Excited states of Eu153

Nuclear Physics 5 (1958) 187--194; @

North-Holland Publishing Co., .4ms~erdam

to be reproduced by photoprlnt or microfilm without written permission from the

Not

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publisher

O F E u lsa

C. W. M c C U T C H E N

Cavendish Laboratory, Cambridge Received 29 A u g u s t 1957 The fl- decay of S m z " a n d t h e electron c a p t u r e decay of Gd z" h a v e been studied w i t h a N a I scintillation spectrometer, w i t h a x e n o n p r o p o r t i o n a l c o u n t e r used w i t h and w i t h o u t escape g a t i n g 1), a n d w i t h a conversion-electron p r o p o r t i o n a l counter. The t w o decay schemes h a v e been verified and, in one case, extended. F r o m a m e a s u r e m e n t of its internal conversion coefficient t h e 97 keV 7 in the Gd z58 decay is s h o w n to be E l . The decay schemes and multipolarities are s h o w n to be inconsistent w i t h a n y likely spin sequence for E u l~a and g r o u n d s t a t e spin of Gd z".

Abstract:

1. I n t r o d u c t i o n

T h e excited states of E u 163 can be reached b o t h b y the fl- d e c a y of 47 h Sm zSa a n d the electron c a p t u r e d e c a y of 236 d Gd ~53. F o r a s u m m a r y of the situation on Sm ~Sa the r e a d e r is advised to consult the recent p a p e r b y D u b e y , Mandeville, a n d R o t h m a n a). All groups agree t h a t the first and second excited particle states are at 103 and 172 keV respectively, t h a t the cascade transitions are M1 + E 2 a n d t h a t the cross-over transition is v e r y weak. e2 S m ~

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C. XV. M c C U T C H E N

T h e r e is no a g r e e m e n t on the place of the 540 keV 7 (and the 600 keV 7) in the d e c a y scheme. Fig. 1. shows the conclusions of D u b e y et al. 3). T h e 103 keV level has a half-life of 3.4 × 10 -9 sec ~). Mrs. M a r t y s) reports t h a t the 84 keV r o t a t i o n a l level (previously observed b y Coulomb excitation 4)) is w e a k l y excited. T h e d a t a on Gd ~63 are more conflicting. Conversion electron studies ~) h a v e found w h a t are a p p a r e n t l y the same 103 and 70 keV transitions as are excited in the Sm 15~ decay. However, Church and Goldhaber 7) h a v e also seen a 97 keV line. McGowan e) reports t h a t within a few keV of the delayed 103 keV 7 there is a p r o m p t 7 (T½ =< 10-9 sec). This is p r e s u m a b l y the 97 keV transition r e p o r t e d b y Church and Goldhaber. H e f u r t h e r states t h a t in the Gd ~Ss d e c a y the 70 keV transition leads to the m e t a s t a b l e level r a t h e r t h a n to the short-lived one.

2. S o u r c e s T h e sources were n a t u r a l Sm203 and Gd~O 3 i r r a d i a t e d with t h e r m a l n e u t r o n s at the A . E . R . E . at Harwell. T h e Gd sources were allowed to age for several weeks before use to allow the 18 h Gd 1~9 and the 7 d T b 1~1 activities to die. F o r the conversion electron work the sources were either deposited from a w a t e r slurry of the finely divided oxide or v a c u u m evap o r a t e d as the chloride.

3. E x p e r i m e n t a l P r o c e d u r e and R e s u l t s First the 7 spectra of Sm and Gd were c o m p a r e d to v e r i f y which ~'s were c o m m o n to b o t h decays. Two escape gated 1)t x e n o n c o u n t e r runs were t a k e n in quick succession to avoid possible gain shifts. These (fig. 2) show clearly t h a t the 103 and 70 keV ~'s are those c o m m o n to b o t h decays. These runs also place an u p p e r limit of 10 ~ on I~(97)/I~(103) in the Sm decay. An o r d i n a r y x e n o n c o u n t e r run, in b e t t e r statistics, on Gd showed t h a t the 97 keV y was 1.74 times as strong as the 103 keV~. McGowan, separating the y's b y means of time resolution, gives this same i n t e n s i t y ratio as 1.4. A coincidence e x p e r i m e n t on Gd in which receipt of a 70 keV ~ b y the scintillator g a t e d x e n o n c o u n t e r pulses to the kicksorter showed (fig. 3) t h a t in Gd the 70 keV y is in coincidence with the 103 keV 7 a n d not with the 97 keV 7. This provides direct confirmation of the conclusion reached b y McGowan b y less direct reasoning. An u p p e r limit for the K conversion coefficient of the 97 keV transition was o b t a i n e d b y c o m p a r i n g I~(97)/I~(103) with Ie(97)/Ie(103). T o find t Escape gating suppresses all the full energy peak in the xenon counter spectrum; the resulting s p e c t r u m is m a d e up entirely of K X r a y escape peaks.

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the l a t t e r q u a n t i t y the conversion electron s p e c t r u m was e x a m i n e d with a p r o p o r t i o n a l c o u n t e r of 4 in. d i a m e t e r filled with 1 to 2 a t m of argon a n d

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a few cm of ethylene. ,Fig. 4 shows that the highly K converted transition of the pair a t about 100 keV is that at 103 keV. As the two 7's are very close in energy some care was taken to be certain of the identification. For this purpose the counter was equipped with an aluminium window near the source to let out y's and X rays. These could then be counted by an external y counter. That the supposed K conversion line really was a K T

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line was shown by enhancement of the peak when the proportional counter was put in coincidence with K X rays caught b y a scintillator just outside the window. The assignment to 103 rather than to 97 keV was first made on the basis of energy by calibrating the counter with an external X ray source. The assignment was verified by showing t h a t the conversion electron line was in coincidence with 70 keV y's. The K conversion line, of the 97 keV transition, though not clearly resolved is certainly less than ½ as strong as that of the 103 keV transition. The measured K conversion coefficient for the 103 keV transition is about 1.2 7, 9). Combining these data with m y value for Iy(97)/I~,(103) gives an upper limit of 0.37 for 0tK(97) which definitely classifies the transition as E l . The theoretical K conversion coefficients (as given by Sliv and Band 8) are 0.226 and 1.10 for E1 and E2 respectively. Since this work was completed Bisi, Germagnoli and Zappa 9) have published a magnetic spectrometer spectrum of these conversion electrons. Though t h e y appear not to notice it, their curve (and as far as I know this is the first time that the

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actual spctrum has been published) clearly shows the K line af the 91 keV transition. From their data the ratio Ie(97)/Ie(103) seems t o ,be between 0.25 and 0.35. This would put the 97 keV conversion coeffieierrt ~etwveen 0.185 and 0.26 which agrees well with the theoretical value. The escape gated runs in fig. 2 also show that, in the Gd decay, there is electron capture directly to the 103 keV state. In the Sm decay there is a fl branch to the 103 keV level. Fig. 2 shows that Ir(70)/(Iy(103 ) in the Gd decay is no higher than I~(70)/I~(103) in the Sm decay so the 103 keV level must be fed directly in the Gd decay also. If the necessary counter efficiency and wall effect corrections are made I~(70)/Iy(103) in the Sm decay is found to be 0.2. This should be compared with 0.25 found by McGowan and 0.24 found by Dubey et al. My value is probably poorer than theirs as the various corrections are harder to estimate for the xenon counter than for a scintillator. The agreement does, however, provide some confidence in the method. In the Gd decay the results of fig.2 must be used with care because any full energy counts from the Eu K X ray peak which manage by informal processes to get recorded despite the K escape gating will appear at 71 keV on the escape peak energy scale. Filtering out these X rays with 0.015 in. of Cd gave a value of I v (70)/I v (103) of 0.11 --0.05" +0.o7 The large uncertainty is because the statistics were poor and because there was some geometrical uncertainty in the effective thickness of the Cd absorber. Searches for other low energy 7's yielded nothing. The apparent peaks at 76 keV in fig. 2 (which might have represented a transition from the 172 keV level to the 97 keV level) were apparently lead fluorescence from the lead shield used in the K escape gating apparatus. They disappeared when the plain lead shield was replaced by a lead-cadmium graded shield. The high energy 7% in the Sm decay were verified and their intensity was measured relative to that of the 103 keV 7. Scintillator-scintillator coincidences showed that the 535 keV was in coincidence with the 103 keV 7 and probably with the 70 keV 7 as well. This agrees with the findings of Dubey et al. The relative 7 intensities found in this investigation are summarized in table 1 where they are expressed in terms of the intensity of the 103 keV 7. TABLE 1

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192

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MeCUTCHEN

If the ratio I~(535)/I~(103) is compared with Ie(535)/le(103) from the data of Lee and Katz 10) and the known value of the K conversion coefficient for the 103 keV transition is employed the K conversion coefficient for the 535 keV transition is found to be 0.175 which implies an M3 transition. Various groups disagree somewhat on I~(535)/.1~(103) b u t use of any 7 data makes this transition at least E3. This point was first noticed b y Mrs. Marty 3).

4. Beta Branching Ratios and ?t Values There is reasonable agreement between the most recent workers (Dubey and Mrs. Marty) on the branching ratios in the Sm decay. The numbers in fig 1 are those of D u b e y et al. except that the branch to the 700 keV level has been made to agree with m y idea of ltotal(535)/Itotal(103). In the Gd decay the transition energy has been deduced b y measuring the ratio of L to K capture and then using the theory of Marshak 11). Gupta and J h a 12) and Bisi, Germagnoli and Zappa 13), both using simplified decay schemes, have found the energy difference between the ground states of Gd 153 and Eu 153 to be 225 and 183 keV respectively. Gupta and J h a find that the intensity of the electron capture branch to the ground state is 25 °/o while Bisi et al. make it to be 2 ~o. The simplifications in the decay scheme do not entirely vitiate these results which, roughly, state that there is much L capture and that therefore the decay energy is small. The log fl values shown for the Gd decay in fig. 1 were derived assuming a total transition energy of 225 keV and splitting up the decay b y means of the total (7+conversion electron) transition intensities. To do this the measured 7 intensities were combined with the total conversion coefficients assumed b y D u b e y et al. in the case of the 103 and 70 keV transitions and taking 0.4 as the total conversion coefficient for the E 1 97 keV transition. No fl value is given for the branch to the 172 keV level because the transition energy is then uncertain b y a large factor.

5. Interpretation The ground state spin of Eu 153 has been measured 14) to be 5/2. From the rotational spectrum and Coulomb-excitation cross section 15) it is known that the deformation is large so the g r o u n d state is identified with the 5/2W Nilsson le) orbit 27. The ground-state spin of Sm 153 has not been measured but that of the neighbouring nucleus Gd lss has spin 3/2 17). As Gd 157 has the same spin and nearly the same magnetic moment it is likely that the ground states of both Gd isotopes are Nilsson orbit 52; in Gd 1~5 the hn/2 orbit 28 is empty while in Gd 157 it is filled. Sm 153is there-

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fore assumed to have the 3/2-- Nilsson orbit 52 as its ground state since, like Gd 15s it is a 91 neutron nucleus. Alaga is) has analysed the fl decay of Sm 153 using these assignments and finds it consistent with his asymptotic quantum number selection rules. The ground state fl branch is first forbidden hindered. The fl and ~, selection rules require that the 103 keV and 172 keV states be 3/2~- and ½~- respectively. In the absence of unsuspected selection rules, any other sequence which would attract first forbidden z]] = 0 or 1 branches will not account for the extreme weakness of the cross-over transition. The 97 keV state decays to the ground state by E1 radiation so it can be 3/2--, 5/2-- or 7/2--. Of these, only 7/2-- will not be fed by an allowed fl decay. On present information the 700 keV state poses an insoluble problem. The strength of the fl branch feeding it requires that its spin be ½, 3/2, or 5/2 and yet it appears to decay to the I03 keV state by E3 or M3 radiation. Clearly it is important to recheck the conversion electron data to see if the conversion coefficient really is so large. Only Lee and Katz have seen the conversion line of either high energy transition. Mrs. Marty looked but found nothing detectable in the presence of the fl's. Ignoring this difficulty we move on to the Gd153 decay and find another. Gd153decays to the 97, I03, and 172 keV levels in Eu 153with/t values which indicate spin changes of no more than I. From the decay of SmIs3 the spins of these states are required to be 7/2, 3/2, and ½. No choice for the ground state spin of Gdls3 can feed all of these levels. The situation is even more difficult if we believe that the ground state branch is very weak (as the results of Bisi et al. 13) indicate) for this means that the intermediate spin value of 5/2 is for some reason not fed. The ground state spin of Gd 153 has not been directly measured, nor has that of any other 89 neutron nucleus. Between 88 and 90 neutrons a violent change occurs in the deformation of the even isotopes of Gd 17) so the deformation status of 89-neutron nuclei is in doubt. The spherical shell model would predict a n f7/2-- level while the Nilsson assignment is either the ½+ orbit 60 or the 3 / 2 + orbit 57; the former if the hn/2 orbit 28 is filled and the latter if it is not.

6. Possible Explanations Perhaps Gd is3 has an as yet undetected long rived isomeric state from which one or more of the electron capture branches originate. If the isomeric state possesses the shell model spin and parity (7/2--) and the ground state has either of the deformed model values (½+, 3 / 2 + ) or vice versa then all the electron capture branches can be explained.

194

c.W.

MeCUTCHEN

This supposition; could most easily be checked by an investigation of the ratio 1(97)/1(103) for sources of different ages, for it is unlikely t h a t the initial population ratio for the isomeric and ground states would be the equilibrium values. No change in I~,(97)/I~,(103) was noted in these experiments, but the youngest sources were about a week old so equilibrium m i g h t already have been attained. FinMly, if the E1 97 keV transition leads not to the ground state but to an as yet undetected state the level that feeds it might be of even parity in which case the fl branch to it in the Sm decay would be first forbidden and could possibly be weak enough to agree with its experimental absence. Many choices of the spin of this level would then permit one of the Nilsson predictions to be used for the ground state of Gd 158. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)

C. W. McCutchen, Nuclear Physics 3 (1957) 76 V. S. Dubey, C. E. Mandeville and M. A. Rothman, Phys. Rev. 103 (1956) 1430 N. Marty, J. Phys. Rad. 16 (1955) 458 N. P. Heydenburg and G. M. Temmer, Phys. Rev. 100 (1955) 150 R. E. Hein and A. F. Voight, Phys. Rev. 79 (1950) 783 J. M. Cork, J. M. Le Blanc, W. H. Nester and F. B. Stumpf, Phys. Rev. 88 (1952) 685 F. K. McGowan, Phys. Rev. 93 (1954) 163 E. L. Church and M. Goldhaber, Phys. Rev. 95 (1954) 626 L. A. Sliv and I. M. Band, (Academy of Sciences of the U.S.S.R. Moscow, Leningrad; issued in U.S.A. as Report 57 ICC K1, Physics Dept., Univ. of Illinois, Urbana, Illinois) A. Bisi, E. Germagnoli and L. Zappa, Nuclear Physics 3 (1957) 670 M. R. Lee and R. Katz, Phys. Rev. 93 (1954) 155 R. E. Marshak, Phys. Rev. b l (1942) 431 R. K. Gupta and S. Jha, Nuovo Cimento 4 (1956) 88 A. Bisi, E. Germagnoli and L. Zappa, Nuclear Physics 1 (1956) 593 B. Bleaney and W, Low, Proc. Phys. Soc. A b8 (1955) 551 N. P. Heydenburg and G. M. Temmer, Phys. Rev. 104 (1956) 981 S. G. Nilsson, Mat. Fys. Medd. Dan. Vid. Selsk., 29 (1955) no. 16; B. R. Mottelson and S. G. Nilsson, Phys. Rev. 99 (1955) 1615L D. R. Speck, Phys. Rev. 101 (1956) 1725 G. Alaga, Phys. Rev. 100 (1955) 432