Volume 28B, number 1 P H Y S I C S L E T T E R S 28 October 1968

A N A N O M A L O U S C O U P L I N G ½+ S T A T E I N 7 3 S e

G. MU RR A Y , W. J . K. W H I T E a n d J . C. W I L L M O T T Physics Department, University of Manchester, England

and

I~. F. E N T W I S T L E Physics Department, University of Otago, New Zealand

Received 30 September 1968

The exis tence of a 25.9 ~ 0.3 keV E3 t rans i t ion is repor ted in 73Se with a half- l i fe of 42 min. This together with m e a s u r e m e n t s on the ~ decay of the ground state and i somer ic state of this nucleus, implies that the spins of these s ta tes a re ½+and ~--, respect ively .

A n u m b e r of a n o m a l o u s ~.+ and 4 + s t a t e s h a v e b e e n i d e n t i f i e d in n u c l e i w i th N o r Z = 41, 43, 45 a n d 47 [1] and s e v e r a l p o s s i b l e c o u p l i n g s c h e m e s w h i c h cou ld a c c o u n t f o r s u c h a n o m a l o u s s t a t e s h a v e b e e n pu t f o r w a r d [2]. We p r e s e n t e v i d e n c e h e r e w h i c h s u g g e s t s t h a t a ½+ s t a t e of t h i s t ype e x i s t s in a n N = 39 n u c l e u s n a m e l y 73Se39.

P r e v i o u s w o r k [1] on 73Se h a s e s t a b l i s h e d t h a t t h e r e a r e two l o w - l y i n g p o s i t r o n - e m i t t i n g l e v e l s , one of h a l f - l i f e 7 . 1 h and the o t h e r of 42 m i n . No t r a n s i t i o n b e t w e e n t h e s e l e v e l s had b e e n o b s e r v e d and the o r d e r of t he l e v e l s had not b e e n e s t a b l i s h - ed. A p a r t i a l d e c a y s c h e m e i s s h o w n in f ig. 1. T h e l e v e l s p i n s of t he 73As g r o u n d s t a t e , 67 keV and 428 keV l e v e l s h a v e b e e n e s t a b l i s h e d by a c o m b i n a t i o n of n u c l e a r s y s t e m a t i c s , l i f e - t i m e and

I/" d - 0.026 42m 7/=+ ~ o 7.,h JI

1.29 1~4~2 a ~ ~

9/2+ 5 " /~ r~ . . 1.65 1 7 . 7 1

~ - o.o671~ sons

3}2-- ~ 0 7 6 d

7 3 A s 33 40

Fig. 1. Pa r t i a l decay scheme of 73Se; the l o g f t values of the fl+ decays a re shown in bracke ts .

i n t e r n a l c o n v e r s i o n m e a s u r e m e n t s t o g e t h e r w i th a m e a s u r e m e n t of the 3 6 1 - 6 7 keV y - y a n g u l a r c o r r e l a t i o n [1].

In the p r e s e n t w o r k the 73Se a c t i v i t y w a s p r o - d u c e d by the 70Ge( 'a , n) r e a c t i o n w i t h the M a n - c h e s t e r H i l a c a n d the m e t h o d of B o s w e l l e t a l . [3] w a s u s e d to c h e m i c a l l y e x t r a c t the s e l e n i u m . T h e d e c a y c u r v e s of the 67 keV and 361 keV y - r a y s ,

' I I I I ' I

eOK Buildmup ~xl decay of Ca'-oe ground state activity • e~ $ca~ ÷ K)

l

I O K ' i I i I I I I I i I 2 4 6 8 I 0

11ME [HOURS] F i g . 2 . T h e b u i l d - u p a n d d e c a y c u r v e s o f t h e 67 k e V a n d 361 keV ~ rays following the/3 decay of the 73Se ground

state.

er

Z

8 u 201<

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Volume 28B, number 1 P H Y S I C S L E T T E R S 28 October 1968

4 0 0 C

~ 2 0 0 C z

i ° 200C

I I I

X- ray X-roys 25.9 (L~'M|

I I I I

i I FI9 3(o)

Ek~u'om ond X-rays

. . . i _ _ ~

Fig 3(b}

X-roy X-rays X-toys o ~ y

I I I I I 5 0 I 0 0 150 2 0 0 2 5 0

C I ' I A N I E L ~

Fig. 3a. The e - and X-ray spectrum recorded with a 73Se source inside the proportional counter: energies are quot- ed in keV;

3b. the X-ray spectrum with the 73Se source outside the counter.

o b s e r v e d with a 15 c m 3 Ge(Li ) d e t e c t o r , a r e shown in fig. 2. The r a t e of b u i l d - u p of t h e s e l i n e s i s c o m p l e t e l y c o n s i s t e n t wi th f eed ing of the 7 . 1 h l e v e l by the decay of the 42 min l e v e l thus e s t a b - l i s h i n g the o r d e r of the two l e v e l s in 73Se.

The 73Se e l e c t r o n s p e c t r u m in the e n e r g y r a n g e 0 to 40 keV w a s i n v e s t i g a t e d wi th a con t in - uous f low gas p r o p o r t i o n a l coun t e r as p a r t of a s e a r c h fo r the t r a n s i t i o n f eed ing the 73Se ground s t a t e . T y p i c a l s p e c t r a , ob ta ined wi th the s o u r c e i n s i d e and ou t s ide the c o u n t e r , a r e shown in fig. 3a and fig. 3b, r e s p e c t i v e l y . The K and L + M c o n v e r s i o n l i n e s of the 25.9 ± 0.3 keV t r a n s i t i o n w e r e o b s e r v e d to decay wi th a h a l f - l i f e of 40 ± 5 m i n i nd i ca t i ng an E3 t r a n s i t i o n . The to ta l i n t en - s i ty of the 25.9 keV t r a n s i t i o n w a s c o n s i s t e n t wi th the a m o u n t of b u i l d - u p of the 73Se ground s t a t e . The K - (L +M) e n e r g y d i f f e r e n c e was 11.6 + 0.3 keV which i s in a g r e e m e n t wi th the v a l u e e x p e c t - ed fo r a t r a n s i t i o n in 73Se (11.4 keV) but not in 73As (10.7 keV). The m e a s u r e d I</(L +M) i n t e n s i - ty r a t i o , 0.23 ± 0.03, i s c o n s i s t e n t wi th that fo r a 26 keV 1~.3 t r a n s i t i o n (0.25). F r o m th i s , t o g e t h e r wi th d e t a i l e d s tud ie s [4] on the 42 min fl+ d e c a y , we conc lude that a 25.9 keV l e v e l in 73Se d e c a y s p a r t l y (82%) by an E3 t r a n s i t i o n to the g round s t a t e and p a r t l y (18%) by fi+ e m i s s i o n . The sp ins of the g round s ta te and f i r s t exc i t ed s t a t e in 73Se m u s t t h e r e f o r e be e i t h e r -~ +, 3- o r 27-+, ½- and the t h r e e p i e c e s of e v i d e n c e r e l e v a n t to the c h o i c e of the l a t t e r c o m b i n a t i o n wi l l now be d i s c u s s e d .

36

With one e x c e p t i o n a l l the known g r o u n d - s t a t e sp ins of nuc l e i wi th N o r Z = 39 a r e ½- [1]. The e x c e p t i o n is 8 5 y w h e r e the ½- i s at 40 keV. T h r e e of the nuc l e i have high e n e r g y M4 t r a n s i t i o n s p r e - c lud ing a l o w - l y i n g ~- s t a te . In the s ing ly c l o s e d - she l l n u c l e u s 89 the f i r s t 3- 39 Y50 ~ state is at 1.51 IVI~V. Thus one would anticipate a ground-

1- 9-" state ol- near ground-state spin of ~ but not If the ground-state spin of 73Se is ~+ the 1½%

branch to the ~- state in 73As is a unique first- forbidden transition and its logft value (7.7) in- dicates it to be the fastest such transition yet ob- served in odd A nuclei [5]. However a logft value of 7.7 is not exceptional for a non-unique first- forbidden transition thus favouring the ½+ assign- ment to the ground state.

Previous experiments [1] have indicated the absence of a 42 min/3-decay branch to the 73As 67 keY level. From detailed measurements [4] on the 42 min/3 + decay we have placed a lower limit of 8.0 to the logft value of this branch. This would be extremely high for an allowed transition but is consistent with an assignment of ½- to the isomeric state.

We conclude that the ground-state and first- exc i t ed s ta te sp ins of 73Se a r e ~+ and ~'-, r e s - p e c t i v e l y . The c a l c u l a t i o n s of I k e g a m i and Sano [2] on such a n o m a l o u s ~-+ s t a t e s in nuc l e i wi th N o r Z = 47, 45 and 43 should c l e a r l y be ex t end - ed down to N = 41 and 39 and, in addi t ion , a di- r e c t m e a s u r e m e n t of the 73Se g r o u n d - s t a t e spin would be va luab l e .

Volume 28B, number 1 P H Y S I C S L E T T E R S 28 October 1968

1. Nuclear Data, 1B, 4 and 6 (1966}. 2. H.Ikegami and M.Sano, Phys. Letters 21 (1966) 323

and references therein. 3. G. G. J. Boswell, T. McKee and J. Lynch, University

of Salford, to be published.

4. G. Murray, W.J.K. White, J. C. Willmott and R. F. Entwistle, to be published.

5. C.S. Wu, in Alpha, beta and gamma ray spectroscopy ed. K. Siegbahn (North Holland, 1965) Volume 2, p. 1378.

E R R A T A

Autocor re la t ion effects in the neutron induced f i ss ion c r o s s sect ion of 235U, M.G. Cao, E. Migneco, J. P. Theobald, Phys ics Le t t e r s 27B (1968) 409

The sen tence s t a r t ing at l ine 4 in the las t co- lumn on p. 410 should read:

"Thus, the resu l t ing f i ss ion c r o s s sect ion is composed of a dominant pa r t randomly f luc tua t - ing (3-; 0 channel) and a sma l l par t showing a grouping s t r u c t u r e (3-, 4-; 1 channel) ."

Leve l s t r u c t u r e of 89Zr via the reac t ion 90Zr(p, d)89Zr, H. Taketani , M. Adachi, Y. Yoshida, M.Ogawa, K. Ashibe, Phys ics Le t - t e r s 27B (1968) 499

Page 500 r ight column 25th line should read:

1.09(2p~ 1) ins tead of 1.09(2p 71)

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