The character of the 1315 keV state in Yb174

Nuclear Physics 29 (1962) 100---108; ~ ) North-Holland Publishing Co., Amsterdam N o t to

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THE CHARACTER

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O F T H E 1315 k e Y S T A T E I N Y b tTt

H . J. P R A S K , J. W . B I C H A R D *, E. O. F U N K , Jr. a n d J. w . M I H E L I C H

University o/ Notre Dame, Notre Dame, Indiana *? R e c e i v e d 14 J u l y 1961 A b s t r a c t : T h e electron c a p t u r e d e c a y of L u 1.~ w h i c h p o p u l a t e s levels of 76.5 a n d 1315 k e V i n Y b 1~4 h a s b e e n r e - i n v e s t i g a t e d . A m e a s u r e m e n t of t h e K - c o n v e r s i o n coeificient of t h e 1238 4-7 k e Y t r a n s i t i o n w a s m a d e , a n d a directionM correlation m e a s u r e m e n t p e r f o r m e d o n t h e 1238 keV- 76.5 k e V cascade. T h e d a t a were m o s t c o n s i s t e n t w i t h a n a s s i g n m e n t of 2 - for t h e 1315 k e V level in Y b 17t.

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

The electron capture decay of Lu 1~4 to states in Yb 1~4 has been investigated b y several groups 1-4), and it has been established that there axe two gamma transitions in cascade with energies of 76.5 keV and approximately 1240 keV (refs. 1, ~), and that the decay to the 1315 keV state is b y L capture ~). There is some disagreement as to the half-lives of the two isomers of Lu 1~4, b u t the most recent experiments 4) indicate that Lu 17~ decays with a half-life of approximately 170 d, and a long lived metastable state decays via an isomeric transition with a half-life of approximately 90 d. The coupling rules of Gallagher and Moszkows'ki s) for odd-odd nuclei predict a ground state spin of 1 -- for Lu 174. Because of the absence of observable transitions to the expected second excited state ( 4 + ) or ground state ( 0 + ) from the 1315 keV state, this level was tentatively designated 2,a) as 0 + . The purpose of the present work has been to establish the nature of this level b y determining the multipolarity of the high energy transition and the spin of the level b y means of a directional correlation measurement. A gammagamma coincidence measurement gating on the high energy transition was also made. 2. R e s u l t s 2.1. D E T E R M I N A T I O N

OF THE INTERNAL

CONVERSION COEFFICIENT

The energy of the upper transition was determined to be 12384-7 keV from a measurement of the conversion lines in an intermediate image spectrometer. ? N o w a t U n i v e r s i t y of B r i t ~ h Columbia, V a n c o u v e r , B. C. ** W o r k s u p p o r t e d b y U . S A t o m i c E n e r g y C o m m i s s i o n . 100

101

THE 1815 keV STATE IN Yb 174

The internal conversion coefficient of the 1238 keV transition was determined by obtaining the ratio of the intensities of K-conversion electrons and photons. The activity was produced b y proton irradiation of enriched Yb yielding both Lu ~78 and Lu 17' activities. The Lu lr~ activity did not interfere in this experiment since the highest energy transition is 637 keV (ref. e)). The spectrometer scurces were evalgorated from an HC1 solution onto 1 mg]cm ~ Mylar film with aluminium backing. The same sources were used to obtain the relative gamma r a y intensities. The relative intensities of the conversion electrons were determined with an intermediate image p-ray spectrometer of the SiegbahnSl~itis type having momentum resolution of approximately 2.5 ~o. Fig. 1 displays the electron spectrum obtained. I

I

I

I

I

I

I

300 ~ sr

! o

.J ! t00

mO

~

~

i

30

5i

!

li2

33

~(AeNET

Fig.

1. C o n v e r s i o n - e l e c t r o n

spectrum

I

~t

35

54

CURIW.NT (A)

o f L u 1~' f r o m

1070---1370 keV.

The relative gamma-ray intensities were obtained with an integrally packaged 7.6 c m × 7.6 cm NaI crystal and a 256-channel analyzer with energy resolution of 8 ~o at 662 keV. Fig. 2 displays the high energy portion of the gamma r a y spectrum. A number of transitions for which the internal conversion coefficients are well established were measured in identical geometries for both the conversion electrons and the photons, thus calibrating the geometry of the apparatus. The sources used for comtmrison were Bi s°7 (570, 1064 keV), Cs187 (662 keV) and Coe° (1173, 1333 keV). A value for CK of (6.64-3.3) × 10 -4 was obtained for the 1238 keV transition.

102

H.j.

P ~ S X zt a/.

In table 1 are listed the K-conversion coefficients for Z = 70 for dipole and quadrupole radiation as obtained from the tables of Sliv and Band ~). The I

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I

I

it) FZ :3 0 L)

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",.,I

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too

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~o

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moo

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moo

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taoo

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14oo

I

moo

E N E R G Y (keV)

Fig. 2. High-energy p o r t i o n of g a m m a - r a y s p e c t r u m of L u 1.4 (solid line) t a k e n w i t h a 7.6 can x 7.6 cm N a I crystal. T h e dashed curve is the s p e c t r u m of N a u . T h e t w o p h o t o p e a k s h a v e been superAmposed for comparison.

experimental internal conversion coefficient is consistent with an assignment for the transition of E1 with an upper limit of 5 % M2 admixture. 2.2. D I R E C T I O N A L C O R R E L A T I O N

The gamma-gamma directional correlation measurement was made using t w o 5.1 c m × 5.1 c m N a I c r y s t a l s m o u n t e d on R C A 6342-A p h o t o m u l t i p l i e r s . T h e

THE 1815 key STATE IN Y'o174

103

resolving time of the coincidence circuit was 35 ns. The low energy counter was frontally shielded with 1.4 g/cm z of copper. The high energy counter was frontally shielded with 3 g]cm 2 of copper. The counters were 5 cm from the source which was in an aqueous solution of HC1 in a thin walled lucite holder TABLE 1 Theoretical K-conversion coeificients f r o m t h e tables of Sliv a n d B a n d 7) for a 1238 keV t r a n s i t i o n Multipole

I

Internal con v. coeff.

E1

[

8.6 × 10 -4

M1

I

3.65 x 10 -4

E2

]

1.98 × 10 -~

order

M

M2

1

8.4 × 10-4

1.2 cm high and 2 m m in diameter. The data were least squares fitted using the m e th o d of Rose 8) and the geometrical finite size corrections were extrapolated from the curves of Arns, Sund and Wiedenbeck 9). The analysis of the correlation data taken at three angles yielded the following coefficients: A s ---- 0.064+0.028,

A 4 = 0.032+0.046.

These values include a contribution to the gates from a weak impurity believed to be E u la. This i m pur i t y would affect the experimental values b y no more t h a n about 10 ~oSince the 1238 keV tlansition is predominantly E1 the spin of the 1315 keV level is 1, 2 or 3. If one considers the theoretical correlation coefficients 10) for cascades of the t ype I(D, Q) 2(Q)O, the experimentally determined A2 and At are consistent with quadrupole admixtures between 5.5 and 9.5 % for I ---- 1, 4 and 8 % for I = 2, or 1.5 and 5 % for I -----3. Thus, a unique spin assignment cannot be made from the directional correlation data alone. However, t h e only observed transition de-populating the 1315 keV level is t h a t to the first excited state. If the 1315 keV level were 1-- one would expect observable transitions to bot h the ground state and first excited state. If the 1315 keV level were 3- - one would expect observable transitions to both the 2 + state and the 4 + rotational state which should be at about 250 keV. Experimentally, upper limits for gamma r a y transitions of 1060 and 1315 keV are 3 % and 2 % of the 1238 keV transition intensity respectively. The spin value most consistent with the dat a is therefore 2 although arguments involving the absence of E1 transitions are not infallible. Moreover, a spin of 2 is most consistent with the correlation data if attenuation were present. A half-life of 1 to 2 ns would be expected for the 76.5 keV state, and Yb 174 is a strongly deformed nucleus, so t hat the quadrupole inter-

104

H.j.

P ~ S K ~ aJ.

action could produce appreciable attenuation of the true correlation. Marklund et a/. 11) and Bodenstedt et al. ll) have reported appreciable attenuation of directional correlation measurements performed on the (1380 keV)-(80 keV) gamma r a y cascade in Er le6. If comparable attenuation (Gs = 0.7, G~ ~ 0.6) were assumed in the similar case of the (1238 keV)--(76.5 keV) cascade in Yb lva, the experimental correlation results would confirm the choice of the 2(D, Q)2(Q)O sequence with a quadrupole admixture of 2.5 ~/o to 6.5 ~/o in the 1238 keV transition. IO4

(o) Singles (b) Cclncldoncn (a)

i0 3

0

ENERGY (koV)

Fig. 3. G a m m a - r a y s p e c t r u m c o i n c i d e n t w i t h 1298 keV. T h e p e a k a t 120 k e V i s d u e t o a n i m p u r i t y believed t o b e E u 1is.

105

THE 1315 keV STATE IN Yb174 2.3. G A M M A R A Y C O I N C I D E N C E S W I T H

1238 k e V

Gamma-gamma coincidences were obtained b y gating with a 3.8 cm × 5.1 cm NaI crystal on the 1238 keV photopeak and displaying on a multichannel analyzer the coincident spectra obtained with a 3.8 c m × 2.5 cm crystal. The coincidence measurements were made with the display crystal 2 cm from the source. The gate crystal was frontally shielded with 2 g/cm 2 copper absorber. The resolving time of the coincidence circuit was 0.14 ps. The coincident spectrum obtained is .displayed in fig. 3. 71Lurt4mF:~90d) 170.8

6-

I59.

3-

I

67.1

2--

II 1.7 44.6

44.6

Ec /

,, , 0 71Lu1749(R~170d)

1315

2 -

1238

1

<_t385

76.5

2 + 76.5

O+ I n

7o~'b174

0 .~' E(keV)

Fig. 4, T h e e l e c t r o n - c a p t u r e d e c a y of L u l~am a n d L u 174. T h e levels of L u 174 a r e t h o s e g i v e n b y H a r m a t z , H a n d l e y s n d Mihelich s) . T h e h a l f lives a r e t h o s e g i v e n b y R o m a n o v , I o d k o a n d T u c h k e v i c h * ) .

The relative intensities of the 76.5 gamma-ray and K-X rays obtained from the coincidence measurement gating the 1238 keV photopeak are consistent with those of Wilson and Pool 2). If the theoretical E2 conversion coefficient (atx ~ 1.52) for the 76.5 keV transition is assumed ]3), then the K / L capture ratio to the 1315 keV level is less than 0.15. Conversely, the relative intensities obtained from the coincidence measurement yield a conservative upper limit of 1.9 for the K-conversion coefficient of the 76.5 keY gamma ray. The accuracy

10fl

H . J . PRASK ft al.

of the experiment is not sufficient to exclude the possibility of an anomalously high conversion coefficient 14) for the E2 transition. For this experimental value, the theory of Brysk and Rose 16) for allowed electron capture decay would indicate that the decay energy is less than 79 keV. Therefore, the Lu 174a Yb 1~4 energy difference is less than 1385 keV, the semiempirical mass tables of Cameron 16) predict a value of 1502 keV. Fig. 4 displays the decay scheme. The analysis of the coincidence spectrum shows an indication of a transition of approximately 60 keV in coincidence with the 1238 keV transition. This transition could conceivably be due to a rotational excitation of the I315 keV state. However, no other evidence for such a transition in Yb iv' has been observed. 3. D i s c u s s i o n

Although few 2-- levels have been observed in even deformed nuclei thus far, at least two cases are available for comparison. In the decay of Re is2 to W ls2 a 2 - - level is observed at 1290 keV. The transitions to the 0 + ground state and 4q- second excited state are approximately 0.1 and 0.01 of the intensity of the transition to the 2 + first excited state iv). Ewan etag. 18) have observed 2-- levels at 1264 keV and 1358 keV in the decay of Tb 16° to D y 1~°. From each level strong E1 transitions are reported to the first excited state. No transitions were observed from the 2-- levels to either the ground state or the second excited state. There are considerably more data available on the gamma vibrational bands in even nuclei. The position of the 2 + states of these bands in Er 1~ (787 keV) (refs. 11, iT)), Erl,a (822 keV (ref. 21)), ybl70 (1232 keV) (ref. 3)), y b l , , (1468 keV) (ref. 17)) and W la2 (1~22 keV) (refs. XT,~2)) indicate that such a level in Yb 174 m a y lie fairly high. As would be expected from the decay energy available, no population of such a band was observed in the electron capture decay of Lu174.

The amount of M2 admixture in the 1238 keV transition is worth remarking on. Arns, Sund and Wiedenbeck 19) have investigated the (1175 keV)--(87 keV) cascade in D y le° b y means of ~ directional correlation measurement and report a comparable M2 admixture in the 2----~ 2+t- transition. Shirley, Johnson and Schooley 2o)have performed nuclear orientation experiments on Tb isotopes and found the quadrupole admixture of the 1175 keV transition in D y le° to be less than 0.7 %. A few remarks m a y be made concerning the levels in Lu iv'. The level positions and spins were postulated in a previous publication s) in which it was pointed out that the ratio of the intensities of the L-conversion electrons is consistent with an assignment of M3 for the 59.05 keV transition. The energy

THE 1315 keY STATE IN Ybt?4

107

ratio of the 111.7 and 44.6 keV levels is exactly that predicted by the I ( I + 1 ) interval rule for the postulated spins. If this interpretation is correct, the retardation, of 109 for the M3 transition might be partly accounted for by K-forbiddenness impeding the M3 transition between states of K = 6 and K = 1. The absence of electron capture decay from the isomeric state in Lu 1~4to the ground state rotational band of Yb 1~4 can also be attributed to a high degree of Kforbiddenness (~4K= 6). Electron capture might also be expected to a possible rotational excitation of the 1315 keV state. If one assumes an inertial parameter of the same magnitude as t h a t of the ground state band, decay to only the 3-- and 4-- rotational states would be energetically possible. For both of these possible states the decay would be at least second forbidden and thus not likely to compete with the 59 keV M3 transition depopulating the metastable state. However, if the half lives reported by Romanov et al. 4) for the two isomeric states are correct, the Lu 174mactivity would have been much weaker t h a n the ground state decay activity since an aged source was used in our experiment. I t should be pointed out that Romanov et al ~) did not observe the 67.1 keV transition reported by H a r m a t z et al 3). It is possible that this transition was not resolved in their experiment since the L I line of the 67.1 keV transition would lie 0.4 keV below the intense M line of the 59.05 keV transition. It is also interesting to note t h a t if the half-life of Lu l ~ is taken to be 170 d, the branching ratios of Wilson and Pool ~) lead to log fl values of 8.6, 8.3 and 7.1 for electron capture to the ground state, 76.5 keV and 1315 keV states in Yb 1~. The ratio of transition probabilities for decay to the 0 + and 2 + levels of the K = 0 ground state band is then very nearly 2. Assignments of K = 1 and 0 for Lu 1 ~ lead to theoretical ratios at 0.5 and 2.0, respectively. I t is clear that K = 1 is not the correct assignment unless one admits a considerable admixture of other K-numbers. The authors wish to thank Dr. R. C. Pilger and B. Harmatz for their helpful comments. References 1) 2) 3) 4) 5) 6) 7)

L. T. Dillman, R. W. Henry, N. B. Cove and R. A. Becker, Phys. Rev. 113 (1959) 635 R. G. Wilson and M. L. Pool, Phys. Rev. 117 (1960) 517 B. Harmatz, T. H. Handley and J. W. Mihelieh, Phys. Rev. 119 (1960) 1345 V. A. Romanov, M. G. Iodko and V. V. Tuchkevich, J E T P 11 (1960) 733 C. J. Gallagher, Jr. and S. A. Moszkowski, Phys. Rev. 111 (1958) 1282 J. W. Bichard and J. W. Mihelich, Phys. Rev. 116 (1959) 720 L. A. Sliv and I. M. Band, Coefficients of internal conversion of gamma radiation, p a r t l, K shell (Physioo-Technical Institute, Academy of Science, Leningrad, 1966); issued in USA as Report 57 ICC K1, Physics Department, University of Illinois, Urbana, Illinois 8) M. E. Rose, Phys. Rev. 91 (I953) 610 9) R. G. Arna, R. E. Sund and M. L. Wiedenbeck, privately circulated report

108

10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)

H.j.

PRASK et ~ .

R. G. Arns and M. L. V~iedenbeck, privately circulated report I. Marklund, B. van Nooijen and Z. Grabowski, Nuclear Physics 15 (1960) 533 E. Bodenstedt, H. J. KSrner, C. Giinther and J. Radeloff, Nuclear Physics 22 (1961) 145 M . E . Rose, Internal conversion coefficients(North-Holland Publishing Company, Amsterdam, 1958) F. K. McGowan and P. H. Stelson, Phys. Rev. 107 (1957) 1674 I-L Brysk and M. E. Rose, Revs. Modern Phys. 30 (1958) 1169 A. G. W. Cameron, Atomic Energy of Canada Limited Report AECL, 1957 (unpublished) B. Harmatz, T. H. Handley and J. W. Mihelich, Phys. Rev. (to be published) G. T. Ewan, R. L. Graham and J. S. Geiger, Nuclear Physics 22 (1961) 610 R. G. Arns, R. E. Sund and M. L. Wiedenbeck, Nuclear Physics 11 (1959) 411 D. A. Shirley, C. E. Johnson and J. F. Schooley, Phys. Rev. 120 (1960) 2108 K. P. Jacob, J. W. Mihelich, B. Harmatz and T. H. Handley, Phys. Rev. 117 (1960) 1102 J. j . Murray, F. Boehm, P. Marmier and J. W. DuMond, Phys. Rev. 97 (1955) 1007