"3‘ 2235 E2125 A 5mm OF NUCLEAR ERERQY LEVELS EN “"55 12? AND ELESEEEG 6» AND 312A? spEcmoacopv Sb Thssis fer the Degrae oi FEE. BE ME ECHEC’ ELIE STE/53% UNEVERSEEY RENEW E... Auk-Ea W66 rm This is to certify that the thesis entitled A STUDY OF NUCLEAR ENERGY LEVELS IN 1218b, 1238b, 1258b AND 1271 USING e— AND y-RAY SPECTROSCOPY presented by RONALD L . AUBLE has been accepted towards fulfillment of the requirements for Ph. D. degree in PHYSICS Wag/m H. w; Major professor X Date-£6.35. g; E966 0-169 -. . V~_ I a ' -.'r ,. 1m .‘, “ , Li 13.51:! A 2’ ,. PE'EI‘CE‘... 5a A; Di 34m ; Umversity 4. _ m ABSTRACT A sum or m ENERGY LEVELS IN 1218b, 1238b, AND 1271 USING (3- AND y-RAE SPECTROSCOPY 125Sb by Ronald L o Auble The decay schemes of several odd-A tellurium (A - 121, 127) and tin (A - 123, 125) isotopes have been examined in an effort “to gain information about the nuclear level structure of adjacent antimony and iodine isotopes. Beta- and gamma-ray singles and coincidence spectroscopy were used to determine the ordering of the game-ray transitions, and therefore, the energies of the nuclear levels. Several levels which had been unobserved in earlier studies were located in each of the four isotopes. Angu- lar correlation measurements on the praninent gamma-game cascades 1215b, 1253b and. 1271 were made to study the spins of the nu- in clear states and the mixing ratios of the gamma-ray transitions. The spectrum shape of the beta-rays feeding one of the excited 3 states in 1’23 Sb was studied and found to yield an essentially linear Fermi-Kurie plot. This result, in conjunction with the corresponding log ft value, suggests a non-unique, first forbidden ! assignment for this beta transition and limits the possible spin I assignments for the 1238b state. 4. Comparisons are made of the existing experimental data with ' the predictions of several nuclear models 3 In the low energy regions of 1213b and 127 I , sufficient data on mixing ratios and lifetimes are available to make quantitative comparisons with predictions of the single particle model and with the Ronald L. Auble pairing-plus-quadrupole residual interaction calculations of Kisslinger and Sorensen’. Partial agreement is found between the experimental data and the latter predictions. At higher excitations, only qualitative comparisons can be made due to the lack of gems-ray lifetime and mixing ratio measurements. A model which is found to predict level structure similar to that observed experimentally is one in which the low- lying particle states are coupled to excitations of the nuclear core. However, several high energy states in ,J'ZSSb are found to deviate from the predictions of this model. Thus, other types of excitation must also be assumed to exist. The nature of these states can only be determined after additional experimental data become available. A sum or NUCLEAR ENERGY LEvms IN 1218b, 1238b, 12551: AND 1271 USING E- AND T-RAY SPECTROSCOPY BY :2." 0 Ronald L3.“ Auble A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Physics and Astronomy 1966 ACKNWLEDGEMENTS I wish to express my deepest appreciation to Dr. William H. Kelly for his aid and guidance during the experimean work and the preparation of this thesis. I thank Dr. Herbert H. Bolotin for suggesting this region of the isotOpe table for study and for his aid in understanding the theory and application. of angular correlations. Mr. G. Bernins, Mr. L. Beyer, Mr. R. Etherton and Mr. D. Gollnick aided in the acquisition and analysis of experimental data. ' ' I acknowledge the financial assistance of the National Science Foundation which provided partial support for the experi- mental program. During much of the time in which this program was in progress, I was supported by a Fellowship from the Michigan Institute of Science and Technology. Last, but definitely not least, I thank my wife Shirley for her continued moral support and encouragement. ii ACIQIONLEDGEMENTS. LIST OF TABLES. . LIST OF FIGURES . INTRODUCTION. . . CHAPTER 1. CHAPTER 2 . CHAPTER 3. CHAPTER h. TABLE OF CONTENTS NUCLM MODE-ISO O O . O O O O O O O I 0 O 0 LA. 1.8. 1.0. 10D. LE. The Nuclear Shell Model . . . . . . Th8 Nilssan Mbdel e e e e e o e o o The Collective Model: Even-Even Mueleie e e e o e e e e e o e e o e The Collective Model: Odd-A N11016:“ O O O O O O 0 O O O O O O 0 Residual Interactions . . . . . . . SOURCE PRODUCTION AND PREPARATION . . . . EXPERIMENTAL APPARATUS AND TECHNIQUES . . EXPERD’EENTALRESULTS.......... hm. h.B. 121 The Decay of 1211‘s and mTe . . . h.A.i. The Gamma Ray Singles SpGCtrume s o e e o o e e o hmAeiie 001n01d8nce StudieS. e e e h.A.iii. Summary of 1211‘s Results. 123m Tho Decay 0: Sn 0 o e o e o e L heBeie The Singles SpBCtra e e o e heBoiie CainCidance Studies. 0 e 0 iii Page ii 15 22 22 22 21; 3h 38 38 h.c. . hone h.s.iii. Beta Spectrum Studies. . . . . b.B.iv. Discussion of Proposed Decay Scheme..........o.. 125 sneoeeeeeeeeo The Decay of h.C.i. The Gamma Ray Singles Spectrum . 14.0.11. Gama-Gama Coincidence Studies h.C.iii. Beta-Gamma Coincidence Studies h.C.iv. Construction of a Proposed DecaySChelneeeeeeeeseo h.C.v. Angular Correlation Measurements 1271‘s and m'E‘e. . . . . . h.D.i. Gamma Ray Spectrum of 127Te Iamraoeeseoeeeeeee The Decay of h.D.ii. Coincidence Studies . . . . . . 1271‘s Angular Correlation Measurementseeeeeoeoe h.D.iv. Summary of 127Te Results. . . . h.D.iii. CHAPTER 5. DISCUSSION or EXPERIMENTAL RESULTS AND COMPARISONSWITHTHEORY. . . . . . . . . . . . SJ. 5.3. TheLOWFhergyStatBB. e e e e o e e e e 5.A.i. Comparison of Electric Quadrupole MSLtion Rates 0 e e e e e .e e 5.A.ii. Comparison of Ml Transition ' Ratea............o_ 5.A.iii. Energy Level Systematics . . . 5.A.iv. Beta Transition Comparisons . . TheHighEnergyStates. . . . . . . . . BMIWM O O O O O O O O O O O O O O O O O O O O I. O 0 iv h5 h? h? 50 S9 59 63 69 69 69 75 79 82 87 91 95 95 100 Table l. 2. 30 7. LIST OF TABLES Page Summary of the results of measurements on.photons emitted.in.the decay of 121Te isomers. 35 Energy and.intensity measurements on gamma rays emitted.in the decay of leSn using a Ge(Li) detector. - 51 Summary of angular correlation.measurements on photons from 12551:. 65 Summary of data onuphotons emitted in the decay of 127Te. 71 Summary of angular correlation.measurements on 127Te photons. 76 Properties of low energy states of 1218D and 1271. 83 Transition.rates between low energy states of 1218b. - 85 Transition rates between.low energy states of 1271. 86 LIST OF FIGURES Figure Page 1. Photo-efficiency vs. energy for a 14 mm x 2 an Ge(Li) detector. 18 2. The gamma ray singles spectrum of 1211‘s plus 1231,90 23 3. K X-ray-gamma coincidence spectrum of 1211‘s. 25 11. Low energy photon spectrum in coincidence with the 1102 kev transition in 1211s. 28 5. 'The high energy gamma spectrum in coincidence with photons in the 80 to 120 kev region of the 1211’s gamma spectrum. 30 6. Spectrum in coincidence with the 970-10h0 keV region of the 1211‘s spectrum. 31 121 7. The proposed decay scheme of 1211‘s and mI‘e. 37 i 123 8. Singles gamma ray spectrum of 125 day Sn plus 113 Sn obtainedwlth a NaI(Tl) detector. 39 9. Gamma spectrum of 1238n (125d) plus 113Sn obtained with a )4 mm by 2 cm Ge(Li) detector. to 10. Fermi-Kurie plot of electron spectrum in coincidence with the 1089 keV gamma transition in 123811. 143 - 11. Proposed decay scheme of 125 day 1238n, compared ' . 121 . with the decay scheme of mTo. E46 Figure ' Page 12. Ge(Li) spectrometer spectrum of low energy photons emitted in the decay of 9.7 day 125.31. 1:8 ' 125 13. High energy gamma ray spectrum of 9.7 day Sn taken with a 1: mm x 2 cm Ge(Li) detector. 1:9 11;. Spectra observed with a NaI(T1) detector in coincidence with (a) 1089 and (b) 1067 kev 125 transitions as seen by a Ge(Li) detector; Sn. 53 15. Ge(Li) spectrum of 125$n in coincidence with the unresolved 1067 and 1089 keV photopeaks detected with a NaI(Tl) crystal. Sh l6. Gamma spectra in coincidence with the (a) 332 plus 351, (b) E70, (c) low side of the N70, (d) Ihzo and (e) 1806 keV energy regions. 55 17. Coincidence spectra obtained by gating on the (a) 272, (b) 332 plus 351 and (c) the 1:70 1:67 regions 125 of the Sn gamma spectrum. 58 18. Proposed energy level scheme for 1253b as seen in the decay of 9.7 day 125 . 60 127 19. Gamma ray spectra of Te taken with a (upper. curve) 7.6 cm x 7.6 cm NaI(T1) crystal and (lower curve) a 1; mm x 2 cm Ge(Li) detector. 70 20. ' Spectra of 127Te taken in coincidence with (a) g 57.6, (b) 360, (o) 591 and (d) 657 kev photons. 73 Vii Figure Page 21. 22. 23. Coincidence spectrum obtained by gating on the unresolved 203-211.; keV photopeaks in the 127Te spectrum. 71; 127 , Proposed decay scheme of 1271‘s and 11‘Te. 80 Energy systematics of the 2d5/2 and lg.”2 states in odd-i antimony and iodine isotopes. , 9h viii INTRODUCTION Among the more interesting nuclei to be studied both experi- mentally and theoretically in recent years, are those having odd- A and spherical equilibrium shapes.l’2 ’3 The spherical shape is deduced from the properties of the ground states of these nuclei, » especially their small electric quadrupole moments. Of particular interest are those isotopes which have a single nucleon outside a closed shell. The ground states and, in most cases,.the first few excited states of such nuclei, have been studied experimen- tally and the spins and parities, where known, are in reasonable agreement with Single Particle Model predictions. Many of these states, however, have no spin assignments. Even less is known about the higher energy states ( > 600 keV) of these nuclei and it is obvious that additional data will have to be obtained be- fore any comparisons can be made with theoretical predictions. This is especially true for the antimony isotopes which have 51 protons. In order to study the excited states of these isotopes, it is necessary to populate them by means of the radioactive decay of adjacent isobars. This requires that the parent nucleus have a _ reasonably long half life (> 1 day). The antimony isotopes 1215b, 1238b and 1258b meet this requirement and are therefore well suited for this study. (The isotope 1198b is populated by long-lived 1191.0. However, the parent nuclide could not be pro- ‘ duced by the methods available to us at the time.) In addition to the antimony nuclei, which differ from each other by the addition 1 2 of pairs of neutrons, it is of interest to examine a nucleus having an added pair of protons. Thus, the levels in 1‘71 are studied. This particular isotOpe is chosen since the known low energy level structure appears similar to that of 12le and it meets the requirement of having a long-lived parent. The experimental methods employed in these studies are those of beta and gamma ray spectroscopy. The introduction of more sophisticated electronic instrmnentation and the development of new detectors in recent years has greatly increased the value of these techniques. Two particularly important advances have been the introduction of multichannel-wiltiparameter analyzers for accumulation of large quantities of coincidence data in a minimum of time, and the development of high resolution semi- conductor gamma-ray detectors. The use of these new deve10p- ments, as well as more conventional instruments and methods, in these investigations, makes possible more accurate and complete descriptions of the nuclear level structure of the isot0pes under study than were previously possible. CHAPTER 1 NUCLEAR MODELS Since one 'of the objectives of experimental studies, such as theSe, is to compare experimental data with theoretical pre- dictions, a brief outline of several nuclear models and some of their predictions will be presented. l.A. The Nuclear Shell Model As already pointed out, the low lying states of many odd mass nuclei are described, at least in part, by the Single Parti- cle Model!"5 In this model, which is the simplest form of the nuclear Shell Model ,h’6 the nucleons are considered to move in- dependently of each other in an average static potential, for ' example, a harmonic oscillator potential. In order to reproduce the experimental "magic numbers" (deduced from binding energies, magnetic moments, etc.)h it was necessary to include in the Hamiltonian a spin orbit term, that is, a term proportional to, ‘ sol, which splits the levels having 3 - 1 + g:- and j - ,( - %" . The Sign of this interaction is found empirically to be negative since the states having 3 - 1 4- EL- lie below those with j - X - 3.5:- . The result is a set of levels described by the quantum numbers (n13). For the nuclei being studied here, which have both N and Z between 50 and 82, the single particle states available are the (2d5/2)’ (lg7/2), (2d3/2), (331/2) and mull/2). This is the order suggested by Mayor and Jensen for an unpaired proton. The excited states of the nucleus, in this model, are assumed to be ‘ 3 due to the promotion of the unpaired nucleon to the higher energy single particle states. As may be expected, such a simple model cannot, and indeed does not, explain the more detailed properties of nuclei, even in those isotopes near closed shells. The classic examples usually given are the E2 transition rates and electric quadrupole 7 These are almost invariably larger moments in certain nuclei. than the single particle estimates, often by several orders of magnitude. One model which has been employed in attempting to account for these effects is the "Extended" Shell Model (or Intermediate Coupling Shell Model). The first attempts to extend the range of validity of shell model calculations consisted in the removal of the requirement that the particles move independently by including two-particle interactions. This was first done for particles in a single configuration, namely that giving the lowest energy. This restriction was later removed and "configura-' tion mixing“ introduced. This corresponds to using a wave func- tion which is a linear combination of wave functions for single particle states having nearly degenerate energies and the same total angular momentum. The principle difficulty in applying this model was the complexity of the calculations when more than two or three particles are present outside the closed shell. The usefulness of calculations including residual interactions has recently been extended and will be discussed later. 1.B. The Nilsson Model A second technique which has been applied to explain the 5 large quadrupole effects in nuclei is the use of spheroidal poten- tials. In this case, the nucleons are still considered to move independently but the potential well in which they move is no longer spherical. This model has been develOped primarily by Nilsson (hence, the Nilsson Model).8 In the calculation by Nilsson, the particles are assumed to move in a potential given by vi =- vo{(1 +-2§ 6)(x: + y?) + (1 “13‘- mi} + 0&1 . 51 + mg: . The first term is the axially symmetric spheroidal potential, the second is the spin orbit coupling term as used in the single particle model and the third term is introduced, on semi-empirical grounds, to lower the energy of high spin states. The single particle energies are then calculated as a function of 5 , the deformation parameter. To apply the model to a given nucleus, one must calculate the equilibrium deformation by minimizing the total energy of the nucleus with respect to 6 and then use this deformation parameter, in general different for each state, to deduce nuclear preperties. Calculations of equilibrium de- formations using the Nilsson Hamiltonian, with slight modifica- tions (principally the inclusion of a pairing force and Coulomb effects) have been made by Marshalek, Person and Sheline,9 by - Bee and Szymanski10 and by Szymanski.n This model has had suc- cess at predicting spins, parities and relative level orderings of nuclear states of odd-A deformed nuclei as well. as accounting for the large quadrupole effects observed in the deformed regions. (The deformed regions are presently taken to include the nuclei 6 with A ~25, 150 < A < 190 and A > 221;, and possibly others.)9 1.0. The Collective Model: Even-Even Nuclei Even with the refinements mentioned, many nuclear properties cannot be predicted by shell model calculations. It appears that certain nuclear properties, for example the level structure of many even-even nuclei, can only be explained by the collective motion of the nucleons. This leads to the collective model, in- troduced earlier by Bohr,12 in which the nucleus is assumed to undergo two basic motions, rotation and vibration. For even-even nuclei far from closed shells, the observed spin, parity and ordering of the levels are in reasonable agreement with the re- sults obtained from the rotation of an axially symmetric rotator. Such a model is characterized by a series of levels with angular momenta I - 0, 2, h, 6,.. whose energies are given by 112 EI'z-‘rl-ICIZ-tl) where $1 is the moment of inertia about the "1" axis (the "3" axis being the symmetry axis). The energy levels of a non-axially symmetric rotator have been studied by Davydov and F'ilipov.13 One major difference be- tween the results for axial and for non-axial symmetry is that the latter shape gives rise to 3+, S+,... states, as well as even Spin levels. A relationship which may be applied to test this model is that the sum of the energies of the first two excited 2+ states should equal the energy of the 3+ state. There appears to be some evidence that this model may be particularly useful in the "transition" nuclei between closed shells and regions of large deformation. The second type of motion to be considered, vibrational, has been introduced to account for certain higher energy states and the levels observed in nuclei between the rotational regions and the closed shells. The nucleus is usually assumed to have a sharp surface whose shape changes with time. The quanta of these shape oscillations are referred to as phonons. In the regions near closed shells, the nucleus is assumed to undergo vibrations about a spherical equilibrium.shape, while elsewhere the vibrations will, in general, be about a deformed shape. The'resultant level structure in the Spherical case, assuming only quadrupole vibra- tions are present, will.be a 2+ state at energy”hw, a triplet, produced.by coupling twijhonon , with.spins 0+, 2+, h+ at 2hw, etc. Many spherical even-even nuclei show structure similar to these predictions, although the cases where all three of the 0, 2, h triplet states have been observed appear to be quite scarce. In the deformed regions, vibrations can still take place although the energies are usually'much larger than those of rota- tional levels. In general, the two types of’motions will be - coupled with the result being the construction of rotational bands on each of the vibrational states, much as in molecular spectroscopy. l.D. The Collective Model: Odd-A Nuclei Thus far, we have considered the collective excitations only in the even—even nuclei. Similar excitations can be expected in the odd mass nuclei as well, requiring the coupling of collective and particle states. In the deformed regions, the coupling will be between single particle states obtained using a deformed well (Nilsson levels in the axial symmetry case) and the rotations of the deformed, either axially symmetric or non-axially symmetric, core. This, the strong coupling limit, has been discussed for the 11" The energy level spectrum axially symmetric case by Kerman. obtained is given by l 2 J+ h 2' 1 EJ,K 6K +m[J(J +.l) -2K2 +6K g;a(-) (J+2-)] 3 where CK - single particle energy; J =- total angular momentum; K - projection of J on the symmetry axis ; Jr - moment of inertia. The decoupling parameter, a, expresses the strength of the coupling between particle and rotational motions. The structure is there- fore that of a rotational band built on each of the particle states. This model has been very successful in predicting the levels in odd-A deformed nuclei. The excited states of odd mass nuclei having non-axially symmetric deformations has been considered only recently by Pashkevich and Sardaryan.ls The several comparisons with experi- . ment made by these authors are quite good for nuclei in the de- formed region: A - 25, 150 < A < 190, and A > 2214. Calculations 1195b. The experi- were also made for the "spherical" nucleus mental data are still very incompletefor all but the first few excited states. However, several of the observed states fit \o quite well into the predicted level scheme. Additional spin- parity measurements will need to be made before the applicability of the model to nuclei outside the deformed regions can be ascer- tained. At the other extreme of coupling, that is weak coupling, the nucleus consists of a particle coupled to a core which can be excited to various vibrational states. In general, the lowest lying states will be just the single particle states. At higher energies, the core can be excited to its first excited state, which,when coupled to the single particle state j, will give rise to a multiplet of states having J - J + 2, j + l, ... J - 2. The center of gravity of this multiplet should have an energy 'hw, that is, the energy of the vibrational phonon. Similar re- sults are also possible for higher excitations of the core. Possible experimental evidence for such a coupling scheme has been examined by de-Shalit.16 Intermediate to these situations, the problem is apparently much more complex due to the mixing of single particle states due to the interaction with the core. Calculations have been made, for example, by Choudhury}7 by Glendenningl8 and by ,Bannerjee 19 for specific nuclei with a limited nmnber of particle and Gupta configurations. The applicability of such calculations is not clear at the present time due to lack of comparison with experi- mental data 0 LE. Residual Interactions The models discussed above, engendering both particle and lO collective features of the nucleus, have effectively replaced the particle interactions by a potential which is, in general, time dependent and non-spherical. This potential cannot take into account the entire interaction between the particles and a weak interaction can still be assumed to exist. Utilizing techniques introduced in superconductivity theory, Belyaev20 studied the effect of a pairing force between the particles and found that the inclusion of this force could explain the energy gap (i.e. , the absence of excited states below ~ 1 MeV) observed in even- even nuclei. The calculations were extended by Kisslinger and Sorensonfl (KS) who included, in addition to the pairing force, a long range quadrupole interaction. These two forces have somewhat Opposing effects, the pairing interaction tending to couple nucleons to zero angular momentum producing a spherical shape while the quadrupole force tends to correlate the motion of the nucleons giving rise to collective features (quadrupole vi- brations) in the energy level spectrum. The resultant wave func- tions for the nuclear states are therefore a linear combination of particle and particle plus phonon wave functions. These authors have calculated many nuclear properties for nuclei from nickel to lead, not including the strongly deformed region ~ 150 < A < 190. The agreement with experiment is, in many cases, very impressive. For example, in the even-even isotopes, the . agreement between theoretical and experimental reduced electric quadrupole transition probabilities, 1.6., the B(E2) values, is usually within a factor of two. The predictions for the odd mass, isotopes are even more interesting. The experimental electric ll quadrupole moments of many such nuclei are an order of magnitude greater than thetingle particle prediction, whereas the values calculated by KS are in considerably better agreement. As might be expected, the states which have large phonon contributions are usually connected by particularly fast E2 transitions, in agree- ment with.many of the observed transition rates. However, the agreement between observed and calculated Ml transitions, as pointed out by Geiger, et al.,53 is generally somewhat less imp pressive. Although the discussions given here do not exhaust the various models which have been tried in predicting nuclear levels and their properties, they may serve to indicate the types which have been found to give results hearing at least some resemblance to the experimental data. Of particular interest for the experimental results to be reported here are the calculations of KS. The work by Bannerjee l9 and Gupta, which is concerned with energy levels in the iodine isotopes, will also need to be considered. CHAPTER 2 SOURCE PRODUCTION AND PREPARATION The radioactive isotOpes employed in these studies were produced by either charged particle or thermal neutron irradia- 121 tion of stable isotOpes. The first nuclide to be studied, Sb, - is populated in the 151; day and 17 day electron capture decay of 121mm and 121Te, respectively. These were produced by irradiat- ing natural antimony metal, 57 percent 12le and h3 percent 1235b, with 10 MeV protons and 20 MeV deuterons in the Brookhaven National Laboratory Cyclotron. The powdered antimony metal was packed into a ~ l/h" w. x 1/32" dp. x 3" long slot milled into a water cooled aluminum target and covered with ~ 10 mils of aluminum. Typically, irradi- ations of 200 to 1:00 microampere hour were required to obtain sufficient activity. Another 1211‘s source, produced by thermal neutron irradiation of enriched 120Te, was placed at our disposal by Dr. G. B. Board of Wayne State University. In order to reduce the contamination from other elements (and also to remove the inactive antimony from the cyclotron targets) the sources were chemically purified. The technique was _ essentially that given by Fink, et al.23 125 The levels in 123Sb and Sb were populated by the negaton decay of 125 day 1238n and 9.7 day J‘ZSSn, respectively. These parent nuclides were produced by irradiation of 10 mg quantities 122 121: of Sn and Sn enriched to 91.; to 96 percent. The irradiations were carried out in the ORR reactor at the Oak Ridge National 12 l3 Laboratory for periods of one or two weeks in a thermal neutron flux of approximately 2 x 1011‘ c1112 sec. These sources were chemically purified using a technique given by Newton and McDonnell.2h ' The last nucleus to be studied, 1271, is fed in both the 127 127 electron capture decay of Xe and the negaton decay of Te. However, only the latter decay pOpulates the higher energy states 127 and was therefore chosen for this study. The Te was produced by thermal neutron irradiation of 10 mg samples of 9b persent enriched 126Te in the ORNL research reactor for a period of two weeks. The target was chemically separated using the same tech- nique referred to in the case of 12lTe. The main contaminants in 12th and 110Ag. After repeating the this source were found to be chemistry several times, no trace of these contaminants could be found in the source. Two types of mounts were used in preparing a source for counting: for singles counting on an NaI(Tl) detector, where it is desirable to use a source geometry which is easily reproducible, the source was dried on a microscOpe slide cover glass and mounted in an aluminum frame by means of Scotch tape. These frames fit into an aluminum holder and could be readily interchanged with standard sources mounted in a similar way. For angular correla- tion measurements, liquid sources are used to reduce the possi-' bility of perturbing the correlation (e.g., see Chapter b.D.iii.). Therefore, the sources were contained in thin walled Teflon cups made by drilling a 1/8" x 1/2" hole in one end of a l/h" x 1" length of Teflon rod. A 5/8" length of the drilled and was then 1h turned to make the walls as thin as practicable (approximately 5 to 10 mils). CHAPTER 3 EXPERIHENTAL APPARATUS AND TECHNIQUES 121, 123, 12 The energy levels in the four isotopes sSb 127I were studied using [3- and y—ray spectrometry. The ex- and perimental apparatus and techniques were more or less identical in the four investigations and are therefore described here only. The bulk of the gamma ray singles and coincidence work was performed using NaI(Tl) scintillation detectors. These detectors were commercially packaged (Harshaw Chem. Co.) and were of two basic sizes. For high energy photons (> 100 keV) detectors 7.6 cm diameter x 7.6 cm high were generally utilized. These represent a reasonable compromise between efficiency and resolution and are particularly convenient for use since extensive tables and curves of efficiencies and peak-to-total ratios are readily available.25 Finite solid angle correction factors required in analyzing angular correlation data have also been calculated and measured for such crystals.26 The crystals were originally mounted on Dumont type 6363 photomultiplier tubes using conventional mounting tech- niques.27 The gain of these tubes proved to be very. sensitive to both temperature and counting rate and were later replaced by ,_ EMI type 95783 photomultiplier tubes. The second type of NaI(Tl) detector used was designed to detect low energy photons ( ~ 6 to lOO keV). The NaI(Tl) crystals were 0.6 cm thick 2: 3.8 cm diameter and had windows of either 1 mil aluminum or 5 mil beryllium. These were mounted on either RCA 63h2A or EMI 9536 photomultiplier tubes. Both the large and small detector units proved to be quite 15 l6 temperature sensitive and had to be kept at constant temperature (I 0.50 C). This was accomplished by performing most of the ex- periments with the apparatus inside a styrofoam box (~ 5' x 5" x 5') with the temperature being controlled by a bi-metallic regula- I. tor which operated a lightbulb used to provide heat. This proved to be a satisfactory low budget answer to a very serious problem. Gains stabilized in this way were found to be constant to within 1 0.5 percent over a period of several days. In addition to NaI(Tl) photon detectors, a xenon-methane filled proportional counter (Amperex 300 PC) was used in one ex- periment. This detector has the advantage of reasonable resolu- tion ( ~ 10-20 percent) in the energy range from ~3 to ~hO keV. However, it also has a serious disadvantage in its low efficiency. In the later stages of the experimental work ( ~ Dec.', 1961;), a new type of gamma ray spectrometer became available which has created somewhat of a revolution in gamma (and particle) spectros- copy. These are the semiconductor detectors.28 Thus far, only silicon and germanium crystals have been used successfully, with silicon being used primarily for particles and germanium for pho- tons. Prior to the introduction of the germanium gamma ray detec- tor (designated Ge(Li)) precision energy measurements had to be - made on crystal diffraction spectrometers or in [3-ray spectrome- ters using internal or external conversion. Both methods re- quired, in addition to very expensive equipment, extremely intense sources. The Ge(Li) detectors used in these studieshtypically had a resolution of 5-6 keV for'the 662 .6 keV gamma rays from 137C8 (compared to ~50 hell for NaI(T1)). The efficiencies of such 17 detectors have so far been rather small compared to NaI(Tl) detec- tors because of their small size and low Z. One detector, which has been used here, has a sensitive volume approximately 2 cm diameter x 11 mm deep. The photo efficiency vs. energy curve for this detector is shown in Figure l. The standard intensities were obtained from a similar curve for NaI(Tl).25 In addition to the use of scintillation detectors as photon counters, similar detectors were used to study the electrons emit- 123 12SSn. The detectors in this case, ted in the decay of Sn and were plastic (Naton 136). Two types of source—detector geometry were employed: In the case of l23$n, which will be seen later to have a very simple decay scheme, it was desirable to study not only the energies of the electrons but also the shape of the spectrum. Since considerable distortion can result from back- scattering of electrons incident on the face of the scintillator, the source was sandwiched between two detectors and the unit mounted on an RCA 63142A phototube. The joint between the detec- tors was sealed with reflecting adhesive tape and aluminum foil wrapped around the detectors served as a light reflector. Good performance was obtained with this system down to electron energies 60 29) < 100 keV (as indicated by a study of Co electrons . The second configuration that was used, which is more convenient when only end point energies are desired and some distortion can be tolerated, was with the source external to the detector. Experi? 125Sn ments of this type were used in the study of the decay of with a 2.5 cm thick x 5 cm diameter plastic scintillator with a 1.0 mg/cm2 aluminized mylar window to reduce absorption and to Photo - efficiency 18 6. .b. IOO IOOO , Ioooo' Energy (keV) Figure l. Photo—efficiency vs. energy for 8. 1mm x 2mm Ge(Li) detector. 19 provide good light collection efficiency. The data taken with these detectors were of two types, singles and coincidence. Singles studies, applied here primarily to the study of the gamma-ray spectra, give the energies and in- tensities of many of the transitions but are usually insufficient for determining a unique level scheme. The analyses of the data are quite different for NaI(Tl) and Ge(Li) detectors. The resolu- tion of the NaI(Tl) detector is generally insufficient to resolve the peaks in the spectrum and one must rely on spectrum strip- 25’30 to locate the weaker lines. In the Ge(Li) BPBCW: on ping the other hand, the individual lines are usually well resolved, affording better energy and intensity measurements and obviating, in most cases, the need for spectrum stripping. In order to aid in the construction of the decay scheme, coincidence relations beWeen the radiations were studied. A fast-slow coincidence circuit (Cosmic Radiation Laboratories, Model 801) having a variable resolving time was used to gate the multichannel analyzer (MCA). The MCA used for most of the work (Nuclear Data 150 FM) had a 102).; channel memory and two—parameter analysis capabilities, which allowed the simultaneous study of spectra in coincidence with several gamma ray transitions. In obtaining and analyzing coincidence data, one must usually record two spectra; one including both true and chance coinci- dences and a second containing only chance coincidences. The difference between the two is then taken to obtain the desired true coincidence spectrum. In order to eliminate the need for recording a separate chance spectrum, the analyzer was modified so 20 that an equivalent chance spectrum could be subtracted as the true plus chance spectrum is being recorded. This is accom- plished as follows: The analyzer is normally programmed to "add-l" to an appropriate memory location whenever a pulse is analyzed. However, if the pulse is accompanied by a control sig- nal, the analyzer automatically switches to the "subtract" mode and will "subtract-l" from the memory. Upon carrying out this one operation, the analyzer again returns to the "add" mode. The required control signal is obtained by the use of two coincidence circuits having identical resolving times, but one of which de- tects only chance coincidences. The coincidence output from the "chance-only" circuit is then used to control the mode of the MBA. One of the main problems encountered in gamma-germs coinci- dence studies was the existence of crystal-to-crystal Compton scattering, leading to false coincidences and distortion of the spectrum. In order to eliminate, or at least reduce, this prob- lem, a scattering shield was designed to fit over the NaI(Tl) crystals (used in most of the gamma-germs coincidence work). These consisted of two parts: a lead cylinder 10 on LB. 3: 15.5 cm O.D. x 15.5 cm long, and a lead cone whose inside dimen- sion tapered from 10 cm to 1 on while the outside diameter went from 15.5 to 3 cm. All surfaces were covered by 0.05 cm Cd or Sn and‘0.05 cm Cu to absorb fluorescent Pb X-rays. The two detec- tors were normally placed at an angle of 90° and were therefore separated by ~ 10 cm of lead. Even at 180°, very little scatter- ing could be detected. 21 In addition to providing a clue as to the locations in the level scheme oflines strong enough to be seen in the singles spectrum, coincidence studies usually make it possible to ob- serve additional weak transitions. This was found to be true in all of the decay scheme studies made here. Also, by measuring the coincidence rate as a function of angle between the detectors, that is, by studying the angular correlation of coincident gamma rays, one can usually learn something about the spins of the nuclear states and the character (dipole, quadrupole, etc.) of the radiations. A sumary of angular correlation theory and methods of data analysis have been given previously.31 CHAPTER h EXPERIMENTAL RESULTS - 121 h.A. The Decay of 121To and mTe h.A.i. The Gamma Ray Singles Spectrum 121 123 The gamma ray spectrum from the combined To and Te activities is shown in Figure 2. The source was mounted 10 cm from a 7.6 x 7.6 cm NaI(Tl) crystal, the unit having a resolution of 8.7 percent for the 662 keV photopeak from 137C8. The relative intensities were obtained using line shapes obtained by interpola- tion between spectra from standard sources. These were taken with the same geometry and approximately the same counting rates as were used in obtaining the tellurium spectrum. The peaks at 160 and 211; keV are from known isomeric transi- 123 121T6, respectively.22’33 The 506, 572 and tions in To and 1102 keV lines had been reported by previous investigator?"3 in addition to a 68 kev transition which is obscured by the Compton distribution from the intense higher energy transitions. By plac- ing a graded lead absorber over the face of the crystal to absorb most of the 160 and'2lh keV photons, it was found that the peaks at 730 and 790 keV were due to the accidental summing of the 572 with the 160 and 2114 keV photons. However, the absorbers changed the ratio of the 910 and 1000 keV to 1103 kev peak heights only by an amount consistent with the difference in absorption coefficients of the three photon energies. In addition, these peaks were found with the {same relative intensities, within experimental error, in four 22 23 L IOOO IO 4 O 0 8.223 xtqtk\mtcc what .3300 2 l0 l 1200 . 600 800 ENERGY (keV) 400 200 Figure 2. The gamma ray singles spectrum of lal‘I‘e plus 123%. 2h different sources. 01' the sources used, three were from proton or deuteron bombardments, while the fourth was the neutron produced source. The consistency, both in energy and intensity, with which the 910 and 1000‘ keV lines are found, regardless of the mode of source production and despite repeated chemical separa- tions, is believed to be strong evidence that these transitions are in the decay of 121Te. In addition, the decay of theseurce was followed for several months and all of the peaks , with the exception of that at 160 keV, were found to decay with the same halflife. h.A.ii. Coincidence Studies The spectrum in coincidence with the unresolved antimony and tellurium K-I-ray peaks is shown in Figure 3. Two NaI(Tl) crystals with axes at 90° were employed with the source 10 cm from both crystals. The spectrum was corrected for chance coin- cidences by delaying the signals from one of the detectors. When the singles spectrum was superimposed on this coincidence spec- trum and normalized on the 572 keV photopeak, it was found that the 910, 1000 and 1102 keV peaks were enhanced in the coincidence spectrum by a factor of 1.96 I 0.06. Since the theoretical K to L capture ratio is essentially the same for capture to the high -- energy states as it is for capture to the 572 keV state, this would indicate that the 910, 1000 and 1102 keV transitions are in coincidence with a highly converted transition. In order to give the observed enhancement, this transition must have a K-shell conversion coefficient> 9. Assuming that this transition COUNTS/ 6 60 min 1111111 I0 \ 'comc. WITH 3 \ Sb K-X RAY : \ SINGLES " /\ ~ \ , \ \ x /' '0 :' \ ‘\a_\ I, “j :\ I \ Jr" /’ j \ _ _ . \ .1 s... \ I \ , \ \\\ 1 g .4 r— \ \ ‘\ q _ u / \[x ‘\ . / \ /\ \\ lo I I l l l I ll \I \ l 400 600 800 IOOO |200 ENERGY (keV) Figure 3. K X-ray-garmna coincidence spectrum of 121Te. 26 1215b predicted by the shell is from the low lying 7/2+ state of model and expected from level systematics, one would expect the radiation emitted to be primarily Ml. Under this assumption, 3. K shell conversion coefficient of 9 suggests that the energy of the transition be about 30 keV. It was also found that the 506 keV peak is only slightly enhanced in the coincidence spectrum. This indicates that the low intensity of the 68 keV transition cannot be explained by a high conversion coefficient and that it must therefore precede the 506 keV photon. The 1470 keV region was also found to be enhanced in the coincidence spectrum showing that there is a MC keV transition which is also in coincidence with this highly converted transi- tion. This lends support to the assumption that the ~30 keV transition is from the first excited state since the MO keV will be shown later to be in coincidence with the 68 keV transition and therefore probably connects the 506 and ~30 keV states. 1 The energy of the converted transition discussed above was determined by observing coincidences between 1102 keV photons and the low energy portion of the gamma spectrum. The xenon-methane filled proportional counter was employed as the low energy. de- - tector while the 1102 keV photons were detected by a NaI(Tl) crystal (unless indicated otherwise, the NaI(Tl) detector used is the 7.6 cm x 7.6 cm unit). The detectors were 180° apart with the source 1.0 cm from the proportional counter and 10 cm from the high energy detector. Approximately 0.03 cm of poly- ethylene, placed betwaen the source and proportional counter, was 27 used to stop the conversion electrons from the highly converted ' transitions. Due to the low efficiency of the proportional counter and the high conversion coefficient of the low energy transition, the experiment had to be extended over a period of several days. However, frequent gain checks showed that drifts were negligible ( < 1 percent). The results shown in Figure 1; indicate that the 1102 keV transition is in coincidence with a 38 I 2 keV transition as well as Sb Ks" K -, and L- X-rays at 26.2, 29.7 and 3.6 keV, respec- tively. The Fimproved resolution of the Sb Kx- ray lines is due to the absence in the coincidence spectrum of the Te Kc X-ray. The peak at 6.5 keV is due to Fe K-X-rays arising from scattering in the stainless steel body of the proportional counter and the peak at 11.1; keV is due to a small amount of selenium carrier remaining in the source. The enhancement in the 16 to 22 keV’ region is believed to be due primarily to Compton scattering and, to a lesser degree, to Is L-X-ray escape. The chance coincidence spectrum was found to have the same shape as the singles spec- trum which is shown for comparison. The coincidence data and the 1102 keV singles intensity indicate that approximately 2 percent of the 1102 keV photons _. were in coincidence with 38 keV photons. This is an agreement with the high conversion coefficient suggested by the K-X-ray 3h coincidence data. Monaro, et a1. , have published an account of lifetime measurements on the 38 keV transition which confirms our placing this transition from the first excited state of 1218b. They find the half life of'the 38 keV state to be 3.5 1 0.2 uses COUNTS/5600mm I I r I COINC. WITH ‘- 3 ”03 keV IO :' .___ I 1 2 IO C.— '2': t‘ _ _. 0 .i __ 4.0 _ 6 0 l0 :- j _ SINGLES " q _— .. _ | l l 'l I - , 0 IO 20 30- 40. ENERGY(keV) Figure 1+. Low energy photon spectrum in coincidence with the 1102 keV transition in 1211‘s. 29 and assign the transition to be a probable M1. Beard and Snyder have recently observed this 38 keV transition accompanying the negaton decay of 121Sn and have performed Mossbauer experiments on it.35 A two parameter analysis was made of coincidences between photons in the 25 to 150 keV region and the 7001b 1100 keV region using the 6h,x 16 channel mode of the 102k channel analyzer. Two NaI(Tl) detectors were employed to detect the photons, with the crystals at 90° and anti-scattering shields in place. The results are shown in Figures 5 and 6. Figure 5 shows the high energy portion of the spectrum in coincidence with photons from.80 to 120 keV. The dashed line shows the chance coincidence contribu- tion. Figure 6 is the low energy region in coincidence with photons in the 970 to th0 keV region. These data indicate that the 1000 keV transition is in coincidence with a transition having an energy of 103 keV. The only low energy coincidences obtained with the remainder of the high energy region were with K—X—rays, the 38 keV peak being obscured by the intense X-ray peak. A study was also made of the 150-300 keV region in coinci- dence with the 700-1100 keV region. Any coincidences, if present, -were masked by chance coincidences with the intense 160 and 21h keV photons. The 103 keV photon intensity, determined from the 1000-103 keV.coincidence and 1000 keV singles count rates, was found to be approximately 10 percent of the 1000 keV photon intensity. Since. the 103 keV transition is not highly converted, as evidenced by COUNTS/240 hr IOO IO I 1 I l I 1 I 1 I 700 800 900 I000 II00 ENERGY (keV) Figure 5. The high energy gamma spectrum in coincidence with the photons in the 80 to 120 keV region of the 121Te gamma Spectrum. COUNTS/240hr Figure 6. Spectrum in coincidence with the 970-10140 keV region IOO I I I III 50 IOO ENERGY (keV) of the 1211‘s spectrum. I50 32 the similar enhancements for the 1102 and 1000 keV transitions in coincidence with the K-X-rays, its low intensity requires that it precede the 1000 keV transition. As already noted, the 506 keV transition is in coincidence with a 68 keV transition. However, when the analyzer was gated on the 68 keV region, it was found that the h70 keV region, as well as the 506 keV peak, was enhanced in the coincidence spec- trum and that the relative intensfiies of the h70 and 506 keV I peaks remained essentially the same as in the singles spectrum. A search of the high energy region (> 572 keV) was made, and no coincidences with the 68 could be found as had been reported by other groups.36 The angular correlation between the 68 and 506 keV photons was measured using two NaI(Tl) detectors enclosed in scattering shields. The source, in which the 17 day and 15h day activities were essentially in equilibrium, was in liquid form.and located 12 cm from both detectors. The lead cones used for shielding extended to within 2 cm of the source. The multiparameter feature of the multichannel analyzer was used to obtain an accurate cor- rection for the close lying, intense K-X-ray peak. Small correc- tions for source decay and source asymmetry were made and the - least squares coefficients computed and corrected for detector solid angle.31 The result obtained was W(e) - l + (0.066 1 0.009)P2(cos e) + (0.00 I 0.02)Ph(cos 6). This result is con- sistent with a 3/2 assignment for the 506 keV state and either 1/2 or 3’2 for the 572 keV state. However, the mixing ratio for the 506 keV transition has been determined recently to be 33 (32/141) - +0.29 _+_ 0.09 from angular distribution measurements on 37 nuclear resonance fluorescence. Using this value, it was found that only two cases were possible: a) spin sequence 3/2, 3/2, 5/2 with a 68 keV mixing amplitude 5!- -O.3h 1 0.02; and b) spin sequence 1/2, 3/2, 572 with a 68 keV mixing amplitude 6 - +0.17g: 2 0.03. These give E2 to Ml mixing ratios of 6 - 0.11 I 0.01 and 0.03 1 0.01 for a) and b), respectively. Recent conversion elec- tron measurements38 show that for the 68 keV transition 62 4 ' 0.02, thus the only spin sequence and mixing ratio which is con- sistent with all available data is case b), requiring a spin as- signment of 1/2 for the 572 keV state. The que stion3 9 of possible positon branching in the decay of 1211‘s has been studied by detecting 511-511 keV photon coinci- dences for various angles from 900 to 180°. An extremely weak annihilation radiation was found to exist, since coincidences were found at 180° but not at 90° or 135°. An estimate of the positon branching which would be necessary to account for the observed annihilation radiation was obtained from the coincidence data and found to be approximately 0.003 percent of the total 121mTe decay. The possibility of pair production from the 1102 keV photons was studied and could account for less than 10 percent of the observed annihilation radiation. Attempts to determine which states are fed by the positon decay and to measure the end point energy were unsuccessful due to the extremely low intensity of the transition. 3h h.A.iii. Summary of 121Te Results The results of these measurements on 121To are summarized in Table l. The relative photon intensities given for the 21h,’ h70, 506, 572, 910, 1000 and 1102 keV transitions were obtained from singles spectra with corrections being made for the net de- tector efficiencies and the peak to total ratios.25 The 68 keV intensity was calculated from the 68-506 keV coincidence data and the 103 keV intensity was obtained from the 103-1000 keV coinci— dence measurements. The equilibrium transition rates were determined by making photon emission rate measurements on a source in which the 17 day component had essentially decayed out, so that only a small cor- rection was necessary to obtain the equilibrium transition rates. It was found that the K-X-ray emission rate could not be completely explained by the transitions discussed above and the highly con- 123To. This was to be verted isomeric transitions in 121To and expected to some extent since the K-capture to positon-emission ratio is expected to be approximately'looo for transitions from 121 12 mTe to the 38 keV state of lSb (which is the only position in the decay scheme which can be assigned to the observed positon transition). The electron capture branching to the 38 keV state required to account for the high.X-ray intensity is approximately 116 percent, which is a factor of AIS larger than the feeding ex- Ioected from the positon intensity and theoretical capture to zaositon ratio. This may indicate that additional highly converted transitions are present which have gone unobserved. The decay scheme proposed on the basis of these results is 35 Table 1. Summary of the results of measurements on photons emitted in the decay of the lrlTe isomers. Equilibrium Coincident Relative Transition (c) V geese.) misses. ..... “miss ’ g:X-ray - 11? 21h 1 2 LT. 100 81 1102 i 2 3.1. 1 0.1 2.6 38 1000 f. 5 0.11 _+_ 0.02 .085 (38), 103 910 2‘. 1.0 0.1 _+_ 0.05 .08 (38) 103 1 I. 0.02 2‘. 0.01 .008 1000 38 1 2 20(1)) (h70,910,1000) ' 1102 S72 :3 S 100 65 506 1 S 23 i 1 11.7 68 L70 1 5 1.8 1 0.3 1.16 68, (38) 68 i 2 0.5 1 0.1 0.96 506, Mo (a)Absolute transition rates for the case where the 15k day and 17 day states are in equilibrium and.normalized to 100 decays of 121 15h day mTe. Correction for internal conversion was made by assuming the transitions are pure multipoles of lowest order con- sistent with the proposed decay scheme. Theoretical conversion coefficients were taken from tables by‘Roseho and by Band and Sliwpoa The X-ray rate is the number of K-X-rays obtained per 121 100 decays of 15h d mTe. (b)Based on the assumption that the high.X-ray count rate is due 121 to direct capture from mTe to the 38 keV state of 1218b. (0) Those transition energies given in parentheses are inferred from coincidence measurements with the K-X-ray. 36 shown in Figure 7. The arguments for the given time ordering of the transitions are as follows: i) the 68 keV transition must precede the 506 in order to explain.its low relative intensity.‘ since the X-gamma coincidence results rule out the possibility of a high conversion coefficient. This time ordering is in agreement with the nuclear resonance fluorescence measurements of Metzger and Langhoff.37 ii) Since the 38 keV transition is in coincidence with both the high energy (910, 1000 and 1102 keV) and.the h70 keV transitions, and since the L70 keV transition is also in coinci- dence with the 68 keV transition, the 38 must follow these transi- tions. This is in agreement with the results of other recent investigations?!“38 iii) The low relative intensity of the 103 keV transition suggests that it precedes the 1000 keV transition and that there is direct capture to the 1038 keV state. The spin assignments l/2 and 3/2 for the 572 and 506 keV states, respectively, are required by the results of the measure- ment of the angular correlation between the 68 and 506 keV photons. hl The log ft values given in Figure 7 were obtained assuming the ground states of 121Te and 1218b are separated by approximately 1300 keV.32 This value is from nuclear systematics. The log ft values found for the electron capture transitions to the S72 and 506 keV states indicate that these are allowed transitions and are therefore in agreement with the above spin assignments. The allowed nature of these transitions also indicates that the 506 . and 572 keV states have positive parity. The 7/2... assignment for the 38 keV state is based on shell model.predictions and energy level systematics and is in agreement with the recent lifetime - 296 'V2 l54d // g 0 , ~9 ant 5m // e K: + ' 0 l/2 |7d m 47 Ten“ v 09 52 69 7 9... fl 2 H41 (5’2”? 8 8 0.}, IT ~8l% * : - P (gel-.1) . ., Memo/......) (1 9.11)’ ' 8 2° 948 2'2'2 o ~560(O.2%€.~9.7) m ~650(0.2°/.e,~10) ~ 600(~0-003% 13*, x « o469$£,871 a g? I/2’ \ 575 to a) “ a , '8 ‘° ‘3’ 35.0 an .1 9 F3 '0 v "’730(8|.7°/o6 .63) / ~790(l8.3% 6 ,7.0) / 7/2* :1 r r . r as] 5/2” 3 0 um ' Sb 5| 70 Figure 7. The proposed decay scheme of 121Te and 1211mm. 38 3h measurement of this state by Monaro, et al., and the approxi- mate internal conversion coefficient given above. The log ft values for capture to the llhO and 1038 keV states indicate first forbidden transitions and suggest spins of 7/2+, 9/2+ or 11/2+ for each of these states. The mode of feeding the 9h8 keV state has not been determined and has tentatively been assigned direct feeding from 121mTe on the basis of log ft values. 123 h.B. The Decay of mSn h.B.i. The Singles Spectra The gamma-ray spectra were studied using both NaI(Tl) and Ge(Li) detectors. The spectrum obtained with the NaI(Tl) detector is shown in Figure 8. This spectrum.was taken with a 3 mm thick lead absorber to reduce chance summing of the high energy photons with the much stronger 392 keV linen2 from 11.38m The source was also sandwiched between 7 mm thick carbon absorbers to reduce bremstrahlung from the intense beta radiation. The high energy line has been stripped using a peak shape interpolated from lines in ésin (1115 keV) and 20781 (1063 keV). This shows that the high energy "peak" actually contains lines from two transitions. The energies and intensities of these transitions were - measured with the Ge(Li) detector, the spectrum from this detec- tor being shown in Figure 9. The energies were found to be 1089.1.1 and 1032 :_1 keV, with the intensity of the latter being 0.056 I 0.006 times that of the first. I The small peak at h28 keV is due to 125Sb (from the decay of‘ 125Sn) and the intense lines at 392 and 255 keV are from 113Sn. 39 .mOpompoe flan—”ma o a» ..... donflepeb nmm: mafia As mmfiv :mmmd .Ho Sappoomm hem seesaw moamsfim .m chew?” A>mxv rommzw CON. 000. 00m 00m 00? CON 0. n _. a _ _ _ n u I I \/ H .. 1 \a / 1 I ’ L U \ / n o. 4 / III v 0 O \ n N I 1 1. 1 / \ s L / I O f L H V I c 1 N 1 N u 1 3 n \/ 1 .I .r \ n. o. / e ./ . /\.\\ 1 1:] v. o_x I. N r x n _ _ _ r _ _ . o. to. «03% AfiHVmo find an 5...: w Sufi.» whiwsppo cmmju mama“ Am .313 an ..mmH mo sshpoomm assoc .m gamma ~- b[IP BR and BM .52qu ON 8. LO u 1‘ - ~0- CW2. new NMO. WENNVHD/SiNnOO ____. ...—...- Mm-..— —- " ' ...-'— mmo. 3:. .. L hl The relative intensities of the 1032 and 1089 keV peaks were‘ studied over a period of 7 months and it was found that the two transitions decayed with the same half life, in agreement with 123 the assignment of the 1032 keV transitions to Sn. 14.3.11. Coincidence Studies In order to determine if additional low intensity transi- tions could be detected, coincidence measurements were made. All regions of the spectrum were studied, with particular attention being paid to coincidences with the 160, 380, and ShO keV regions. These are regions in which transitions had been observed in the decay of the low spin (3/2+) isomer of 123 tion studies}D Sn or in Coulomb excita- In addition, the 1032-1089 keV region also re- ceived careful study. In only one case was a positive result obtained, that being with the gate on the low energy side of the 1032-1089 keV peak. "With the gate set on this region, a very weak line was observed at lSS‘:_lO keV. When the gate was moved to the high energy side of the peak, this line did.not appear. These results are interpreted as indicating a coincidence between the 1032 keV transition observed in the singles spectrum and a lSS keV transition having a relative intensity of approxi- mately h percent of the 1032 keV intensity; The intensity was deduced from singles and coincidence counting rates and tables 25 Since the of detector efficiencies and.peak to total ratios. 155 keV transition is not highly converted, as indicated by the absence of K-X-ray-gamma coincidences, this low intensity suggests that it precedes the 1032 keV transition, thus requiring a state at oullB? keV. h.B.iii. Beta Spectrum Studies Although the shape of the transition to the ground state 123 of Sb had been meaSured and found to be consistent with AJA" - 23,83, no information was available on the {5 transitions to the excited states. It was soon evident from the measured relative intensities of the gamma transitions that only the 1089 keV level was populated with sufficient strength to allow one to make any meaningful beta-gamma coincidence measurements. Due to the high relative intensity of the ground state beta transition, very weak sources had to be used in the beta detector. The hn electron detector described in Chapter 3 was used to reduce the effects of backseattering. The gamma detector was a 7.6 omflx 7.6 cm NaI(Tl) crystal at 1800 with the plastic scintillator. A 3 mm thick graded lead absorber was placed over the face of the NaI(Tl) detector to reduce the number of backseattered photons entering the two crystals. The spectrum obtained was corrected for detector resolu- hS \v tion“ and a Fermi-Kurie plot obtained. This plot is shown in Figure 10 and indicates that the spectrum has an allowed shape. This would indicate that the transition is allowed or non-unique first forbidden}5 The energy calibration was obtained from Compton edges of several standard gamma transitions and 113Sn " K-conversicn electrons from the 392 keV transition in (gating on the K-X—ray in the NaI(Tl) detector). The end.point of the spectrum was found to be 330 ILlO keV, in good agreement 0C9 omma aw cowfimcshv «Elam >ex mmoa one no? eoqeewocfioo 5 fifipommm coopoeao .wo wean mflsxnficfiom .OH euzwwh A>mxv soumcm 00m OON 00_ a — _ _ O his i In I ,l N / 4.. 1. 1m. d M 1.... n [©— hh with the 1h20 keV end-point energy that has been reported for the ground state transition and the 1089 keV measured for the gamma ray energy. In order to check the reliability of the scintillation 1235n spectrometer in reproducing the fi-spectrum shape, the was replaced by a 6000 source of comparable intensity and thick- ness. This source is very convenient in this case since it has an allowed p transition, with approximately the same endspoint energy29 ( ~13lh keV) as 123 Sn. The 5 Spectrum was recorded in coincidence with the 1173 keV gamma photopeak and a Fermi- Kurie plot of the spectrum.was again found to be linear with an end.point energy of 306 1.15 keV. This is taken to indicate that the 123 Sn results are correct. The relative intensities of the 5 transitions were de- termined from the ground state beta singles intensity and the 1089 keV gamma intensity using the known efficiency of the gamma 25 ' detector and assuming 100 percent efficiency for the B- detector. It was found that only 0.6 percent of the 1235n decays go to the 1089 keV state. From the gamma ray relative intensities given above, it was concluded that only 0.0h percent and 0.001 percent of the {3-decays take place to the 1032 and 1187 keV levels, respectively. The log ft values)41 obtained from these intensities are 9.1, 10.5, 9.0 and 11.3 for transitions to the ' ground, 1032, 1089 and 1187 keV levels, reSpectively. These values suggest a first forbidden character for these 9- transitions, which is consistent-with the shape measurement on the hS transition to the 1089 keV level. h.B.iv. Discussion of Proposed Decay Scheme The results described in the previous sections have been utilized to construct the decay scheme shown in Figure 11. The 123Sn parent and l23Sb spin and.parity assignments for the ground states have been taken from previously reported measure- ments.h6 The assignment of a spin of 9/2 or 11/2 for the 1089 keV level is suggested by the allowed (or non-unique first for- bidden) shape of the beta transition to this state and by the prompt decay to the ground state. The only evidence for assign- ment of spins 7/2 through 1l/2 to the 1032 keV state is the log ft value for the beta transition to this state and the presence of a prompt ground state gamma transition. The spin of the 1187 keV state may be > 11/2, deduced from the absence of a ground state transition. However, no positive information is available on this level and therefore no assignment has been made. The parities of “these states are suggested to be positive on the basis of 10g ft ‘Ualues. Included in Figure 11, for comparison, is the decay scheme 0x 50..” any one mmoa 63 new; 005039.80 E.” nepoep0c 28:02 .0 as? 3.50090 sapoemm .3” 0.3m?“ 38: 320cm 53 com. 000. com ooo 00¢ 08 08. 000. com com oov cow _ a a i _ _ i a a a _ _ I ll L H H H m , ..- 7 .1. 7 I- L F 1.. l n H H H H u a 1 1 H 02.8.1728. $0. 2: H 0250.013; $0. 3. H 1| Jl I. fil IT ..I 7 _ _ _ _ P _ 11 _ L _ _ _ _ 1 O. 0. 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C 1.1 l w .1 . 11 . 11 11 1.1 W 1 11 1 1 11 1 U 1 11 0.. 1n .1 1 e 1 11 11 oo: L1 11 1 ..1 - _. 1 .. .- -- . o. nnv 11 11 1 1 z o... 11 1 11 1|. . 1|. .11 11 1 1. 00... 11 our 11 1 11 o... 11 .0250» 11 .0250» 11 .0250» 1 1 11 11 >3. 0:. :3 11 >3. 9:. 11 31.31.93 1 1. .o. 11 .3 11 .8 11 .3 11 .3 .1 H ”1.1. 11 11 11 1 _ _ _ .11. _ .11. r .11 . .1 o. 56 unresolved 332 and 351 keV transitions. Chance corrections were made as mentioned above. Corrections for underlying Compton distributions were made by subtracting coincidences obtained with regions slightly above and below the peak being studied. This coincidence spectrum shows that, even after these correc- tions, a weak coincidence between the 11120 and either 332 or 351 keV transition still remains. Part (b) of Figure 16 shows the spectrum in coincidence with the 1170 keV photopeak. Here it can be seen that, in addition to the previously observed 1170-1089 and 1170-11120 keV coincidences, there is a very weak line at approxi- mately 1800 keV. In order to study this coincidence more thor- oughly, the gate was moved to the low energy side of the 1.170 keV photopeak, giving the spectrum shown in Figure 16(c). Here the ratio of the height of the 1800 keV peak to that of the 11.120 keV peak is obviously enhanced over that of part (b), indicating that. the transition in coincidence with the 1800 keV photons has an energy flightly less than 1170 keV. In order to confirm this con- clusion, the analyzer was gated on the 11.120 and 1800_keV regions separately, giving the spectra shown in parts (d) and (e), respect- ively, of Figure 16. Here it was observed that, whereas the peak in coincidence with the 11120 keV transition came at 1.170 keV, the " peak observed in‘coincidence with the 1800 keV region was shifted down to 1435 keV. This coincidence, which had not been reported previously, is in good agreement with the appearance of the 1806 keV peak in the singles spectrum. The fact that a 1135 keV peak is not observed in the Ge (Li) singles spectrum is. not surprising since the intensity of this transition was found to be only 1 I 57 approximately 50 percent of the intensity of the very weak 1806 keV line. With this intensity, the peak height would have been much less than the statistical fluctuations in the 1135 keV region of the Ge(Li) spectrum. Multi-parameter coincidences with the 272, 332 and 1170 keV regions with photons between 700 and 1200 keV were also studied 8 using two NaI(Tl) detectors. ,Spectra obtained in coincidences with these regions are shown in Figures 17 (a), (b) and (c), respectively. The weak coincidences of the 272 keV transition with the 915 and 1075 keV regions have not been reported pre- viously. The fact that the 1075 keV region is somewhat enhanced over that of the 915 may indicate an incomplete subtraction of the coincidences with the partially unresolved 332 keV peak. However, the absence of an 822 keV peak, within statistics, indicates that the correction for the underlying Compton distribution was reason- ably accurate. The spectrum obtained in coincidence with the 332 keV peak is shown in part (b) of Figure 17. It was found that, in addition to the previously observed 1089 keV coincidence, peaks at 822 and 1017 keV were also evident. Again, corrections for coincidences with the strong underlying Compton distributions have been made and the absence of a 915 keV peak shows that the correction was accurate. Unfortunately, the 332 and 351 keV peaks are unre- solved so the peaks in the 822 and 1017 keV regions may be in co- incidence with either or both of these transitions. . The 820 and 1017 keV peaks are reduced by a factor of approximately 5 over the 1089 keV peak, indicating that only a fraction of the 332' plus 53000st 051.30 cm 05 0o mcowwoh >00. 8... 05 A00 mma 080 Hmm mad“ mmm T5 «NE 03 0%. co 0:300 ho. vocwwpno 0.50090 00.00.030.60 .0...” 0.9000 322. >820 58 ... 0.0 50 ... 0.0 5.0 _._ 0.0 5.0 _sna _ ..s... ....o_ 1 1 , 1 1 o. N O O n w. ..I 1 I 1 s l 0 11 11 1 u- D u l. .1 I u m. m >Hm m c -- < / ... 1 02.001,» 1>0x Ohv 3.1 02.00» )3. _mm+Nmm 3. 1 .02_00b 1>0x CNN 3.1 1 IL . F . _ _ . . _ . . _ _ _1 59 351 keV photons are in coincidence with these transitions. The coincidences obtained by gating on the 1170 keV peak, shown in part (c) of Figure 17, are identical to the results ob- tained in previous studies, and shows only the strong 1089 keV peak. The enhancement seen in the Compton valley is due to the Compton distribution of the 11120 keV photons. h.C.iii. Beta-Gamma Coincidence Studies Several studies of beta-gamma coincidence relations have been made to help confirm the location of a number of transitions in the decay scheme. The beta detector geometry utilized an ex- ternal source, as discussed in Chapter 3, while a NaI(Tl) crystal was used to detect the photons. The observed end-point energies of beta rays in coincidence with the 332, 1170, 822, 915 and 1089 keV photons were: (1150 plus 9110), (1150), (1120, (350) and (1300 plus 500) keV, respectively. Due to the inaccuracy of the calibra- tion curve, obtained by using Compton edges of known gamma rays, an error of ~50 keV is assigned to each of these end-point energies. The end-point energy obtained from the beta singles spectrum for the ground state transition was 2360 i 50 keV. These values are in 117 good agreement with previous measurements. 11.C.iv. Construction of a Proposed Decay Scheme The decay scheme constructed on the basis of these studies is shown in Figure 18. Many features are quite similar to those proposed previously.b’7 The location of the states at 1089.0, 1890.6 and 1982.0 keV are fixed by the beta-gamma coincidence data obtained both in this and other studies. In addition, beta-gamma 6O 70,0.l%,6.4 22”) 90.0.1%.6.o 372.0 a/ ‘ 120.0.2%.7.o 2199.7 1eo.9 360..9'%.7U.\ 200200 ”0.1.2963. 1902.0 7/2. I II: 470.2.0%.0.0 IOOOJ 7/2.|l/2 sso.1o 1000.9 940.03%,93 MID-I 9/2 IOI0,0J%,IO.3 (1350.3) 1270.1.3%.9-7 1039.0 1/2. 9/2 5/2 . 7/2 .9/2 (1017.2) 1290,l l _— (1017.21 2360.94% .a.9 0 7/2" l25 S b . 125 - . th Figure 18. Proposed energy level scheme for Sb as seen in e decay of 9.7 day laSSn. 61 coincidences with the 2002.0 and 1hl9.8 keV transitions have been reported and.determine the position of these transitions. Since no gamma-gamma coincidences could be observed with the 2276, 2200, 1890 or 1350 keV transitions, these probably go to_the ground state. The high intensity of the 1067 keV gamma indicates that it also is a ground state transition. The assignment of levels at 1017 and 1350 keV, which is _only tentative, is based on the observation of the 1017 keV transi- tion in coincidence with the 332 keV region and assumes that a 333 keV transition, which would be unresolved in the Ge(Li) spectrum, exists between these states. The presence of such a transition is also indicated by the fact that the energy differ- ence between the 1h19.8 and 1089.0 keV levels is 1.1 keV less than the 331.9 keV obtained for the transition between these levels. ‘This is believed to be outside the precision of the present measure- ments. (It seems unlikely that the proposed 333 keV transition is 125 the same as the transition from the first excited state of Sb 125Sn. This has been seen in the decay of the 9 min activity of reported to be 326 keV.h7 The latter value should be accurate to better than h-S keV while the separation of the 1017 and 1350 keV peaks obtained here is accurate to better than 1 keV.) The level at 22h0 keV is suggested by the 1151-1089 and 1173-1067 keV gamma-gamma coincidences. The enhancement of the 822 keV region in.coincidence with the 330 keV region_can also be explained by assuming that the 351 keV transition connects the 22h0 and 1890 keV levels, thus adding additional evidence for the 22h0 keV state. 62 A level at 2288 keV has been preposed to explain the existence of the 1221 keV transition in coincidence with the 1066.9 plus 1089.0 keV photopeaks. The 1221 keV transition is believed to proceed to the 1067 keV state since a very broad peak is produced in the 1173-1221 keV region when gating on the 1066.9 keV photons. This is compared to the much narrower peak obtained in this region when the gate is set on the 1089 keV photOpeak. A 913 keV transition connecting the 2002. 0 and 1089.0 keV levels has been proposed to explain the existence of the 1089- 915 keV coincidence, which was found to be much too strong to be due solely to. the weak, unresolved 893 keV transition. Such a transition can be expected since the 9311 keV transition to the 1066.9 keV state was observed and it appears, from the angular correlation studies to be described later, that the 1066.9 and 1089.0 keV levels may have similar spins. This assignment is also supported by the fact that the intensity of the 915 keV peak, relative to that of the 822 keV peak, is reduced by ap- proximately 10 percent in coincidence with the 1066.9 keV transi- tion. In addition, such a transition can explain the existence of the 272-915 keV coincidences. The 76 and 811 keV transitions are placed between the 2276 and 2199.? keV states and the 1890.6 and 1806.9 keV states, respectively, on the basis of energy differences. One rather'large discrepancy still exists in the energy measurements. The 821.8-1066.9 and 799.7-1089.0 keVl'cascades both add up to 1888.7 keV, whereas the energy measured for the transition which is assumed to proceed to the ground state is 63 1890.6 keV. This may indicate that additional levels exist near 1890 keV. One possible test would be the detection of 351-1890 keV coincidences. Unfortunately, this coincidence is extremely weak and no conclusions concerning its existence could be made in this study. 111 Also shown on the decay scheme are the lag ft values for _beta transitions to the various levels in 125 b. The relative beta intensities were determined from the beta singles and beta- gamma coincidence spectra in conjunction with the gamma ray in- tensities. These log ft values suggest that the transitions are probably first forbidden, which, in conjunction with an 11/2' 125 assignment for 9.7 day 3'1, suggests positive parity for the states in 125 125 Sb which are populated in the beta decay of 9.7 day 14.0 .v. Angular Correlation Measurements The angular correlations between all of the prominent gamma ray cascades were measured using two NaI(Tl) detectors and the coincidence circuitry discussed in Chapter 2. The detectors were enclosed in anti-scattering shields and care was taken to keep the source-detector geometry such that the detection solid angle was defined by the NaI(Tl) crystal and not by the opening in the lead cone of the shield.. Coincidences were recorded on the multi- parameter analyzer in 115° steps from 90° to 270°. In this manner the analyzer could generally be used to measure several correla- tions simultaneously. Equivalent chance coincidences were sub- tracted automatically and corrections for underlying Compton 6h distributions were obtained.from coincidences with regions above and below the photopeak being studied. The results, after cor- rection for source decay and finite detector size, are given in Table 3. Many of these correlation functions are limited in accuracy by the presence of unresolved transitions or by intense Compton distributions. For example, the 332-1089 and h70-1089 keV corre- lations had to be corrected for the Compton distributions due to the 822 and 915 keV photons arising from coincidences with the unresolved 1067 keV photopeak. These corrections were made using the technique mentioned above. Similar corrections were necessary for the 332-h70 keV coincidences due to the presence of the 1067 and 1089 keV transitions. Even.more serious than the Compton correc- tions is the existence of unresolved photopeaks as is the case for both the 332-1089 and the 822-1067 keV correlations. The 332-1089 keV coincidences contain approximately an 18 percent contribution from the 351-1067, 1089 cascades while the 800-1089 keV cascade contributes approximately'ZO percent to the 822-1067 keV correla- tion data. Un ertainties introduced by these unresolved transi- tions in the A2 coefficient of the desired correlation function were estimated by assuming a function of the form 1 +.A2P2(cos9), withLA1< 0.2, for the unresolved cascade. This appears to be a reasonable assumption in view of the magnitude of the correlation coefficients obtained for other cascades in this nucleus. The co- efficients and errors given in Table 3 for the 332-1089 and.822- 1067 keV cascades include the corrections thus deduced for these correlation functions. No corrections were made for the Ab .0nm>00 use 0wm>00.mn :0rww 0am m0sH0> 000:9 00 A0 a .8210. mad“ amo.ouamo.01 Hmo.onsma.o+ ease o.amoa1e.amaa mHo.qnwoo.o+ mao.0MWNH.o+ aso.eunmo.o+ aoo.0Homa.o+ m.eooa1a.maa mao.qumao.o+ mao.eunma.o+ 0:0.0H0No.o+ omo.onowa.o+ a.oooa1w.amw Hmo.quoao.01 emo.ouomo.01 040.0H0No.o+ omo.ouo~o.01 o.mwoe1a.amm nmo.onmmo.o+ omo.oum~o.o+ m.maea1o.mea oeo.quoNo.O1 amo.qumao.01 o.amoa1o.see sao.ouwoo.o+ mso.onwso.01 Hmo.onsmo.o+ amo.ouomo.01 a.Hmm 10.no: a m e N Am 4 am < 4 < 0000000 .90 scum mfiovonm so 00:05099000E mowwmaohmoo hmasmce mo mmmssdm .m 0Hnma I, CC) coefficients since the experimental values are very small; there- fore, these values are not to be considered very reliable. The h70—lh20 and 915-1066.9 keV cascades are relatively free from interfering radiations and no such corrections were made. The 9lS'keV peak does contain a small contribution from the 893 and 3h keV transitions (and a possible 913 keV transition proposed above) but the net contribution is less than 10 percent and should have little effect on the measured correlation function. ' No attempt was made to analyze the 1151-1089 and 1173-1067 keV correlation data since the two are completely unresolved and have approximately the same intensity. Also included in Table 3, for comparison, are several coef- ficients obtained by Devare and.Devare.So The values obtained for the h70-332 and 915-1067 keV cascades are in good agreement; those for the 332-1089 and 822-1067 keV correlations are in better agreement if comparison is made with the coefficients obtained.in' this before correcting for unresolved.peaks. These'were 1 - (0.0h6 : 0.029)P2(cose) + (0.01h : 0.036) Pb (cose) and l + (0.15? i O.OlO)P2(cose) + (0.02 i 0.02)Ph(ccse) for the 332- 1089 and 822-1067 keV cascades, respectively. ‘ The large uncertainties introduced by these corrections make it impossible to find a unique set of spin assignments for the nuclear levels and.mixing ratios for the transitions between them. In an effort to gain some insight-into the nature of the transitions, the following spin assignments, obtained in.previous h7,So studies, have been assumed. The ground state and 1089 keV excited state have been assigned spin-parity7/2+ on the basis of 67 beta spectrum shape measurements and arguments presented above concerning the measured log ft values. The beta transition to the thO keV state has also been studied50 and the results indicate that the transition has a statistical shape, suggesting that the transition is either allowed or non-unique first forbiddenJ"5 The log ft value is in agreement with the latter assignment which suggests a 9/2+ or ll/2+ spin-parity assignment for the lh20 keV state (higher values being ruled out by the strong ground state transition). Under these restrictions, the ll/2 assignment for the lh20 keV state can be ruled out on the basis of the h70-332 keV and the h70-lh20 keV correlation functions since, for this assign- ment, the two functions would be identical (assuming negligible octupole contributions). The possible spins for the 1890 keV state were limited to 7/2, 9/2 or ll/Z since there is a beta branch (probably first forbidden) to this state. This limits the possible spins to 7/2 through 15/2, and a transition to the 2/2 ground state, which suggests spins assignments 3/2 through ll/Z. Only the 7/2 and ll/2 values were found to be consistent with the angular correlation data. The two remaining spin sequences for the 1890, luZO, 1089 and ground states, 7/2-9/2-7/2-7/2 and ll/2-9/2-7/2-7/2, were analyzed using the double mixture curves of Arns and Weidenbeck.31 The re- sults for the first sequence suggest quadrupole mixtures of O to 12 percent (-), 99.5 to 100 percent (-), 6 to 12 percent (-), and 99.5 to 100 percent (-) for the 1089, 332, h70 and lh20 keV transitions, respectively. In the second case, the respective 68 mixtures are u to 10 percent (-), 99.7 to 100 percent (-), 15 to 3h percent (+) and 99.5 to 100 percent (-). The signs in.paren- theses refer to the signs of 6, the mixing amplitudes, with the sign for the 332 keV transition being for the case where it is the) 51 first transition in a cascade. This sign.must be reversed when analyzing the h70-332 keV correlation. Analyses of the 822-1067 and 915-1067 keV cascades were at- tempted.but only a limited amount of information could be deduced. Possible spin assignments for the states involved were limited by ‘ the following arguments: the 1982 keV state could be limited to 7/2 through 15/2 by the existence of a first forbidden (from the log ft value) beta transition to this level while the values for the 1890 keV state were limited to 2/2 and ll/2 by the analysis of. the h70-332 and h70-lh20 keV correlation functions given above. The possible assignments for the 1067 keV state were limited to 3/2 through 11/2 in view of the prompt decay to the ground state. With these restrictions, it was found that the only spin assign- ments consistent with the 915-1067 keV correlation function were 7/2 or ll/2 for the 1982 keV state and 5/2, 7/2 or 9/2 for the 1067 keV level. The latter values are also consistent with the 822-1067 keV correlation function. The quadrupole mixtures in the 822, 915 and 1067 keV transitions could.not be determined since nearly all values were found to agree with the correlation data. 69 127 h.D. The Decay of 127Te and mTe . . 1° h.D.1. agma Ray spectrum of “YTe Isomers 127 The Y ray spectrum of the Te source is shown in Figure 19. The tsp curve shows the spectrum obtained.with a NaI(Tl) crystal having approximately 8.5 percent resolution for the 0.662 Kev line from 13703. he bottom curve shows the spectrum obtained using a Ge(Li) detector. Singles spectra were recorded over a S'month.period and it was found that the peaks at hSB, 69h and 728 keV decayed with a half life of approximately 30d 129 and are believed to be due to Te. By comparing spectra, it was found that the 572 keV line is due to a strong transition in 321% and that the 159 keV line is due to 123 32 127 Te. The remainder of the peaks are due to Te. Of particular interest are the two at 591 and 657 keV. Only the latter transition had been.previously observed.52 The lower energy transitions have all been previously, observed, although the 203 and 21h.keV peaks had.not been resolved. 12? The relative intensities of the gamma rays due to Te are given in Table h. The th keV peak is not observed in the singles spectrum due to its low intensity. However, its presence has 52 been established in previous studies and in coincidence studies discussed below. h.D.ii. Coincidence Studies Coincidence measurements were made using two NaI(Tl) crystals. These detectors were separated by 900 and enclosed in lead shields to reduce the effects of crystal to crystal Compton scattering. Copper absorbers 0.25 mm thick were placed on the TO H J« 4. _ ‘- « a i:: 2:. : 3: i; i .7. w 7: t. .. . .fi 4.4. ._ . rm.” 4. M .g "__u“. _. . . ~ _- ”ii: 6. _ Q _ -- g-LL 1- r4..-_.-.- a Meal- : --.l -. 3-.“ .. - .4- ... -- :x 1 _- 4 . Ti: 4 F; M _ g . _ _ c: : :2 J _ .m H _. . w_.m_m_ _____ m “ :__ i f.” n m.__ L: i . J; _ ._,._ it»-.. . x .- In-t -:|1v+h+_xi rL :ipl- .. ..H . h- .. . . L a - ...; ..-}. 3 ii: . . __:M_ __ i _ 4 _W .. . _ ...m a. i._: .. _ ._ ._.. _ . _ .MW.W.. “m m M . :4 “WWW w i _ . H. u _ m“ . . . N... m“. ... :i.. 4 4m ” m :0 . _ .:4 Wm _ _ w M u ti? f- C Flt-FILL..- riiv Lu? hie-+1. 131* 4. +.. :._M . i. _ _ _ ... _ i .3. u. . m . _ . T1“ _ m. _ .1.“ _ M _ mil . i “L .7 . _ y . a _ .7 .._ . ___ _ _ : . .i . M _ . _t_ _m_ .__ :1 . H“ n ,. __ .. . l _J . ... .“ .. _ _. . .IT. - iii-Afl‘ E. I. . m ...... f . 4. 711- .l .- :4 ci+ _. if.-- -.iwltilufi. -. El. . J -I- - i . : _. ”w. _ _"Tm.. 3.: a. M ____ 4f 0 __ u n _. a. “I... w H m... . mm . . . _. .:j __ .5 . . g g . .u _ 4d _ fl _ . . . :m4__ W . “u l :q _ . .. _ _ as; . i i: 2 . 3 _ i _ .L: .. .W- 4.2-2; , L .uei-:,-.---..-:C.-_« a W _ .i W. 3:? ”:14 m . _ ELM. :_ . _"_._. . ... in“ . ”_ ... 5:. 5...... . 5:. 2 .“ ...i_--.-.t--T inw.__o.a » a ...J _ . . r :3... . 2:74 . 0 g.” l i: i rim” ii”. l :4 . r. m L; n . ..: .. I“... :__i_~ _ M. :m . _ _ _ .: eI+-+-IT+-i|£-f_ i w i??? m .im . W “WmW”m . _“Mmmwm .7 H miff. 3:: .i_.. _ M: ._ w _H._:l u__~ . W..* _ _w ..i 0 F: _ l f_ B? _ “Clrr 0 3 2 IIO IO ' l0 ENERGY (MeV) ‘ Figure 19. Gamma ray spectra of 1271's taken with a (upper curve) 7.6cm x 7.6cm NaI(Tl) crystal and (lower curve) a hmm x 2cm Ge(Li) detector. 71 127 Table 1;. Summary of data on photons emitted in the decay of Te. We . it“; .....- Energy (Mafia) Intensitya) Ratesb) ‘ (I-ieV) 0.0576+0.0005 61+1 0.89 0.115, 0.2114 " - "' 0.360, 0.591 0.657 0.087:0.001(I.T.) 25:1 99.2 0.1hS_—=;0.005°) 0.51:0.06‘3) 0.0023 0.0576, 0.211. 020310.001 S.h_+_0.2 0.018 0.211; 0.21h+0.001 3.9+0.2 0.013 0.0576, 0.115 " ' 0.203 036030.000; 1h.8i0.1 0.01.65 0.0576 0.1.17io.0005 100 0.313 0591:0001 0.22:0.0‘4 0.00062 0.0576 0.657:0.001 1.h3«_r_0.06 0.001.31. 0.0576 8‘)These data, with the exception of the 0.116 MeV transition data, were obtained from the Ge(Li) runs. b)Nmnber of transitions, photons plus conversion electrons, per 127 100 disintegrations of mTe. Corrections for internal conver- sion were made using the conversion coefficients calculated by to Rose assuming the lowest multipole order for the transitions consistent with the preposed decay scheme. c)Energy and intensity obtained from coincidences with the 0.2114 MeV transition. 72 faces of the crystals to reduce the intensity of the strong K-X-ray. The coincidence spectra 0 tained between the 0 to 100 keV and 70 to 720 keV regions are shown in Figure 20. The data were recorded on the multiaparameter analyzer operating in the 6h channel by 16 channel mode. Part (a) shows the spectrum seen by the first detector in coincidence with 57.6 keV photons entering the second detector. The 591, as well as the previously Observed 360 and 657 keV transitions can'be secn.l These results suggest the presence of excited states at h17, 6h9 and 715 keV. The lh5 and 21h.keV lines, expected on the basis of previous measure- 52 are too weak to be observed above the Compton distribu- meats, tion of the strong 360, 591 and 657 keV photopeaks. Similarly, parts (b), (c) and (d) are spectra seen.by the second detector in coincidence with the 360, 591 and 657 keV photons seen by the first detector, respectively. Parts (c) and(d) have not been corrected for the overlap of the 591 and.657 keV photopeaks. They serve only to indicate that there is no appreciable broaden- ing of the 57.6 keV'peak in coincidence with the 591 keV region as ‘would be expected if there were a transition connecting the 715 and 6&9 keV states. The spectrum in coincidence with the unresolved 203 and 21h keV transitions is shown in Figure 21. This spectrum.has been corrected for chance coincidences butnot for coincidences with the. underlying Compton distribution. .Any such correction would.have limited accuracy since the 203-21h keV peak lies so close to the Compton edge of the 360 keV Compton distribution. Coincidences 73 .3893 sex Rm A3 e8 an 3 6% A3 6.1% as so? 883866 5 emcee manna no «seamen .8 muse-rm $2): >ommzm . _ moo eoo moo eoo moo eod me he we no we no No .5 4 00. O. 31:13 .LN000 0. no. o? )4. COUNT RATE *0 0.: 0.2 0.3 A ENERGY (MeV) Figure 21. Coincidence spectrum obtained by gating 0n the unresolved 203- - 1 . , , . . 1 . 211+ hell nnoccpeexs in ten 2Y‘l‘e spectrum. 4 75 were recorded at both 900 and 1800 and an average of the two sets of data used to obtain the approximate relative intensity of the lh5‘kev transition. The 57.6 keV peak is also visible, but an accurate intensity measurement could.n0t.be made due to the large contribution from the Compton distribution of the 360 keV transi- tion. The peak at approximately 86 he? is due primarily to back- scatter of the 203 and 21h.keV gamma rays. These results suggest 127 127 To and mTe decay to excited states at 57.6, 203, L17, 127 that 6h9 and 715 keV in I. h.D.iii. 127 Te.Angular Correlation Measurements .Angular correlation measurements were made using the same experimental apparatus as in the coincidence studies. The multi- parameter feature of the multichannel analyzer was again.utilized in correcting for underlying Compton distributions. Coincidences were recorded at angles of 90, 135, 180, 225, and 270 degrees be- tween detectors and corrections made_for source decay and source asymmetry. The resulting correlation function coefficients were corrected for finite detector size.26 The results are given in Table 5. The source used to obtain the coefficients for the cas- cades invOlving the 57.6 keV transition had to be kept very dilute since it was observed that the anisotropy of these correlations was strongly dependent on the source density. .A ser'-quantitative study of the effect of source density was made by adding or evap- orating conc. HCL to change the volume of the source. The volumes used ranged.from approximately 0.02 cm; to 0.1 cm; and contained.approximately 51mg of source material. The measured 76 .epme defipmaoamoo headmom moons msfinthsm cfi poafiawps was powwow mp oo>fiw one: mm ahaobflpoedmoa amassed”... so: named es note 23 so meooéflmomod- Es mo.owmm.o+ do 8103393 agenda? Swab-Q: 8.630.? .8 5.03%?- mksmbsfim mod-mad? . ... .- .8 8.6.3.6- Nanak-uh _ 36.6.266- ~.mo.o+w8.o- Sued-Red Sew-aim. .8 2.63%? {Tab-NR 06.? . a- weak-Nb- madumdoi. 8 dug? shod-Rm .0 Loan .- I aN.N- as mo.o+ma.o- mxm-m\a-mxm mo.o+oo.o No.o+mm.o+ 6emo.o-oem.o 68 A .8 genome... N\m..m\m-N\m moo.o.+-moo.o.. mood-$8.0. 8.5.6386 1 ... . )-..i .) .3 N .1 .. - AmAH «enemy oosczeom . 4 ¢ N meadoxa assoc mes fies-a manna Sum memomwo .maopomm ea so assessesmeee.cowpmamaaoo anSmmm Ho hamsssm .m magma NNH 7? anisotropy varied from 0.33 :_0.08 for the smallest volumes used to 0.58 1 0.08 for the largest volumes. The larger value is be; lieved to be approximately the unperturbed value since the an- isotropy was essentially constant for the more dilute sources employed. The possibility that this effect is due to increased small angle Compton scattering at the higher source densities was investigated by placing a 3 mm thick aluminum absorber around the dilute source. N0 attenuation of the anisotropy in this and other similar experiments was observed. The attenuation of angular ‘correlation anisotropies in liquid sources is believed to be due to the interaction between the electric quadrupole moment of the nucleus in its intermediate state and electric field gradients existing at the nucleus.Sh These gradients may be expected in radioactive nuclei since, in the beta-decay process,-the nucleus acquires a net positive charge which leads to a rearrangement of 127 the atomic electron structure. The 57.6 keV state of I may be susceptible to such an interaction since it has both a large 55 53 quadrupole moment and a relatively long lifetime. The effect of the interaction will be dependent upon the length of time during which the nucleus exists in its intermediate state. It is exp pected that the atomic rearrangement process would occur very quickly. The degree of attenuation of the anisotropy is, there- fore, somewhat surprising. By measuring the anisotropy as a function of the delay between the two radiations, one may be able to determine the magnitude of the interaction.more directly. An 56 attempt was made by w. Chaffee to make this measurement on the . 360-57.6 keV cascade in 1271. Unfortunately, in order to obtain 78 useful results the resolving time of the apparatus must be much shorter than the lifetime of the nuclear state. The apparatus available did not have this capability'and no positive results could be obtained. . The 2lh-203 correlation did.not show'any attenuation ef- fects. This result is expected on the basis of the short life- time of the intermediate state (as measured by Geiger53). In analyzing the results of the angular correlation experi- ments, spin assignments 5/2, 7/2 and 3/2 were used for the ground states and the 57.6 and 203 keV excited states, respectively.53 In addition, the E? admixtures 0.614 i 0.10 percent and 21 1 3 percent obtained by'GeigerS3 for the 57.6 and 203 keV transitions, respectively, were used. ”With these restrictions, it was found that, while the 21h-203 keV correlation is consistent with.l/2, 3/2 or 5/2 for the spin of the h17 keV state, the 360-57.6 keV correlation, Obtained with a dilute source, was consistent with only the 5/2 assignment. These results require that the mdxing amplitude 5(E2/Rfl) of the 203 keV transition be positive. The 21h ke7 transition is required to be either h.h‘: 0.7 percent or > 99.9 percent 32 (assuming positive parity for the hl7 keV state as indicated by the log ft values given below) with (332/Ml) positive in the first case and either positive or negative in.the second case. The 360-57.6 keV correlation also required that the mixing amplitude for the 57.6 keV transition be negative and that the 360 keV transition be either (3.1 I.l°h) percent or (8b.: 3) percent E2(again assuming positive parity of the kl? keV state) with the mixing amplitude negative in both cases. Unfortunately, 79 very little can be concluded from the 657-57.6 keV correlation since it is consistent with 5/2, 7/2, 9/2 or 11/2 for the 715 keV state spin. Similarly, the 591-57.6 keV correlation is con- sistent with the 5/2, 7/2 or 9/2 assignment for the 6h9 keV state. A summary of the spin sequences and mixing amplitudes sug- gested by the angular correlation measurements is included in Table 5. The 5/2 spin assignments for the initial state in the 657-57.6 and 591-57.6 keV cascades, although consistent with the angular correlation results, can be excluded on the basis of log ft values as discussed in sec. h.D.iv. An attempt was made to measure the 2lh-lh5 keV anisotropy but the strong 2lh-203 MeV peak made such a measurement impractical. 127 h.D.iv. S'nmary of Te Results 127 The decay scheme of Te suggested by these studies is 52.53 shown in Figure 22 and is similar to that previously proposed 127 with the addition of a e branch from ”To to the 61.9 keV 127 state of I. The angular correlation.measurements, in conjunc- tion with previously published conversion electron data,53 allow unique spin assignments for the first three 127I excited states , populated by ‘27 Te £3 decay. They also limit the possible spin - assignments for the 6&9 and 715 keV states. I Log ft values,141 obtained using {3 energies and intensities deduced from Yaray'energies and intensities, suggest that the transitions froleYmTe to the 57.6, 6h9 and 715 keV states of 127I are probably first forbidden. This suggests a spin seam.” e8 3.5,.” go O IOIN H a N _ l , m . 0.0 . me n: .0 .0 .0 e aéosne/va J / J .nt 6/ O/ &N@ %v _ .0 M 4 O. 2/9 a0 0 at 99 ( a0 r9 magnum havoc .memomonm .Nm eyewwm \ m.m..eee.mm.mmm.o \ \m.o_.smmoo.o.mme.o \ \ ofieoeeoJemd \ \ x e \ \mdganmodwfio \ s \ \ Netsmeoooddflo \\ m.m.o\.eeoo.o.mmo.o /. \eogdmfiH oHNN_ O uzmd IN. +m $ $06 .0 Bo. . 0 o e 9/ ,me 81 assignment of 7/2, 9/2 or ll/2 and.positive parity for these states. This is consistent with the angular correlation results. n. . . .— 127“ . . ins log ft values obtained for the decay of the is ground state suggests that the transitions to the ground and bl? keV states are allowed which would require a l/2, 3/2, or 5/2 spin assignment and.positive parity for these states, in agreement O - fl r + - ~ ‘ ‘. - x with tne S/z aSSignments mass to those states on the basis oi 53 other data. cumin 5 DISCUSSION OF EXPERIKEHTAL RESULTS AND C HPARISCES'WITH T”“ORY With the excep Hi0 of the ground or fr wcitcd, 7/2+ state, the energy levels of the nuclei studied hei e can be avided into two groups a) those populated by the low spin (l/2+ 3/2+) isomer of the parent nucleus, and b) those excited in.the decay of the ll/2 eate of the parent. The discussion of the states will be divided accordingly. Those in group a) will be confined to the low energy region (< 600 keV), because first, only such states could be studied he: a, and second, relatively little data are availaole on the higher energy states pOpulated by the low spin iso: we 3. In addition to the work reported here, seve ml other experi- 7 mental studies on the low-lying states of 1218‘s and 12' I have been made recently (see secs. h.A. and h.D.) and much of the data now available are summarized in Tables 6, 7 and 8. Table 6 lists tates themselves while [0 some of the infor nation available on the 32 transition rates ()1 Tables 7 and 8 are concerned with the m. an between these levels. T1.e dipole n;cments (u) and quadrupole moments (Q) given in Table 6 are compared with the single proton values (“5p and Q5 p>57 and the values obtained by Kis sslinger and 20p 0 O '3 Q Q Q Sorenson (“KS and QVS) using pairing—plus-quauru,0le resaoual AX forces. The agreement between the experimental a.d the la eter pre- dicted values of the quadrupole mements is obvious. 82 83 Am...,eonmowocwoo ©0233 Homam 3&5 mm“. .bem .393 «The a0 «28 .H .waw I .I I 1| ‘1“ ' egl 'I . r. h- we 5 3-36.: m2 3.. AHMVQ‘On—HVAMN oo . AMVmIdemm .V N\M©N «\m MON H5 3:725; when N} NR Efibflfim {new «\m 8m 3:40.. Embodm NEE N? R s n _ d Rm 9W“-.. m, a MW 88 “but: 1(1 3 is 3 a s a a... .3“;“mewwfimlMeMHSssv 11 A- H53: Enamflwmmewflmwuec Amomwumawwv pemsdmwmmd simian Choc , .... ....H on; ..Q o .3 cam ea $.50 8H engage“ Howe: deem. Queen I - l ..i. hmvmommm QnHNH i . 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LHMnMS Nam 932 mafia: .3.“ 33.5% HEEAmSEH 533%.; .3 no mggmthmhmmm .33“ a332, nova.” 5335.99 .L. $.33. HNH 86 ..Omflo Hut.» ... 4.09 H0000 #3 $3. was .3900.“ H8329“? was $300.02” 9230 28 0.39 5" 000.00pr2me hmhmco umagm 90 0.30.0. By :0 ohms 0950 00:0“? 3.3080 05H. AM ..A a. a .000 003 3.9009 5330.300 Madam c0 93 .3 0030.30 0.3 $ng .855? $830303. ..0 908.903 Hood 3&3... :0... Sum .0093 .2? pm .mflrmn .3 .ma “09.800033.“ 00:05.00“ .Hmmfiodcv .30 .000 «mmonwg 0.8.2000 cum 53.03980 .Hmdfirm «mozmcwofioo @ohwflog m: Amomd M ‘ 20300.. mowmhgm «.8366 .m .3 0 mac 3955560000 :95 n mm: 33.3 mm wofimmam Hmoaosz 2390.003 .m 08. 0396 05308.5 95: mm 003 08... Sum 3003 mm .>0m .thE «000000398 .d .m 090 mm hp 55% 23.039308 mafia: 000.00.93.00? 0.0 0H 00.0 830. 0 H320 .0 300.0- A30H..0me.m m\m0m-m\m 02 030 00.0 Ema «$00th Em -..: o o o o .l o N\N.m I H t .10 0 8.08 0 0H 0 $va 0 33-03. m «\m H «\m 00m 0 mm 0.0.0 908.0 34 300.0 32-0084 00-0% mHm a . . HER . .. . NR .. wk 0 09 Q0 0 000 0 300.. HA 3% 0 300-013 0v 0m 0.... H00 . . 33% H «0. -0300 0 H 08 0 30 A£09.0H06w 0H \0. EH 0 o o o I o N\m IN a m 00 .00 0 M00 0 m H 3000 0 300-3% 0 .00 0H 0 R A0090 dd mm 530.3 98 on» 5.00 mama? . 98 Amwcoo omv . A >93 Adam? 293m H A: mnfipflnmfim he mfiém 033m 03.8% 00.30.0038 Hz 00050....0m0HfiH0. SEEK 0032 33.0.8: Hm£-HmEH0H 83350.00 EH .Ho 0 0.0.0.0 .6928 0.60” 080509 03.0.“ 08330028 . m 0.3.09 87 . 5.A.i. ‘cmparison of Electric Quadrupole Transition Rates The reduced electric quadrupole transition.probabilities are presented.in Tables 7 and 8 as B(E2)/(2jf + l), in units of lO-so e2 cmp, for ease of camparison with theoretical.predictions, in particular those of Sorenson.58 These values are for the case of de-excitation of a state. The corresponding reduced transition rate for the population of a state by'Coulomb excitation will be denoted.B(E2)ex. The two alues are related by 3(32,3-+ 3 >= -§-;iil_aex . The experimental B(E2) values are obtained from the paritial E2 58 mean-lives for gamma ray emission by use of the expression 60 -5 B(E2)exp = 0.0825'x 10' x EY x TYr1(E2) e2 cmy where EY is in.MeV and TY(E2) is the partial mean-life in seconds given by TY(EZ) =- 1.hhe'1Tl/2(l + arm + 535. ) Here, 6 is the branching ratio, Tl/Z is the measured half life of the nuclear state, GT is the total conversion coefficient and 62 is the EZ/Ml mixing ratio. The experimental B(E2)/(2:jf + l) are cor pared.with.the single proton estimate given bys 1307:2032> 23 j£§77"' zfié'ljjfifi'[5(23f+1)(21f+l){W(ijfoip§ 2) x (xfeoolezxio>}21e2 . The qm ntity in square bl ackets is the "statistical factor,“ S, 88 59 which, for many cases, can be approximated by'S - l. The quanp tities “(lfjf‘iji5 %fi2) and.(xf20011f2110) are Racah and Clebsch- Gordan coefficients, respectively; The expectation value of the square of the nuclear radius, ‘' , is often.60 taken as (3/5)R°2 whereRo - 1.2 1110-13 A1/3 cm is the "nuclear radius." This value has been used to calculate the single proton estimates given in column 6 of Tables 7 and 8. The experimental B(EZ) values, given in column 5, are, in ' every case, considerably larger than the single proton value. This result is not unique to these nuclei -- similar results having been obtained throughout the regions of spherical nuclei. (The same goes without saying for the deformed regions, where, as already mentioned, very large quadrupole effects are observed.) The B(E2) values for 121Sb have also been calculated by SorcnscnSB using KS wave functions and the expression . 1 . 2 J,- B(E2)a 31 3r (r) 1?. 1 1 2""'“"jf+1" jiOOijOOeeff VET" M (3:13: '2' " 2' 1313120) .x l . 3(32) 1/2 j.-j ' j j O++2+ i f , Q. f i (UiUr'Vin) * ( 5 > H (2%“) ijoocjfiz * 2 -% j j . r i (23i+l) OjiIZCjiOO . -l The coeffidents Cng are the contributions to the wave functions of the state 3' of configurations consisting of quasiparticles of spin 3 coupled with N phonons with angular momentum R to the resultant j' . These eigenvectors are given by KS for a large 121 number of states including several in Sb and 1271. The 89 quantities U and V are defined in such a way that Ui2 gives the probability that the state (i) is unoccupied and Viz has the op- posite meaning. They are restricted by the relation U12 + V12 a 1. Methods for their calculation are given by K8. (They may also be determined experimentally, as has been done by Cohen and Price61 for the Sn isotopes, using stripping and pickup reactions.) The B032) values obtained for 12le , given in column 7 of Table 7, agree quite well with the experimental ones for the 1:69 and 572 keV pm'e E2 transitions. However, that obtained for the 506 keV transition is still an order of magnitude too small. The experi- mental value for the 68 keV transition is much larger than ex- pected on the basis of values obtained for other transitions in this nucleus and may indicate that one or more of the data used in its calculation are in error. To bring this 3(22 )/(2jf+l) into the range of other values (i.e., < 1) would require an increase by a factor of > 20 in the partial mean-life of the 68 keV transition. The possibility that the mixing ratio of the 68 keV gammas deter- mined in this study is in error was .;::<-‘-nined as follows; the mixing ratio required to give B(22)/(23f+1) < 1 is 6 < 0.014. Using this value to calculate the angular correlation coefficient of the 68-506 keV cascade and esploying data from other studies as discussed in Chap. 3.A., one obtains A2 > 0.17 as compared to the experimental values A2 = 0.066 1'. 0.009. This is well outside the error of our measurement. Thus, the suspected error is expected to exist elsewhere. 8 Similar comparisons with KS values of 8(32) were made in 12 . . 71 using Sorenson’s expression, given above, and eigenvectors 90 for the 127 I states calculated by Sorensen and quoted by Langhoff.53 The We and We were calculated using the expressions given by KS, except for the 1418 keV state for which the values U = l, V - 0 were assumed. The results are given in column 7 of Table 8. Only the values for the 116, 375 and 1418 keV transi- tions are seen to be in reasonable agreement with experiment (within a factor of ~3). There appears to be a slight incon- sistency in the data on the 203 keV level: the lifetime of the 203 keV level has been measured by Geiger53 as < 0.55 x 10-9 sec, while the value deduced from the Coulomb excitation measurements of Davis, et al.,61 (using the 132/141 mixing ratio 6203 = +0.52 obtained by GeigerS‘B) is 0.75 x 10-9 sec. This result is reflected in the discrepancy in the B(E2) values for the 1&5 and 203 keV transitions. It is interesting that a similar disparity exists in 123Te, as has been pointed out by Schmorak, et al.62 The devia- tion in that case is in the same direction as found here, namely, 1 the MM) obtained from Coulomb excitation cross section measure- ments is less than that from half life and mixing ratio measure- . ments. Quantitative data are also available for the 161 keV first 123 excited state of Sb from other studies. This state was not ‘ populated in the long-lived isomer of 123511, when it decayed. 113 Coulomb excitation cross section measurements by Fagg give B(E2)/(2jf+l) = 0.0’4 e2 x 10-50 emu, which is ~ 0.9 x 8 [B(EZ)SP/(2jf+l)]. The half life of this state was determined by -10 Schmorak, et al.,62 as 6.2; x 10 sec. The EZ/Ml ratio has not - yet been determined for the 161 keV transition so the two 91 measurements cannot be compared for compatibility. 5.A.ii. Comparison of Ml Transition Rates It is also possible to compare experimental and theoretical values for the probability for emission of Ml radiation. The method used to make these comparisons is to present the ratio of the theoretical over the experimental reduced transition proba- bilities. The experimental values are obtained from the life- 63 times of the nuclear states by use of the expmssion _ 1.113 x 10'60 ex? E3(MeV)TY(I'fl.) B (I-fl.) where TYO'fl) is the partial mean-life for Ml omission given by '1: (Ml) . l.hhe'1 T (1 + )(1 + 52) . Y x A 1/2 “'1‘ In comparing the experimental result with the single proton esti- mate, it is found that the statistical factor for Ml emission is a rather cumbersome combination of 9-3 and Racah coefficients. A reasonable estimate is given, however, by setting 8 - 1. This then 6h yields the Weisskopf estimate. The ratio of the Weisskopf esti- mate to the experimental value is given by sun), 13 31‘ (El) - 1" -—- Y Y - , where E is in MeV and T is in sec. Bzrfilezp 2.21; x 10-“; Y Y These quantities, the 1-11 retardation factors, are given in column 8 of Tables 8 and 9. The experimental transition rates are from one to two orders of magnitude slower than the Weisskopf estimate. This is consistent with the Ml retardaticns found in other nuclei in this region of the isotOpe table. 92 127I‘with respect 53 The retardations of the M1 transitions in to KS predictions have been calculated by Langhoff and are given in column 9 of Table 9. Similar computations have been 121 63 made for the [-forbidden.Ml transitions in Sb by Sorensen and these results are given in column 9 of Table 8. A similar retardation factor was obtained by Schmcrak62 for the 161 keV transition in 123 Sb. Using the half life quoted above, this gives a retardation of lhO over the single proton estimate. The cor- responding value obtained by Sorensen is only I” 3. Although it has been.pointed out byGeiger53 that the KS predictions are not, in general, as good.for Ml as for E2 transi- tion rates, it appears that in these two nuclei, the agreement is rather good. In fact, all of the predicted values are within a factor of 10 of the corresponding experimental ones, which is better than for many of the theoretical E2 transition rates. 5.A.iii. Energy Level Systematics In addition to predicting electromagnetic transition rates, as discussed above, any usable model should be able to give the I spins, parities, and relative energies of the excited states of 121 127 the nucleus. The low energy states of Sb and I are in quali- tative agreement with.predictions of several models. The single particle levels available to nucleons in the 50 to 82 shell, given in.Chap. l, are consistent with all of the observed low energy 127 levels with the exception of the h18 keV (5/2+) state in I. The KS calculations have been carried out only for the ground.and first three excited states of 1218b. Since these three states are 93 essentially pure quasi-particle states, according to the KS calcu- lations, the predicted spin sequence is essentially the same as for the single particle model. They do predict, however, the ob- served variation of the energies of the. states with the addition of nucleon pairs. This variation is evident from the observed cross- ing of the 5/2+ and 7/2+ ground and first excited states betweeen 121 123 Sb and Sb. Similar computations have been made by 65 Silverberg for different values of the model parameters. The experimental variation of the energies of the dS/2 and g7/2 states in the odd-A antimony isotopes is shown in Figure 23, as is the corresponding trend in the iodine isotopes. The KS calculations have been carried out for a much larger number of states in 127 I. The results are not too encouraging, however, since the first predicted spin 5/2 excited state lies considerably higher than is observed eagoerimentally. .Calculations have also been made using an intermediate coupling unified model in which the particle motion is coupled to collective vibrations of the nuclear core. This model has “been applied to the study of 127 129 19 I and I levels by Bannerjee and Gupta. Although these calculations predict the approximate energies of a number of ob- served levels in 127’129 I it also predicts a preponderance of low- ] lying states which, as yet, have not been observed experimentally. One possible explanation of the poor agreement is that only the 2.d5/2 and lg,”2 single particle states were considered, with the remaining low energy states being treated as phonon-plus-particle states. It is believed that a more realistic interpretation of the observed levels, on the basis of this model, would be to 91+ J .monmoubmun musing Mme smesflpnm 4..va E.” mopmpm thadsm N\mem 93 .Ho mofipmsepmhw rammed/m .mm osswfim .9 mm. mm. Mm. ME .2 a: k: _ _ _ _ _ _ _ . _ I meet I com: / height? in. TI 29/ O _ ./ . o . 3 v:_.eeH m$ /. Ma I now may M ,. / a. i use um: EMU an new / Cow _ a? _ _ P _ _ _ _ _ 9S consider the ground and first three excited states as predominantly single particle states and treating those states at higher excita- tion energies as phonon-pluseparticle states. This interpretation is based on the similarity between experimental observations and single particle predictions of the spins and.parities of the 127 ground and first three excited states of I. 5.A.iv. Beta Transition Comparison It is also interesting to compare the allowed electron capture (llee) and beta (127Te) transitions to the low-lying states in 121 127 Sb and I, respectively. The log ft values for the transi- 121 tions to the l/2+ and3/2+ states in Sb and to the two S/2+ 127I are in the high end of the range usually considered bl states in for an allowed transition. However, the transition to the 203 keV, 3/2+, state in 1271 is at least 3 orders of magnitude slower than expected (log ft ,~.1o,2). The same is true for the transi- 1271 at 375 keV which, within.experi- 127 tions to the 1/2... state of mental error, is unpOpulated.in the decay of Te. Both of these states are populated in.the electron capture decay of the 3/2... l27Xe with transition probabilities well within.the range state of for allowed transitions. The reason that these transitions are so unusually hindered is presently unknown. 5.B. The High.Energy States At higher excitation energies, very little information is available, making it impossible to make any quantitative compari- sons with theoretical.prediotions. ‘Ono can then only Speculate 121, 123 as to the character of the nuclear states. In Sb, Sb and 96 1271, the decay energy of the ll/Z' state of the parent is suffi- cient to populate only two or three of the multitude of high energy levels expected on the basis of comparisons with other nuclei in this region, for example, 1198 . Those which are popu- lated in these three nuclei are found to be quite similar in that they all lie near the energy of the first excited state of the even- even nucleus corresponding to the core of the oddnA nucleus. These first excited states lie at 1180, llhO and 665 keV in 12OSn, 1223n and 126Te, respectively. This result is in.qualitative agreement with the level structure one would expect by coupling the low- lying single particle states to core excitations. The resultant "core-multiplet" should have its center of gravity near the energy. of the excited state in the corresponding even~even.nucleus, since the states should be only slightly perturbed by the interaction ‘with the oddnucleon.l6 ‘ One interesting feature of the higher energy states observed experimentally is that they all decay to the 7/2+ ground or first- excited state. This fact may have a plausible explanation: if we assume the ll/2' state of the parent nucleus is essentially a lhll/Z single particle state, the beta or electron capture transi- tions to multiplets built on the 2d5/2 proton state would be ex- pected to be [-forbidden.(i.e., tax a 3), whereas, those to states formed from coupling the phonon to the lg.”2 state would not be [-forbidden. his assumes that the particle-core coupling is suf- ficiently weak that the particle character of the state is main- tained and 1 remains a good quantum number. The same arguments can be applied to the levels in 1258b. 97 125Sn Here, sufficient energy is available in the beta decay of to populate states which may arise from coupling the particle motions to higher excitations of the core. The qualitative agreement with.the core-coupling model is again obtained, with groups of levels being observed in the vicinity of excited states in 12hSn. One possible interpretation of the observed l25Sb levels is to consider those up to and including the lhl9.8 keV state as due to coupling of the lg7/2 particle state to the first excited state of the core, while those at higher energy can be considered as due to coupling to the second and higher excited states of the core. However, all of the observed states may not be explainable on the basis of core coupling: for example, the l806.9, 2002.0, 2199.7 and 2276.0 keV states which decay primarily to the ground state. Such transitions would correspond, in the even-even nucleus, to transitions from the second or higher excited state to the ground state, whiCh are usually observed to be weaker than transitions to the first excited state.66 Such states may, therefore be due to some other type of excitation whose character can only be decided.when more quantitatte data on these levels become available. One is also tempted to interpret the lhl9.8 and 1890.6 keV levels as members of core multiplets on the basis of the E2/M1 mixing ratios for the h69.6 and 1890.6 keV transitions. These were found to be >»l03 times the'Weisskopf estimate, in keeping with the expected enhancement of the 82 transition rates from the second to the first multiplet and.from the first multiplet to the ground state. This result appears to lose some of its 98 significance, however, when it is found that a similar enhancement is obtained for the 331.9 keV transition, which, in the present interpretation, is assumed to be between two states of the same 16 multiplet. Such transitions, according to de-Shalit, are ex- pected to proceed predominantly by emission of Ml radiation. One disconcerting fact, which tends to cast some doubt on the present interpretation of the high energy levels in 121Sb 1238b as core-multiplets is that, as far as can be determined, and they have not been excited in Coulomb excitation studies.16 For a multiplet built on the first 2+ excited state of the even core, we should have16 2: B(sz;J + J) = 303250“ + 2*) J g That is, the sum of the reduced transition probabilities from the ground state, Jg, of the odd-A nucleus to the multiplet states, J, is just equal to that for exciting the 2+ state in the even core nucleus. The probability for exciting a given J state can then be written as 2J+l 5(2J +1) “323 0+ " 2+) g B(BZ;Jg -* J) = The values of mm; 0+ -* 2+) are found to be relatively large; for example, ~ 20 x 10-50 62 cmh and ~hO-‘65 x 10"50 e2 cml‘l for the even tin and tellurium isotopes, respectively. Thus, at least the high spin multiplet states should be strongly excited. This assumes, of course, identical experimental conditions for both the odd-A and the own isotope cases. The cross sectionfor E2 Coulomb excitation, as given by Alder, et al.,66 is 99 ( mov) 2 .. -... . v9 0232 me x B(“")ex x ng where the function 1‘32 of the incident ions. In the cases being considered here, that is, 12le and 123 decreases rapidly with decreasing energy Sb, the earlier studies by Fagg were made with ' ~ 5.2 IvIeV alpha particles, while ~ 10 MeV alphas were used in 67 Using the the measurements on the corresponding tin isotopes. fE2 curves given by Alder, et al., this change in energy by a factor of ~ 2 is found to result in a change by a factor of ~10 for fEZ' It may be, therefore, that the non-observation of the higher excited states in these earlier works was due to the reduc- tion in the cross section by the decrease in sz rather than to a decrease in B(E2). No conclusions can be drawn on the later report by Robinson, et al., using 7-9 MeV alphas, since no spectra are presented and no indication given of whether high energy transitions were sought. The need for additional informaticm on the higher energy regions of the level spectrum of these antimony isotopes is clearly indicated. In contrast, levels in 127 52 I at 630 and 750 keV have re- portedly been excited by Coulomb excitation using ~3 MeV pro- tons. The relatively large values B(E2) ex - 10 x 10'"50 e2 cm)4 are found for both states. These can be compared with the B(}2:2)(_3x for exciting the first 2+ state in 126To which is ~ to x 10"50 e2 cmh. These states could therefore constitute part of a multi- plet built on the S/2+ ground state of 127I. (l) (h) (5) ‘ <6) (7) (8) B IBL IOGRAPHY Nuclear Data Sheets (The National Academy of Sciences - National Research Council, Washington, D. G.) Nuclear Theory Index (ALAS-NRC, Washington, D. C.) Nuclear Science Abstracts (USAEC, Division of Technical In- formation) Physics Abstracts (The Institution of Electrical Engineers, London) 9“ M. G. Mayer and J. H. D. Jensen, ..lsr'entarz' lite-dry of Nuclear Shel Structure (John Wiley and Sons, Inc. , Her»: York, 1955) H. A. 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Academy of Sciences, Moscow-Leningrad, 1956, 1958); also Reports 57 ICC K1 and 58 ICC L1 (Physics Dept., Univ. of Illinois, Urbana, 1957, 1958) (kl) The use of the quantities, ft, to compare (3 decay rates is discussed, for example, by: E. J. Konopinski and M. E. Rose, The Theory of Nuclear Decay in a-(s -Y Ray Spectroscopy, 10c. cit., Chap. 23 A nomogram from which the value of log ft can be immediately 10h determined for a given end.point energy and lifetime, is given by: A. H.'Wapstra, G. J. Nijgh and R. Van Lieshcut, Nuclear Spectroscopy Tables (North-Holland Publishing C0., Amster- dam, 1959) A comparison of the experimental values is given by: (h2) (h3) (bk) (AS) (L6) (A?) (AS) (19) (50) C. E. Gleit, ChungAWai Tang and C. D. Coryell, Beta Decay Transition Probabilities, Nuclear Data Tables (National Research Council) Nuclear Data Sheets, NRC 60-2-99, 105, 106 L. w. Fagg, Phys. Rev. 1922(1958) 100 R. L. Robinson, P. H. Stelson, F. K. McGowan, J. L. C. Ford, Jr., w. T. 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