Very slow M4 transitions and shell-model intruder states in 199, 201Bi

Nuclear Physics A349 (1980) 61- 67; @ North-Holland

El l.E.l: 3.A

Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

VERY SLOW M4 TRANSITIOtiS AND SHELL-MODEL INTRUDER STATES IN 199*201Bi R. A. BRAGA, W. R. WESTERN ‘, J. L. WOOD ++ and R. W. FINK School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

and R. STONE *, C. R. BINGHAM

and L. L. RIEDINGER

Department of Physics, University of Tennessee, Knoxville, Tennessee 37916, USA, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA

Received 18 June 1980 The isomeric decays of the +’ shell-model intruder states in ‘99’ “‘Bi have been investigated. In the case of ““Bi we show that the s,,~ + h,,, isomeric transition is pure M4 within experimental error and is 2200 times slower than any other M4 transition observed in odd-mass nuclei. Its I-forbiddenness and the possibility of an E5 admixture and penetration effects are discussed.

Abstract:

E

RADIOACTIVITY ‘99.201Po [from “+I. ‘931r(‘4N, 6n), E = 116 MeV], ‘99~20’Bi [from 199*201Podecays]; measured E,, I,, I,,, yy-, yX-coin. ‘99, *‘lBi deduced levels, J, ?T,(EC+j’)/IT branching ratio, ICC, K/L, y-multipolarity, B(M4). Ge(Li), Si(Li) detectors, mass-separated sources.

In the course of studying the decay schemes of 20’m~gPo(8.9 min, 15.3 min) and ‘99”~sPo(4.2 min, 5.2 min) to 201’ 199Bi we have observed isomeric first excited states with J” = +‘. These states are str&gly populated in the low-spin (J” = $-) PO ground-state decays both directly and through band structures consisting of states with J” = 3+, 4’. Some confusion has existed over the location of the $’ state in 201Bi, Alpsten and Astner ‘) placing it at 846 keV and Korman et al. 2, placing it at 1115 keV. We support the former, confirming the recent evaluation of Schmorak 3). Here we report on the determination of the branching ratio and the multipolarity of the $’ + $- isomeric decay in 201Bi and discuss its uniquely strong retardation compared to all other M4 transitions observed in odd-mass nuclei. We also present evidence for a similar case of retarded M4 isomeric decay ’ Present address: Nuclear Data, Inc., Golf and Meacham Rds., Schaumburg, Illinois 60196, USA. +’ Present address: School of Physics, Georgia Institute of Technology. * Present address: EG & G Ortec, Inc., 100 Midland Road, Oak Ridge, Tennessee 37830, USA. 61

R. A. Braga et al. 1 ‘99.20’Bi

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in 199Bi. The primary interest in this isomerism is the fact that it involves the shellmodel intruder state s+; and, that although the large retardation is unprecedented for M4 transitions, it is very similar to retardations of lower-multipolarity transitions de-exciting shell-model intruder states. The sources of 199*201Po used in these studies were produced by bombarding metallic foils of natural Ir with 116 MeV ’ 4N ions from the Oak Ridge Isochronous Cyclotron and were mass separated on line with the UNISOR isotope separator. Gamma-ray and conversion electron multispectrum scaling, yy- and yX-coincidence spectroscopy were performed. Full decay schemes will be published elsewhere 435). In fig. 1 we show the systematics of the s+ intruder state and associated levels in 199-205Bi. These data are taken from the present work and ref. 6, (2033205Bi). In their investigation, Alpsten and Astner ‘) concluded that the 846 keV isomeric transition in “‘Bi has a multipolarity of either M4+(54f20) % E5 or E4+(26+ 11) % M5 (with a preference for the former), based on their measured 5/2+

(970

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Fig. 1. The systematics of the positive-parity bands built on the sl,* 2plh state in 199-205Bi. The deexcitation modes of the band heads and the intraband transitions are shown. The half-life of the band head in ‘03Bi has not been measured, but is estimated to be z 5 s (see text). The band heads in lg9* ‘OIBi both decay by #I+/EC. The ‘A a-branches in 199*“‘Bi are taken from refs. lo, 3), respectively. (The a-decay of the s~,~ isomers in the odd-Bi isotopes has been discussed recently by Vakhtel et al. “).)

R. A.

Braga

et al. 1 ‘g9~20’Bi

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value of aK = 0.13 + 0.04 and a K/L ratio of 2.3 f0.4. In the present study we have taken particular care to determine c1k and K/L accurately for the 846 keV transition. The relative electron detection efficiency of the 3 mm x 80 mm* Si(Li) detector used in these measurements was calibrated using sources of 133Ba, 13’Cs and *“Bi covering an (electron) energy range of 2461700 keV. The normalization ,of the electron intensities to the y-intensities [measured with a 13 ‘A Ge(Li) detector, 2.0 keV FWHM at 1332 keV] was made “internally” using the M4 transitions in the 201mPo and *OimPb decays at 417.9 and 628.8 keV, respectively. The most notable difference between our results and those of Alpsten and Astner ‘) is that we deduce the 846 keV line to be a doublet of 846.4 and 847.7 keV. An 847.70 keV y-line has been seen by Richel et al. ‘) in a study of the *O’gBi + 201Pb decay scheme, and they assign it as a transition between levels at 936.17 and 88.53 keV in *‘lPb. From a comparison of the y-ray intensities reported by Richel et al. ‘) and those obtained in our work, we conclude that the level in *OlPb at 936.17 keV is populated in the P-decays of both *‘igBi and *OlmBi. Although we have been unable to resolve the 846.4/847.7 keV doublet adequately, we can accurately estimate the intensity contribution of the 847.7 keV component by utilizing the fact that the 936.17 keV level in *OiPb also de-excites by a 936.17 keV transition : it is then simply a matter of using the relative intensities for the 847 and 936 keV lines reported by Richel et al. ‘) and normalizing to our 936 keV line intensity. We note that Richel et al. ‘) produced *‘lBi by the *06Pb(p 6n) reaction for example, thus only slightly populating the low-spin *OlmBi. The;r electron spectrum shows a weak line corresponding to an 847.7 K line in Pb, and they deduce a multipolarity of E2: From K-binding energy considerations and intensity considerations this strongly supports a negligible production of *““‘Bi in their experiment. As a result, we deduce from our data that c(x = 0.201 f 0.032 and K/L = 3.1 kO.4 for the 846.4 keV transition. These values can be compared with theory ‘) for M4 (Q = 0.211, K/L = 3.30) and E5 (aK = 0.070, K/L = 1.45). To estimate the branching ratio between *““‘Bi P-decay and the M4 isomeric transition, we determined the feeding intensity of the 3’ isomer in *‘iBi from 201gPo (J* = 3-) decay. Assuming that the reactions used in this work enter the mass 201 chain at 201mP~ (J” = y’) and 201gBi (J” = 3) (and at lower Z), only; it is then possible to apply a correction to the decay chain *01”Po(8.9 min) + 20’gPo(15.3 min) --f *Ol”‘Bi(59.1 min) (bearing in mind source collection and counting times) to relate the feeding intensity of the 3’ isomer to its M4 decay intensity directly and hence obtain the branching ratio for the M4 isomeric transition. The half-lives adopted by Schmorak 3, were used. The only unknown in this procedure is the direct population of the 3’ isomer in *OIBi by p-decay of *OlgPo. We assume that it is equal to the direct /?-decay population of the 3’ state in *O’Bi at 1087 keV. As will be seen below, such an uncertainty does not affect the conclusions of this work. Our result is that the 3’ isomer in *OIBi decays 6.8 o/0by M4 y-emission, yielding a partial M4 y-decay half-life of 14.5 h, which can be compared

64

R. A. Braga et al. / ‘9ps20’Bi

46

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Fig. 2. The reduced half life (T = log,,[T,,,(M4)AZ~/0.693]) for all known M4 transitions between one-quasiparticle states in odd-mass nuclei (76 cases) plotted against mass number. Also shown are the reduced half-life (T = log,,[T,,,(M3)A4/3E~/0.693]) for the one known case of an M3 transition between one-quasiparticle states in a spherical odd-mass nucleus ( “%); and the reduced half-lives (T = log,, U-1 ,@)A 1”‘3E’1/0.6931) for the five known cases of E5 transitions between one-quasiparticle states in odd-mass nuilei (depicted as A). The Weisskopf and Moszkowski single-particle estimates are included. The half-life expressions are for TIi2 in seconds and E, in MeV.

with a Weisskopf single-particle estimate of 23.8 s, i.e. a retardation of 2.19 x 103. (If we have failed to observe significant feeding to the ++ state in the ‘OIPo decay, then this retardation is a lower limit.) The reduced M4 transition probability for this transition is shown in fig. 2, where it is compared to all known 9, M4 transitions for odd-mass nuclei (strictly speaking, all known M4 transitions between onequasiparticle states). We also show M3 and E5 transitions between one-quasiparticle states for comparison (and see discussion below). In the 199gPo 4 19’Bi decay scheme we see a level pattern very similar to *“Bi (see fig. 1). Evidently the 24.7 min +’ state is the a-decaying species reported by several investigators lo). Our decay scheme for lgggPo + lg9Bi differs completely from that of Korman et al. *). We searched carefully for an M4 isomeric transition between the 3’ isomer and 4- ground state and found no obvious candidate in either the electron spectrum or the y-spectrum. We searched the (transition) energy region 400-900 keV, which covers the range of estimated energies of the +’ state made by Halperin lo) (x 800 keV), and by us (x 600 keV) using the energy systematics of the S+ intruder state in the heavier Bi isotopes. Thus, we arbitrarily set an upper limit on the intensity of the M4 isomeric transition by asserting that the corre-

R. A. Bruga et al. / ‘99.20’Bi

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sponding K conversion electron intensity is less than, or equal to, that of the strongest unidentified electron line (corresponding to a transition energy of 667 keV) in the energy range searched. With this limit and a procedure similar to that used in the ‘O’Bi case for estimating the feeding and decay intensities of the +’ isomer (in this case, because we have only a limit, we did not assume any direct B-feeding of the l+ state), we deduce a partial M4 y-decay half-life of 2 1302 min. This can be zompared with a Weisskopf single-particle estimate of 2.30 min for a 667 keV transition, i.e., a retardation of 2 380. The reduced M4 transition probability limit for this transition is shown in fig. 2. We note that if the retardation of this transition is similar to 201mBi, then for a transition energy of 667 keV, the M4 isomeric transition decay branch would be six times weaker than the upper limit set by our data. Consideration of fig. 2 shows that the M4 transitions in 199*“‘Bi are unique in their retardation. Shell-model intruder states are known to exhibit strongly retarded decay to “non-intruder” states (see below). However, we first make some remarks about the I-forbiddenness of the s3 + h, transition, E5 admixtures, possible penetration effects, and our measured ak and K/L values. The I-selection rule for an M1 transition is Al 5 (A- 1) [see ref. “)I. Thus, the s+ + h, transitions in 199,201Bi are I-forbidden M4 transitions (in fact they are the only known l-forbidden M4 transitions in odd-mass nuclei). To obtain an estimate of the retardation of this M4 transition due to the violation of the above l-selection rule, we considered all M,I transitions (A >= 2) in spherical odd-mass nuclei between fairly pure shellmodel states (1 a good quantum number) and remarkably, we were only able to find one case: the g, + s+ M3 transition in l 13Sn [ref. ‘)I. We thus estimated the retardation of this transition (it is the same degree of I-forbiddenness as the s+ + h, M4) by comparing with a Weisskopf estimate (see fig. 2). The M3 transition in l1 3Sn is 38 times slower than a Weisskopf estimate, suggesting that the retardation of the M4 transition in 199,201Bi is not due to an I-selection rule. We also considered the possibility of E5 admixtures in the s+ + h, transition, since they are I-allowed. The centroids of our ctk and K/L measured values for 201Bi allow maximum 7 % and 11 % E5 admixture, respectively. A 10 % E5 admixture would result in a T(E5) of 12.8 [T(ES) is defined in the caption to fig. 21. This corresponds to a minimum retardation of - 14 relative to a Weisskopf estimate (see also fig. 2); and, a minimum retardation of - 25 compared to the E5 transition rate that would be expected from the systematics of Raman et al. 13). The hindrance of the M4 matrix element raises the question of whether the penetration matrix element 14) makes an observable contribution to the S+ + h, decay. We can estimate the incidence of a penetration effect in the 201Bi M4 transition by using the c1kvalue and tables of Hager and Seltzer I’). We deduce a value of the penetration parameter I of - + 0.4, which suggests the absence of any penetration effects. We can reasonably argue that the hole + particle nature of this transition equally hinders the matrix elements for M4 and E5 internal conversion, and M4 penetration processes.

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Shell-model intruder states generally show strongly retarded decay to non-intruder states. Thus, for example, the hi’ + d;’ E3 transitions in I959197,199,201T1 exhibit retardations of N 300 [refs. i6, “)I ; the hi ’ + h&’ Ml transitions in ’ 89319’ 3193A~ exhibit retardations of N 1.6 x lo4 [ref. ’ *)I ; and, the gp ’ + g$ ’ transitions in 115- lz 1I exhibit retardations of N 1 x lo4 [ref. 19)]; all relative to Weisskopf single-particle estimates. This is generally attributed to the particle-hole nature of these transitions; e.g. in *O’Bi the transition can be regarded as s;~@*O*PO + h;’ @*OOPb. We know of only one attempt “) to calculate the hindrance of such transitions in heavy nuclei. (The case of d:’ + fi’ M2 transitions in the A - 40 region has been more widely addressed, see e.g. ref. *‘). It is interesting to note that the 3’ state in *O’Bi decays by an M2 branch (see fig. 1) and from the measured 6, half-life and branching intensity we deduce a hindrance of 5000, relative to a Weisskopf single-particle estimate. Thus, we can predict the half-life of the I+ state in *03Bi (see fig. 1) to be z 5 s. using the branching intensity data of ref. 6, ind a hindrance of x 3500 relative to the Weisskopf single-particle estimate for the I+ + $- E3 branch estimated from the *01s*O’Bi systematics. We 2 note finally that the recent measurement *l) of the half-life of the +’ state in *“Bi (at 2443 keV) shows that the E3 decay of the state is enhanced, presumably due to the state’s admixture of $+{s;’ Q 210Po(O:)} and $+{fi’ 0 208Pb(31)}. Thus, in general, at higher energies we can expect the retarded decay of intruder states to disappear due to admixture of other states. In conclusion, we wish to emphasize that the strongly hindered decay of shellmodel intruder states can give rise to isomerism in nuclei with anomalously long lifetimes (N 103-lo4 times the Weisskopf estimates). The experiments were performed at UNISOR, a consortium of thirteen institutions. It is supported by these institutions and by the Basic Energy Sciences Program of the US Department of Energy, under contract number DE-AC 05-760R00033 with Oak Ridge Associated Universities. Work at Georgia Institute of Technology is supported in part by the US Department of Energy under contract number DEAS05-76ERO-3346. Work at the University of Tennessee is supported in part by the US Department of Energy under contract number DE-AS05-76ERO-4936. The Oak Ridge National Laboratory is supported by the US Department of Energy under contract number W-7405-eng-26 with the Union Carbide Corporation. We would like to thank the UNISOR staff and the staff of the Oak Ridge Isochronous Cyclotron for their assistance in this investigation. We are especially indebted to Dr. W. B. Ewbank of the Nuclear Data Project, Oak Ridge National Laboratory for a search of the Evaluated Nuclear Structure Data File (ENSDF) for all y-rays that have multipolarity M3, M4 and E5.

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