these results suggest the multipolarity of the transition is M2 (as seen in Fig. 30). Since this state decays to the 3/2+ ground state and M2 transitions involve a parity change, this evidence implies the Spin of the 1345.8-keV level could be 5/2- or 7/2'. (If the multipolarity of the transition were either E2 or M3, no parity change would be involved and the level would have positive parity.) A state has been observed at this excitation energy in the (p,d) and (d,t) reaction studies on lusz, and the results seem to suggest 7/2+ for the spin and parity. Jolly and Kashy have suggested (7/2+) and Foster gt_al. also indicate (7/2+), while Chameaux et_al. have given no spin assignment. Evidence from the systematics of energy levels in the even proton N=81 nuclei can also be considered. A 7/2+ state has been identified at 1347.7-keV in l39Ce (Ful 62) (Bee 69c), and a 7/2+ state is seen at l360-keV in 1“38m (Jol 71). From the similarity of these results, one might expect this level in 1ulNd to be a similar 7/2+ state. (_) e— g 4— ‘30" 1 ‘AA L‘n hi; (,1) p. A- ( I) 2,’ a I F." I T) 155 At the present time the evidence pertaining to the spin of this level is not definitive and an unambiguous assignment cannot be made. Our preference is to slightly favor 7/2+ as the most likely choice, but 7/2- and 5/2t cannot be ruled out. The State at 1564.8-keV The level at 1564.8-keV is populated by beta decay with a log ft of 6.6. This indicates an allowed transition, suggesting possible Spins of 3/2+, 5/2+, and 7/2+. This state is observed to feed both the 193.8—keV, 1/2+ state and the 3/2+ ground state. The 7/2+ possibility can be ruled out because this would require the 1371.0-keV Y to the first excited state to be a M3 and the 1564.8-keV Y to the ground state an E2. The experimental ratio of the intensity of the 1564.8-keV y to the intensity of the 1371.0-keV y is 8. The theoretical single-particle estimate predicts a ratio (E2/M3) of 2.4x105. This is clearly in disagreement with the experimental results and there would have to be truly ‘unusual enhancements (and retardation) to obtain this ratio. 131 the case of a 5/2... spin the theoretical predictions in- dicate an (Ml/E2) ratio of 352, while for a 3/2+ assignment (tnyth Ml's) the predicted ratio is 1.5. As a result of tfluese estimates we can rule out the 7/2+. However we cxnuservatively retain both 5/2+ and 3/2+, noting it would zwuquire reasonable amounts of E2 enhancement or Ml retardation (txyth experimentally observed in other nearby nuclei) to favor the 5/2+ spin and parity. 156 The conversion electron results of Charvet indicate the multipolarity of the 1564.8-keV transition is M3. This spin change is somewhat large for a gamma transition, and as a result, this level might be expected to exhibit a measureable half-life. Theoretical single-particle estimates for a M3 transition of this energy suggest a possible life- time of 700 ns for this state. We have made no special effort to determine a lifetime for this level, however we are able to place a limit on its possible magnitude. We have observed this transition in the Sll-Sll-keV-gamma, triple coincidence experiment (resolving time 2100 ns). If this state were delayed, it would not be expected to exhibit many coincidences with the positrons feeding it (within the resolving time of the experiment) and as a result its intensity in this experiment might be expected to be noticeably less than in the singles experiment. In fact however, these intensities are very nearly the same-— suggesting a half-life less than a two hundred ns. In general, except for E2 multipoles, the other multipolarities are hindered (i.e. they exhibit partial half-lives longer than the single particle estimates), hence, if anything we would anticipate a half-life longer than 700 ns for this state. The (p,d) and (d,t) studies populate a level at this excitation characterized by an 1:2 angular momentum transfer, and they are assigned spin and parity of 5/2+. This is quite probably the same level as we observe in the beta decay. As a result of the evidence, a 5/2+ assignment seems most likely, although 3/2+ remains a possibility. 157 The 1596.6-keV State The level at 1596.8-keV (log ft=6.4) populates the 5/2+ level at 1223.3 keV, the 1/2+ first excited state and the 3/2+ ground state. The log ft value suggests an allowed transition limiting the parity to positive. The results of Charvet indicate the 1403.2—keV transition from this level to the 1/2+ first excited state may have either E2, M1, or E3 multipolarity because of the uncertainty in their measurement. The 1596.8-keV ground-state transition is an E2 or M1. The evidence tends to indicate either 5/2+ or 3/2+ are equally likely. A 7/2+ assignment can be ruled out because it would require the 1403.2-keV transition to be an M3. As pointed out earlier this would not be expected to compete favorably with the E2 feeding the ground state. The Remaining States in l“114d We have observed a level at 1820.4-keV (log ft=7.l). This level populates three lower lying levels: the 5/2+ 1223.3-keV state, the l/2+ 193.8-keV state, and the 3/2+ ground state. The log ft value makes it difficult to determine the parity, hence both + and - must be considered. The spin can be limited to 3/2 or 5/2 on the basis that 7/2 would require the 1626.7—keV transition to the 1/2+ state at 193.8-keV to be an M3 or E3 (considered unlikely). A 158 level is observed at this excitation in both the (p,d) studies and the (d,t) work and all three groups favor 5/2+ for the spin. It seems likely this is the same level. As a result,5/2t seems to be consistent with the available evidence but we do not completely rule out (5/2-) or (3/2i). At 1968.0-keV we have observed a level with a log ft=6.4 implying a spin of 3/2, 5/2, or 7/2. This level feeds the 1345.8-keV state strongly and also populates the 3/2+ ground state. The conversion electron data indicate the multipolarity of the 622.2-keV transition to the 1345.8-keV state is El. This would imply a parity change between the levels. If the 1345.8-keV level is 7/2+, this would require the 1968.0-keV state to be 5/2‘ or 7/2‘. If the 1345.8-keV level is 7/2‘ it would require spins of 5/2+ or 5/2‘. Since we favor 7/2+ for the level at 1345.8-keV, we then prefer a 7/2- or 5/2- assignment for the 1968.0—keV level. However, we cannot rule out either 5/2+ or 7/2+. The levels at 1897.1, 2066.4 and 2805 keV have log ft values of 7.4, 7.6 and 7.4 respectively. The decay characteristics of these two levels are similar in that they decay to the same states-—the 1/2+ state at 193.8 keV and the 3/2+ ground state. Consequently we can limit the possible spin assignments to 3/2 or 5/2 (7/2 is ruled out because this would require E3 or M3 multipolarities for deexcitation to the l/2+ first excited state). The parity remains uncertain, however. 159 The levels at 2073.3 and 2206 keV are beta fed with log ft's of 5.8 and 6.8 respectively. The log ft values indicate they are fed by allowed beta transitions and suggest positive parity. We are able to limit their Spins and parities to 3/2... and 5/2+ because feeding from 7/2+ states to the 1/2+ level at 193.8-keV would require M3 transitions, which would be expected to be very weak. The level at 2109.6 keV (log ft-5.8) populates lower lying states of possible spin 5/2+ or 3/2+. The log ft value indicates an allowed transition favoring positive parity, however the spins and parities could be either 3/2+, 5/2+, or 7/2+ with any of these choices being equally likely. With a log ft-8.l the level at 2145.2 keV is probably fed by a first forbidden beta decay indicating possible spins and parities of 3/2’, 5/2', or 7/2'. The level at 2246.6 keV feeds levels with spins and parities of l/2+, 3/2+, and 7/2+ (or possibly 7/2'). The log ft value of 7.1 indicates either allowed or first- forbidden beta decay. This indicates its spin could be either 3/2’, 5/2’, or 7/2’. With the exception of the level at 2303.4 keV, the remaining levels at 2265.3, 2354.4, 2388.4, 2430, 2620, and 2986 keV have log ft's ranging from 6.9 to 7.7. These 160 results are uncertain enough to prevent any definite state— ments about the parities of these levels. Hence we can only say that their spins are 3/2, 5/2, or 7/2. The level at 2303.4 keV is fed with a log ft=7.0 and, as a consequence, we tend to favor a positive parity assignment. 4.2.6. Discussion of the States in lulNd The 3/2+ ground state and the two lowest excited levels in 1141Nd at 193.8-keV and 756.7—keV can be characterized in shell-model terms as (vd3/2)-l,(vsl/2)-l, and (vh11/2)-l, respectively. These states appear to be of a rather pure Single quasiparticle nature as characterized by the large spectroscopic factors associated with their population in the (p,d) and (d,t) studies (Cha 71) and (Fos 71). Our results appear consistent with these findings, although we cannot say anything about the parity of the states. We observed that 88.5% of the B+/EC decay of l“le ;popu1ates the ground state of lulNd with a log ft of 5.5. This appears consistent with the transformation of a d5/2 prwiton into a d3/2 neutron. A similar (Ids/2)+(vh3/2) checay is observed in the case of l39Pr-ga l390e, (Bee 69c) 111 which 98.8% of the B decay is observed to be directly to true ground state of 139Ce with a log ft of 5.6. 161 The first excited state at 193.8-keV receives no apparent B+/EC feeding (<0.26%), and the log ft value >7.9 is con- sistent with its being an (81/2)-l neutron state. The 193.8-keV Y transition from this 1/2+ first excited state is primarily an R-forbidden Ml multipole transition (SI/2+d3/2). Charvet et al.'s results (Char 70) indicate the multipolarity is (87% M1+l3% E2). These values correspond to an M1 retardation of 51 spu (Moskowski single particle units), and an E2 enhancement of 4 spu. The experimental lifetime (1.17 ns) of this odd neutron state is the same order of magnitude as similar R—forbidden, low energy transitions (g7/2+d5/2) in the odd proton N=82 nuclei (Gei 65). 141mN The level at 756.7-keV is the well known isomer d (fl/2=6Os) (Bee 68). This state is one of a series of 11/2‘ isomeric states found in the odd neutron N=81 isotones (Jan 69). These isomers are characterized as (vhll/2)-l neutron states and all are of a relatively pure single quasiparticle character. These isomeric states are characterized by lifetimes ranging from 410m for l33mTe to the order of 60s for the isotones l39mCe, 141m 143m Nd, and Sm. The decay of these states is via an M4 transition directly to the ground state. In this study we have dis— cussed, in Section 3.2.4., the apparent observation of indirect feeding to this level from states at higher ex- citation in l“Nd. It will be of interest to see if a 162 similar effect is observed in the beta-decay of lu3Eu to states in 1“38m. The state at 1223.3-keV appears to contain approximately one-half to one-third of the (2d ) single-particle 5/2 strength, from the reaction studies, indicating that the (d5/2) single particle state is considerably fractionated. In the lule decay the 1223.3-keV state is fed by four higher lying states in lulNd at 1820.4_, 2109.6—, 2246.6-, and 2303.4-keV, with the 886.3-keV y transition from the level at 2109.6-keV being the second strongest line in the gamma-ray Spectrum. This would seem to indicate that these states have some common features in their wave functions. A 5/2+ state is observed in the (p,d) studies at 2.09 MeV (Jol 7l)(Cha 71), and it could correspond to the level seen at 2109.6-keV in the beta decay. However the energy resolution is not good enough to make a positive identification. The level at 1345.8-keV (log ft=7.2) is somewhat of a puzzle. From the results of the (p,d) and (d,t) data of Jolly and Kashy and Foster gt_al. it is tempting to assume it contains some (\Jg7/2)"l component in its wave function. Similar 7/2+ states have been observed in at 1347.4-keV and at 1369.5-keV in 1“3Sm with log ft values of 7.0 (Bee 69c) and 6.5 (Fir 71), respectively. It should be noted that this 7/2+ assignment depends on large 163 2 transfer in the reaction studies, and angular distributions of this kind are difficult to interpret unambiguously. The results of Charvet gt_al. (Char 70) indicate it deexcites via an M2 multipole transition to the ground state, thus favoring a negative parity assignment (probably 7/2'). If it is indeed a 7/2' state, it could result from the coupling of h to a 2+ vibrational state, or a less likely alter- 11/2 native might be that the unpaired neutron is populating the f orbit. This is the next shell to be populated 7/2 after the closure at N=82. Population of the f7/2 has been 1388a(d,t)137Ba observed below the N=82 shell in the reaction (Ful 62) and more recently via the reaction 136Ba(p,d)137Ba (Von Ehr 70); the energy of this state is 1.8 MeV. Such a state is not expected to be observed in a pickup reaction such as (p,d) or (d,t). The 7.2 log ft value makes it difficult to distinguish between an allowed or first for- bidden beta decay. In light of the various results, the spin and parity of this level remains somewhat uncertain, but the evidence slightly favors 7/2+. The levels at 2109.6-keV and 2246.6-keV seem to have some features of their wave functions in common with the lower lying states. The 2109.6-keV state feeds 4 low lying states, the 1820.4-, 1564.8-, and 1223.3-keV levels and tine ground state. The level at 2246.6-keV feeds states 5 lower'lying states at 2066.4-, 1345.8-, 1223.3-, 193.8-keV and the ground state. 164 At the present time little is available in the way of theoretical calculations with which to compare our re- sults. Heyde and Vander Berghe (Heyd 69) have calculated the 139Ce energy spectrum in a unified model with inter- mediate coupling. These results appeared before decay 139 work of D. Beery on Pr (Bee 69c) and they also were done prior to the appearance of the (p,d) and (d,t) studies mentioned earlier. Preliminary calculations 141 (Ree 71) of the states in Nd using the pairing plus quadrupole Hamiltonian of Kisslinger and Sorensen (Kis 63) have been done. The results are for states up to 2 MeV of excitation, and they indicate the three lowest states are very much single quasiparticle states. The lowest 5/2I state seems to have significant admixture of 1/2+ and 3/2+ coupled to one phonon. Similarly the lowest 7/2+ appears to arise mainly from coupling 3/2+ to one phonon (2+) and has little single particle character. The character of the higher lying states apparently arises from coupling 1/2+ and 3/2+ to one phonon (2+). Chapter V Discussion of Results and Systemgtics The previous chapter includes descriptions of the states investigated along with some comparisons with states in neigh- boring nuclei. One of the results of this study has been the identification of a three-quasiparticle multiplet of 141Pm high—lying, high-spin, odd—parity states in . The character of these states is discussed in some detail in Chapter IV. In this Chapter the results of the studies of 1ulSm and l4le are related to other nuclides in the region. r: This thesis then concludes with a survey of energy level systematics of odd proton (52éZé65), odd mass nuclei and odd neutron, odd mass nuclei in the immediate vicinity of the N=82 closed shell. 5.1. Three-Quasiparticle Multiplets in Other Nuclei Well characterized three-particle states in nuclei are relatively rare and their recognition has most often been dependent upon the isomeric properties of a few high—lying, lligh-spin states. As a result, the excitation of a multiplet of such states in one nucleus, with each state feeding a iiumber of low-lying states, is of some theoretical interest. The particular three-quasiparticle multiplet mentioned in Section 3.1.5.C. has some stringent requirements placed on the mechanism involved in its population via the electron 141mS capture decay of m. These requirements include, (1) a 165 166 high-spin parent (e.g. an hll/2 isomer), and (2) sufficient energy to populate states well above the pairing gap in the daughter nucleus. In addition, the ability of the parent nucleus to decay by other modes must be severely hindered. For example, an isomeric transition, if present, must be sufficiently low in energy to allow the B+/EC decay to compete "m”? favorably with it. Finally the nucleus must have an intrinsic configuration that forces the preferred path of decay into the three-quasiparticle states. These particular requirements seem to be satisfied for B+/EC decay of some nuclides with “<82. The other possible regions where these three-quasiparticle multiplets might be observed are discussed in (Bee 69b). It 137m is possible the decay of Nd may populate such a multi- plet in 137Pr, however, no data are currently available for this case. A comparison of the three-quasiparticlermultiplets 139mNd and 1ulmSm has been made populated in the decay of in Section 3.1.6. Here these states are compared with similar states identified in the N=82 region. These involve different particle configurations. The beta decay of 145ng (Epp 71) has been observed to populate a quasiparticle lusEu and a multiplet has been populated in 138 multiplet in 139Lavia the reaction La(d,p)139La (DuB 71). 167 The structure of the three-quasiparticle multiplet in 141 )—1 8 3 -1 Pm is basically [(ng7/2) (ads/2) (\ad3/2 (Vhll/Z) ]J_, where J7 is restricted to values of 9/2‘, ll/2‘, and 13/2_ for states populated by allowed beta decay. The multiplet in 1uSEu populated by the l45ng decay has the structure [("h )2n—l IFT'E‘...—‘«.~L-:‘Q' ;; I -1 11/2 (Vh9/2)(VSl/2) ]J+ where J+ is restricted to 1/2+ or 3/2+ for allowed beta de- cay (the spin of the parent is thought to be l/2+). The J multiplet in 139La is populated via a stripping reaction and the structure of the multiplet involves (g7/2) and (d5/2) proton orbitals and (SI/2), (d3/2), (f7/2), and (p3/2) neutron orbitals. It can be seen that in each in- stance a somewhat different group of states is involved. 5.2. Levels in the N=80 and N=82 Odd Proton Nuclei Figures 17 and 31 Show the positions of the low-lying (<4 MeV) levels in the odd mass N=80 and N=82 isotones. The data for N=9O are from the beta decay studies of (Par 68), (Alex 68), (Bee 71), (Fra 64), and (Bee 69b) and Chapter 4 of this thesis; the results of the reaction study 1“1Pr(p,t)l39Pr (G01 70) have also been included. The N=82 data are from the reaction studies of (W11 71), (Jon 71), (Dav 70), and (Bae 71) and the beta decay results of (Bee 68), (Ber'69), and (DeF 70). Encvgg (MeV) .LUU Levels in N=82. 0er Mass Lso’cones .___:___ (3:) l '\ *‘1/(‘513/L4‘7 .-.—“._- -..-.....4 57/-5/ ————- .mm. _..._3 * _______ _ ____.__-——-:-——g + .... - Z -2527." 2.: ,, + :E“. i we or .___..--—... . _ g V1 ,- 3/ + 3,01 " i/t -._..-— .3/+ J 5/+ ...... —-——— I ‘‘‘‘ '7 3/*———-—-—- 3/1 — :_;_'_—-__——: ___.. ”Ty—v.1»: 1/" a“ 1; W“: 17:5” _‘ " 3/*\."-—:“"‘_.___-.;.;-:/V* , ...fi- -_.__ 5 o)_.__.___._—"—"""-‘ :3--.— - 3/ 21..--..2 ((01: ____:_.___..:_._. (5/‘) 7— “/ “—4 ”i.“ ”I- " ...—......“ 957/12" 2; 33’ 35—7: 3” . g; J ’ t/* 3’/_:-:—_—:::-:= 37+ 3/+ — lI/s ‘ + ”/1 ------ 5 .— / I/‘ "I: ....... . 5,. i 1 57+ -~ -— ' 7/+ 7" 5/" 7/‘ , —— 7/‘— ~———— 7/‘« -—-~———— 7/* ---“ 5"“—--—- .5/1—-——- 5 :_.____ : “-551 :37 139 I4! I45 / 14$ 3 53 81 5 CS La PT‘ 5m Eu 3 5 82 57 82 5‘9 82 4| 82 63 87. j Figure 31. Experimental levels in the N-82 odd mass isotones. 169 A comparison of the two lowest levels in the N=80 and N=82 odd proton isotones, Figs. 17 and 31, shows that in both instances the Spins of these levels are 5/2+ and 7/2+. + and the first excited For Z157 the ground state is 7/2 state is 5/2+, while the roles are reversed for ZES9. In the case of the odd proton N=82 isotones there is a decided gap between the first excited state and the next lowest level. The gap is observed to decrease with increasing Z. At 137Cs the gap is ml MeV and at luSEu it has decreased to No.4 MeV. The gap probably arises because of the hard core formed by the closed neutron shell at N=82. The gradual decrease in the gap with increasing Z may be due to the in- crased proton occupation resulting in a somewhat softer nucleus. In the case of the odd proton N=80 isotones the energy difference between the first excited state and the next low— est level is decidedly less. This is related to the presence of the two neutron holes in the N=82 shell which strongly influences all of the levels. The effect of the relatively inert N=82 core is observable in Fig. 33. Here we show the relative energy spacing between the lowest 5/2+ and 7/2+ states in the odd proton N=80, 82, 84 rauclei. For the N=82 odd mass isotones the energy separation is greater and the curve rises more sharply. The relative :spacings in N=80 and N=84 follow a similar trend. In the case E1723 ‘ E572... (keV) 170 Odd Pro+ovt N‘80,82,84 Iso‘l’onsfi' ° NBC ‘40“) L. . ’4.‘321 W F“: x N084 1 200 t' - -. (3 -200 +— ..4KDC)+— l 1, .L _1 1 1 1 331 55C, 5)" ”9,. up" 63E“ Figure 32. Systematics of the energy level spacings between the low lying 7/2+ and 5/2+ states in the N-BO, 82 and 84 odd mass isotones. 171 of the N=80 the primary part of the ground state wave function is composed of a single proton in the (g7/2) or (d5/2) shell and two (d ) neutron holes. In the N=84 odd proton iso- 3/2 tones the ground state consists primarily of a single (g7/2) or (dB/2) proton and two neutrons in the (f7/2) orbit. In either case the nucleus can be pictured as being softer because of the two neutron holes or particles. The result is that the relative spacing between the 5/2+ and 7/2+ levels is decreased and the variation of the curve is smoother and rises less sharply. 5.3 Levels in the Odd Neutron N=81 Nuclei The level structure of the odd neutron, odd mass, N=81 isotones is shown in Fig. 32. The data are from the reaction studies of (Jol 71), (Cha 71), (Fos 71), (Ful 62), (Von Ehr 70), (Sch 66), (Yag 68) and the beta decay results of (Tre 69), (Mac 70), (Bee 69c), and (Fir 71) and Chapter 4 of this study. From Fig. 32 it can be observed that below 1 MeV of excitation there are 3 identifiable levels in each isotope. These are the 3/2+ ground states, the 1/2+ first excited states and the ll/2’ states. These levels are generally referred to as neutron hole states and in shell model terms -1 )—1 -1 are characterized as (v3sl/2) , (\22d3/2 and (uh ) 11/2 The results of the reaction studies indicate these states Energy (MeV) 172 'Levds N=8l OoLoL Mass 7/' u . 3/f5/“ (5/‘)-————— 7; 9r 3 0-— 7/* (S'lw' (7/') l/' 7/* ._ 7/I --~ (5") ... ”II—‘7 IV', 7”,... “55,37; (9/*)"’"‘__.________“‘— 7” 2 0 ~— (an)..- 5’. - (5'? 547783;: ° 54:3: “-1—" 4. d m; :/ -——-— ~-—-—-——-—- " 7r (1/‘)“_—‘ ( 7/ ') (y/*):;‘;‘.—'“__* n _ ‘7’ 3 ~- 5/ ’ 345,17 5/; (I/O' 5/') . (57),”, ..-. 5/o .. 5,’ _~ 3457““ (9/*) —--- (7P) 1.0 ~(5/*)—---——-—- n; ---....._.___ nx'_.__..__ w- w- * "I " V ‘ ...... 1/*._.__._. O "‘ 5/""""‘—_ 3W 5; -— - 3/‘.‘__..__... 153 :35 :37 :39 Xe 5a Ce 5?. ea: 54 8' 56 BI 58 8| Figure 33. )So’to‘nes ”‘3/,57.77 & 5f 5/,S7 «I 37 5," -—- 1 I *— 407—: .. 141—... H/ —)..___*::_..— ”d 5’)5/~‘—:-‘/' $ ' ,\ 5/ ,(51, 3'! + + s/ ,5 -..—_— smart -..-.__ 7/‘,(7/,'5/L_..__- 5;, ( 3/‘)————— ll/:-____-.__ . -....a.._‘__.-._ ——-...——.- -..-— (9K ( 77.1 :2"__.__.-"~ (Ia/‘1‘“ "“’ .. + 4- /,/,H/ “—.— 7/t~—-—-—— .— 7/ *——-— -—-———- .~——-- 5/*:..__.. -- um- ------ I 5/‘ g +__._:'_...__. yin/.2. .. ---- 5P“: ——————— II/'_.-- .. I/t.--.___.. 3/f.--..__-_ I43 . I 62 he: --—— ..—.4 Experimental levels in the N-al odd mass isotones. 173 are of a fairly pure single particle nature. The (Vhll/2)-l states are isomeric levels which decay to the first excited state via M4 transitions and all exhibit half-lives ranging from 410m for 133Te to 64s for lu3Sm. These states have been studied in detail by Jansen gt_§l. (Jan 69). Above 1 MeV in excitation the density of states in- creases rapidly and the large number of 5/2+ states suggests that the (vd ‘1 strength may be considerably fragmented 5/2) among several levels. These do not represent all the levels that exist in these nuclei. For instance the states observed in beta decay studies depend on the decay energy available and the beta decay selection rules. The energy difference between the 31/2 and the d3/2 states is observed to decrease as a function of increasing Z. A plot of this separation energy versus Z can be seen in Fig. 34. This energy dependence as a function of Z is reproduced quite well by an equation of second order in Z. With the use of such a curve and by extrapolating the fit to Z=64 (i.e. 1uSGd), the results indicate the 1/2+ and 3/2+ cross one another between Z=62 and Z=64 (i.e., the ground state becomes 1/2... and the first excited state 3/2+) and the relative spacing between them at Z=64 is :8 keV. These results seem to be in agreement with those of Eppley et al. (Epp 71) and Newman et al. (New 70) whose 174 N‘G‘ Odd» Mas: “(flames J l J 1 4t!) -’ 33 .11 21K} - V t: :; Lu 0 I ... ‘3 u; 4» - ’m _ Figure 34. l l 54.x“ :55“ ”Ce “Nd 615'“ «GA fl Systematics of the low lying 1I2+ and 3/2"' states in the M1 nuclei demonstrating the apparent quadratic dependence of the energy spacing. value . The value for “Sod is a calculated (predicted) 175 beta decay studies of 145ng indicated the ground state spin of 1uSGd is probably l/2+. The energy of the 3/2+ state has not yet been determined. It is interesting to note that the relative energy spacing of the holes in c13/2 and 51/2 neutron orbits exhibits a parabolic behavior similar to that observed in the antimony Dz (Sb) isotopes and iodine (I) isotopes by Berzins (Ber 67) I and Beyer (Bey 67). 5.4. Characteristics of Similar (Delayed) ll/2’ States in Odd Proton Odd Mass Nuclei in the Region of N=82 In the study of states of lule tflua700 ns delayed state at 628.6 keV was of significant importance in the determination of the properties of the lulmSm decay scheme. This state is certainly an 11/2- state and it deexcites to both the 7/2+ first excited state and the 5/2+ ground state of 141P m. These transitions are presumably of M2 and E3 multipolarity respectively (i.e. assuming the deexcitation to each level occurs with the lowest multipolarity possible). From the measurement of the half-life of the state (700 ns), the partial half—lives of each transition were determined and compared with theoretical estimates (Moskowski single-particle estimates). These results indicated the 431.8 keV M2 transition was retarded by a factor of 58 over the theoretical 176 Table XV Characteristics of Similar Odd Proton (Delayed) 11/2' States Near N-82 Theoretical E3 Transition State Experimental I E3 T (E3) Enhance- Isotope (Ref.) Energy 1:, /2 (ns) Intensities 1042) W ment r"; 1Z§Eu86(ner161) 496.2 (E3) 1.4 0.6 4.8x10'" 1.4 346.5 0H2) 248 24 . 12:8“eu(365161) 625 (E3) 740 4.7 0.12 1.75x10-3 2.1 7 369 (M2) 36.6 _. 145 L- ,- 63EU82(EPP70b) 716 “— 386.6 (M2) 123111182041171) 962 (E3) 689 (M2) lziPm80(Chap.4) 628.6 (83) 700 3.1 0.07 0.78><10'3 1.6 431.8 (M2) 43.6 lgéPrez(st7°) 1117-6 (ES) 10 0.11 0.76x10"3 12.0 (Sho71) 973 (M2) 90 1§§Pr80(nee69b) 821.9 (23) 40 1.4 0.05 0.42xio'3 2.2 708.1 (M2) 27 1:;La80(Van867) 994 (E3) 0.01: z?-8 1004 (Mn+83) 39.41 0.19 0.07 31.12x10‘1 168 (El) 4.35 31.18x10‘“ 177 estimate. However, this is not at all unusual; typical M2 retardations are on the order of 10 to 100. What is more interesting is that the 628.6 keV E3 transition is observed to be enhanced by a factor of 1.6 when compared to the theoretical value. This result is somewhat unusual since in general, all multipolarities except E2‘s are retarded. In the region near N=82 several other odd proton, odd mass nuclei have been observed to exhibit similar (ll/2'-+ 5/2+) enhanced E3 transitions. In Table XV we compare the E3 enhancements and the E3/M2 gamma-ray intensity ratios (Where available) with the theoretical single-particle estimates of similar 11/2- states in nearby nuclei. This somewhat unusual enhancement of E3 transitions (ll/2-,5/2+) could arise from the possible coupling of the d5/2 single—particle state to the 3' core octupole vibration. The amount of admixture necessary to account for this en- hancement is small enough that the 11/2- state retains its significant single-particle character. Some evidence of this possibility has been observed in inelastic scattering studies (E11 69). In cases where the ground state is 5/2+ the 11/2- state seems to be more easily excited via inelastic scattering than in cases where the ground state is 7/2+—— possibly indicating the presence of a small [d5/2 @3-] 11/2- admixture in the 11/2‘ state (w11 71a). .‘l .".J 178 We have included in Table XV the ll/2- states at 962 keV in 1”3Pm and the 716.1 keV state in luSEu as possibly exhibiting decay via enhanced E3 transitions to the ground 141 state. The lifetime of the 1117.6 keV state in Pr has recently been determined to be ”.9i0.5 ns (Sho 71). The partial half-life of (N25 ns) for the E3 ground state transition indicates an enhancement of 12.“. This is the largest enhancement observed in this region and suggests that further such experiments on the odd proton N=82 iso- tones could prove interesting. Little is known currently about what appears to be the corresponding ll/2_ in l“Pm. 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