The energy levels of 121Sb and 123Sb isotopes from the (n, n′γ) reaction

Nuclear Physics Al72 (1971) 215-224; Not to be reproduced by photoptit

THE ENERGY LEVELS OF “‘Sb

@ North-Holland Publishing Co., Amsterdam

or microfilm without written permission from the publisher

AND ‘23Sb ISOTOPES FROM THE

(n, n’y) REACTION E. BARNARD,

N. COETZEE, J. A. M. DE VILLIERS, D. REITMANN and P. VAN DER MERWE Atomic Energy Board, Private Bag 256, Pretoria, Republic of South Africa Received I8 May 1971

Abstract: The low-lying energy level structures of the stable antimony isotopes were investigated with the aid of the (n, n’r) reactions utilising a 40 cm3 Ge(Li) detector, a pulsed neutron source and time-of-flight gating methods. Most of the de-excitationy-rays observed could be associated with the energy level schemes of tZiSb and lz3Sb. Some new levels are postulated and the results are compared with the latest model calculations. E

NUCLEAR REACTIONS 12’Sb, lz3Sb(n, n’y), E = 0.61.9 MeV; measured ‘??b, 1z3Sb deduced levels, 1, n, branching ratios. Natural targets.

EY.

1. Introduction

The low-lying states of the odd antimony isotopes have received a great deal of attention (experimentally and theoretically) in the last few years. (The large body of experimental and theoretical data is referred to in ref. ‘).) An impetus is the fact that these studies yield information on the coupling between an odd proton and the collective motions of a semi-magic core. Various versions and refinements of this eoreparticIe coupling model have been produced ‘) and it is therefore important that the experimental information on the low-lying states should be as complete as possible. The information on the low-lying states of ‘*‘Sb and lz3Sb is mainly derived from b-decay 2-4), Coulomb excitation ‘*6), (3He, d) and (d, d’) experiments 5*‘*“1. As most of these processes are somewhat selective, it was decided to utilise the (n, n’y) reaction and the high resolving power of a Ge(Li) detector to investigate the complete low-lying energy level spectra and decay schemes of 121Sb and 123Sb and in particular to try and resolve some inconsistencies in the energy level and decay schemes of 12’ Sb at about 1 MeV excitation. 2. Exper~ental procedure The standard procedures followed in fn, n’r) experiments in this laboratory have been described ‘, lo). Th e neutron sources were the ‘Li(p, n)‘Be and 3H(p, n)3He reactions, using the pulsed proton beam from the Atomic Energy Board 3 MeV Van de Graaff accelerator. A cylindrical sample (3.0 cm high, 2.5 cm diameter) of natural 215

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YYYV

Y

+-

+-

NC-4

I-\#2 -Ld

et al.

lzl* 123Sb LEVELS

217

antimony was suspended at 10 cm from the ‘Li target and at 0” to the proton beam direction. A 40 cm3 Ge(Li) detector was placed at 85” and a distance of 50 cm from the scattering sample. The detector was shielded from the neutron source by a wedge consisting of 30 cm of lead and 15 cm of paraffin wax. It was also enclosed in a lead

123

SB

1088.4 1030.4

1/2+

3/2+

542.8

s/2+

160.9

7/2+ ,,,,,,, ,

5

/

0.0

Fig. 2. Energy levels of 123Sb based on the present measurements and incorporating data from previous experiments.

sleeve 7.5 cm thick. Two detectors were used in the course of the experiment with energy resolutions of about 3.5 and 2.6 keV respectively for 1.332 MeV y-rays. Time-of-flight methods ‘) were used to reduce general background and discriminate against neutron induced events in the detector. A large number of y-ray spectra were obtained for incident neutron energies between 600 and 1910 keV. Background runs were made at each energy using lead, bis-

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et al.

muth and iodine samples at different times. Spectra were accumulated in a 204% channel analyser and raw data were processed on a CDC 1700 computer. The necessary corrections to the data for detector efficiency and y-ray attenuation in the scattering sample were made as described before lo). Clearly, because one is dealing with two isotopes (57.25 % “‘Sb and 42.75 % 123S)b in the scattering sample, it is not possible to make unique assignments to the correct isotope solely on the basis of the present experiment. This is particularly true where levels decay by a single y-transition and it is not possible to accommodate yrays by means of energy sums. However, many of the levels in each isotope were sufficiently well known from other experiments, that it was possible from energy sums and threshold behaviour, to assign most of the y-rays observed to the correct isotope. Where levels were assigned on the basis of a single y-ray transition, it was done because they are within a few keV of levels known from other sources. The energy levels and decay schemes put together from accurate energy measurement and summing and from a knowledge of level energies from earlier studies are shown in figs. 1 and 2. The y-ray energies are the averaged values for all the spectra measured. From the rms deviations of these averages, it is estimated that the uncertainties in the y-energies and hence level energies are about 1 keV or less for those levels below 1200 keV and about 2 keV or less for those levels above 1200 keV. The relative branching ratios, as measured at 85”, are presented as a percentage of total decays. These have not been corrected for internal conversion and angular correlation effects, which should be small. For those levels above 1250 keV, these ratios may be in error by as much as 20 % due to poor counting statistics and the presence of background peaks. The following y-rays were observed, but could not be accommodated into the level schemes or assigned to the correct isotope in figs. 1 and 2: 627.0 keV (1910 keV) and 1462 keV (1910 keV). The numbers in brackets refer to the incident neutron energy where first observed. 3. Discussion 3.1. THE -‘Sib

NUCLEUS

3.1.1. The levels below 1200 keV. The levels at 37.2, 508.2 and 573.5 are well known from previous work so that no difficulty was encountered in placing these y-rays. The decay modes and branching ratios observed are consistent with the known spin assignments ‘). As previously mentioned, the experiment was particularly motivated by the uncertainty with regard to the levels at about 1 MeV excitation in 121Sb. The situation is illustrated in fig. 3, where previous results from various experimental sources are summarized. Barnes et al. “) investigated the energy levels of “‘Sb using the reactions (3He, d), (d, d’) and (IGO, 160’y). The 948 keV level (known from P-decay “)) was not excited in any of these experiments, while it was assumed that the other levels in

‘*l* lz3Sb LEVELS

219

the region are equivalent ones i.e. the scatter in energy value was within the exparimental resolution (some 30 keV for the (3He, d) experiment and 10 keV for the (d, d’) experiment). The model calculations based on the coupling of the odd proton to the collective motions of the core I1“) suggest that there should be at least four levels at about 1 MeV with spins Sf, e’, 4’ and -‘,’ +. These levels would be predominantly collective, based on the coupling of the 2+ state of the Sn core (at about 1 MeV) to

Fig. 3. Summary of experimental data from various sources for the energy levels of l*lSb at about 1 MeV excitation. Level energies, as derived from the different experiments, are shown in keV.

the lg% and 2d+ single-particle states. It is significant that the levels at 1143 and 1024 keV excited in the (’ 60, ’ 60’y) reactions decay to the ground state (3’) only and the B(E2)f values imply a large 125) component “) whilst the levels populated in /?-decay, decay to the 3’ first excited state implying a large 123) component. This suggests that the levels at 1141, 1038 and 948 keV are levels with relatively high spin, populated in the B-decay of the high-spin isomer Te(y-) and that they are not the same as the levels populated in the (d, d’) and (160, 160’y) reactions. Moreover it is uncertain which of these levels are populated in the (3He, d) reaction. In a recent Coulomb scattering experiment utilizing 14N ions, Gal’perin et al. “) observed, amongst others, y-rays of 1000 and 1105 keV, which they assigned to levels at 1141 and 1037 keV. They assumed these levels to be the same as those populated in the experiments of Barnes et al. ‘). However, it is noteworthy that the levels excited by them decay to the first excited (3’) state. In addition, they found large discrepancies in B(E2)r values (for transitions to levels at 1037 and 1141 keV) between their results and those of Barnes et al. The conclusion is that the levels excited by the Russian group are not the same as those excited by Barnes et al., but presumably the same as those populated in the p-decay experiment “). Gal’perin et al. also observed a 1147 keV y-ray in their experiment, but because of their experimental resolution, they chose

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to attribute it to a ground state transition of the 1141 keV level rather than a level at 1147 keV in 12’Sb. From the above-mentioned it is clear that there are most probably two pairs of closely spaced levels at about 1030 and 1140 keV respectively. In the present experiment, y-rays of energy 909.9, 947.1, 998.4, 1024.7, 1102.0, 1108.3 and 1145.0 keV were observed. As far as the 1102 and 1108 keV y-rays are concerned, necessary cognisance was taken of the fact that background peaks exist at these energies. These originate from transitions in 74Ge [ref. ‘I)]. At incident neutron energies 2 1300 keV these background peaks constituted no problem, as “sample in” and “sample out” measurements clearly indicated that strong 1102 and 1108 keV y-rays originated from the antimony sample (A “sample out” measurement is one in which the antimony sample is replaced by a bismuth or lead sample). However, the 1102 keV y-ray can also be attributed to a 1262 --f 160 keV transition in rz3Sb. To assign it to a level at about 1139 keV in “‘Sb (in accordance with the P-decay experiment of Auble et al.) it was necessary to prove its existence at energies below the threshold of the 1262 keV level. Careful “sample in” and ‘“sample out” measurements at 1250 keV incident neutron energy confirmed its existence although weakly excited, in the presence of the previously mentioned background. In this respect, a comparison of the relative intensities of the 1102, 1108 and 1205 y-peaks lr) in “sample in” and “sample out” measurements was very helpful Based on existing knowledge and our previous remarks regarding the ““Sb energy levels in this region plus threshold behaviour and energy sums, the observed y-rays were assigned to levels at 947.1, 1024.7, 1035.6, 1139.2 and 1145 keV as shown in figs. 1 and 3. The levels at 947.1, 1035.6 and 1139.2 which decay to the first excited (3’) state, clearly correspond to the levels observed in B-decay by Auble et al. 3), with probable spin values 3*, $+ or y+. In the present work the 947.1 keV level also shows a significant ground state transition which makes the 9’ spin value for this state unlikely. Comparison of the B-decay data “) with the branching ratios calculated by Vanden Berghe and Heyde ‘) indicate that a spin of 2” is favored for the 1035.6 keV state and 3_’ for the 1139.2 keV state. The 1145.0 and 1024.7 keV levels are not populated in p-decay and they decay to the ground state (3’). They presumably correspond to the levels populated in the (r60, 160’) experiment of Barnes et al. ‘) with probable spins (g’, 3’) and@+, 3’) respectively. The 1145 keV level shows a transition to the first excited state which is not reported in that experiment. 3.1.2. The levels above 1200 keV. The I386 keV y-ray is assigned to a level in l”Sb. This is done on the evidence of a 1380 keV level determined by Barnes et al. “) in the (“He, d), (d, d’) and (I%, 160’) reactions. The calculations of Vanden Berghe and Heyde “) favor an assignment of 3’ to this level. Based on energy sums, the 1408, 1410 and 1427 keV levels can confidently be assigned to “‘Sb. The 1410 and 1427 keV levels appear to be the best candidates for the first -zl 1 - level predicted by theory ‘f-

12’*=%b

LEVELS

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A level was determined at 1423 keV by Barnes et al. ‘) (with 5-10 keV resolution). The 1473 keV assignment must be tentative due to the low intensity of the y-rays involved and also due to the fact that the 899 keV transition can be accommodated elsewhere whilst the observation threshold of the 437 keV y-ray (above 1700 keV) is not consistent with it originating from this level. Similarly the 1476 keV assignment is 2000 t

121

1900

Sb

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LOO i

100 0

712* 5/2+ EXPERIMENTAL REF.5.

7/2+ 5/2+ EXPERIMENTAL THIS WORK

712+ 5/2+ THEORETICAL REF.1.

Fig. 4. Comparison between energy levels of lzlSb derived from the present work and those of previous workers and with the calculated level scheme of Vanden Berghe and Heyde I). The theoretical predictions are estimated from the figures of ref. I).

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tentative due to the weakness of the 1475 keV y-ray. No levels have been observed in this region previously, the closest being at 1450+ 10 and 1446+ 5 keV from the (3He, d) and (d, d’) ex p eriments respectively “). The 1514 and 1521 keV levels are also assigned to “‘Sb on the basis of energy sums and have not been observed previously. The 1630 keV level is presumably the same as the 1623 keV level previously observed in the (3He, d) experiment. In fig. 4 the energy levels of lzlSb derived from this work is compared with those derived from previous experiments and with the latest model predictions of Vanden Berghe and Heyde ‘). Many features of the spectrum are accounted for by these unified model calculations. In this respect, the main contribution of the present measurement is the fact that there are actually five high-spin states between 900 and 1200 keV and not three. The calculations ‘) predict at least four such states. 3.2. THE

lz3Sb

NUCLEUS

3.2.1. The levels below 1100 keV. The energy levels of iz3Sb below 1100 keV are quite well established 4*‘). No difficulty was therefore encountered in assigning y-rays even where states decay via a single y-emission only such as the 160.9, 552.5, 1030.4 and 1088.4 y-rays. No evidence was found for a 170 keV transition between the 713.4 (+‘) keV level and the 542.8(3+) keV level. This is in agreement with the work of Jeronymo et al. ‘). The decay modes are consistent with the spin assignments from previous work. 3.2.2. The IeveZs above 1100 keV. The 1022.0 keV y-ray is assigned to a level at 1182.9 keV in 123Sb. This assignment is made on the basis of the 1180 keV level known from the (3He, d) and (d, d’) experiments “). In a study of the P-decay of 125 d 123Sn(yf), Auble and Kelly postulated a level at 1187 keV [ref. “)I. Because of the absence of any transition from this level to the ground or first excited state, they concluded that its spin is probably 2 y’. De Pinho et al. 12) made a search for a transition from this state (populated in b-decay) to the first excited 3’ state. They assumed that this state was equivalent to the one at about 1180 keV, known from the (3He, d) and (d, d’) ex p eriments and that it was probably the predicted first 8’ state with structure largely 12%). They could only set an upper limit of 8 x 10m3 % of the total disintegrations.The conclusion is that the state populated in this (and the (3He,d) and (d ,d’)) reactions is probably not the same as the 1187 keV state populated in b-decay. If the 1187 keV state is a very high spin state, not populated in nuclear reactions, it cannot be accounted for by the model predictions. The state at 1182 keV probably corresponds to the first 4’ state, with 1a r g e d s amplitude, predicted by the calculations of de Pinho et al. 12). A 1102 keV branch from the 1262 keV level is tentatively assigned. This is based on the fact that there is a sharp increase in intensity of the 1102 keV y-ray (from the decay of the 1139 keV level in 121Sb) at incident neutron energies 2 1300 keV. This level must be the same as the one at about 1258 keV excited in the (3He, d) and (d, d’) reactions ‘).

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121g12%b LEVELS

The 1338 keV level is assigned to 123Sb on the basis of energy sums. No level has been observed at this energy before. The 1511 keV level presumably corresponds to the levels excited at 1502-&10 and ISlO+ keV in the (3He, d) and (d, d’) experiments respectively 5). The 1577 keV level is assigned to lz3Sb on the basis of the 1574 keV level previously observed in the (3He, d) reaction “), 2000 1900

123 Sb

1800

312:7/2+ 112+

1700 1600

t

llf2"

1500 312%2* 1.400 512f7t2+

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g/2** v/2+

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400 300 200

512+

512+

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-

7/2+ EXPERfMENTAL REF5.

712, EXPERIMENTAL THIS WORK

7/2+ THEORETICAL REF. 1

Fig. 5’. Comparison between energy levels of 12%b derived from the present work and those of previous workers and with the calculated level scheme of Vanden Berghe and Heyde ‘). The theoretical predictions are estimated from the figures of ref. I).

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In fig. 5 the energy levels derived from this work are compared with previous work and with the calculations of Vanden Berghe and Heyde ‘). Note added in proo$ The group engaged in resonance fluorescence studies at Boston University have recently produced measurements on the antimony isotopes 13). The results are in agreement with the present measurements in most cases. Their work does however cast some doubt as to whether the 1485 keV y-ray does originate from the 1521 keV level in “%b. A re-examination of the threshold for this y-ray places it at about 1700 keV (incident neutron energy). The possibility therefore exists that it originates from a higher level in 121Sb or 123Sb.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

G. Vanden Berghe and K. Heyde, Nucl. Phys. Al63 (1971) 478 J. M. F. Jeronymo et al., Nuovo Cim. 55B (1968) 491 R. B. Auble, W. H. Kelly and H. H. Bolotin, Nucl. Phys. 58 (1964) 337 R. B. Auble and W. H. Kelly, Nucl. Phys. 81 (1966) 442 P. D. Barnes et al., Phys. Lett. 23 (1966) 266 L. N. Gal’perin et al., Soviet J. Nucl. Phys. 9 (1969) 139 M. C. Joshi and B. Herskind, Proc. Int. nuclear physics Conf., Gatlinburg, Tennessee (1966) p. 357 P. D. Barnes and C. Ellegaard, Proc. Int. nuclear physics Conf., Gatlinburg, Tennessee (1966) 363 E. Barnard et al., Nucl. Phys. Al67 (1971) 511 E. Barnard et al., Nucl. Phys. Al57 (1970) 130 K. C. Chung et al., Phys. Rev. C2 (1970) 139 A. G. de Pinho et al., Nucl. Phys. All6 (1968) 408 E. C. Booth, private communication