33 E0 Transitions and Intruder States Recent Results
J. Kantele, R. Julin, M . Luontama, and A . Passoja Department of Physics, University of Jyväskylä, SF-40100 Jyväskylä, Finland +
102,104
Results from studies of 0 states and E0 transitions in Pd and Pb are reported. Systematics of low-spin states in Pd isotopes and proton 2p-2h intruders in Pb isotopes are presented and discussed.
Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 10, 2018 at 20:13:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
202,204
Among the most interesting recent topics in low-energy nuclear -structure physics have been the intruder states. As resulting from intense experimental and theoretical studies, a unified picture of them seems now to be emerging [HEY85]. One of the tools in the study of the intruders has been provided by the E0 transitions which are particularly effective in obtaining information on 0 states in even-even nuclei (see [ΚΑΝ84] for a review of the subject). Understanding the E0's requires detailed knowledge especially of the radial parts of the initial and final-state wave functions. For this reason, these transitions are sensitive probes of the various nuclear models: For example, for the different versions of the IBA, the level energies seem to be f a i r l y easy to predict, the B(E2)'s are more d i f f i c u l t , and the E0's usually really hard. In our studies of intruding 0 states and associated i£0 transitions, we use several newly developed special nuclear spectrometry techniques, nota bly in-beam conversion-electron and inter rial-pair spectrometry with various combinations of magnetic and semiconductor devices [ΚΑΝ84], A recent set-up based on an intermediate-image (Siegbahn-Slatis) magnetic electron trans porter i s sketched in F i g . 1. With a 110 mmx3 mm Si(Li) detector, the transmission of this device is about 7 % and the momentum band-width Δρ/ρ=17 %. With a 5 mm thick Ge detector, the system is capable of detecting elec tron lines up to 7—3 MeV. +
+
2
Fig. 1. A magnetic + Si(Li) combination conversion-electron spec trometer based on an "old" Siegbahn-SlStis magnet. 1) beam, 2) target,3) target-changing system, *0 collimator and current measurement, 5) Faradaycup, 6) Pb shield, 7) anti-positron baffle, 8) detector, 10) cold fingers, 13) cylindrical plastic s c i n t i l l a t o r HO light guide, 16) P.M. tube. 0097-6156/86/0324-0221 $06.00/ 0 © 1986 American Chemical Society
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
NUCLEI OFF T H E LINE OF STABILITY
222 102,10Mpo;
a
n
(
j S y s t e m a t i c s o f L o w - S p i n S t a t e s i n Pd I s o t o p e s
From t h e p o i n t o f v i e w o f t h e p r e s e n t u n d e r s t a n d i n g o f t h e i n t r u d e r s t a t e s , t h e e v e n Pd n u c l e i a r e o f s p e c i a l i n t e r e s t s i n c e t h e y a r e e x p e c t e d t o e x h i b i t p r o t o n i n t r u d e r s o f t h e t y p e 2p-6h. Systematic information e x i s t s a l r e a d y f o r t h e 2p-2h a n d t h e 2p-4h i n t r u d e r s i n t h e S n and Cd i s o t o p e s [BR078, BÂC81 , APR84], s o t h a t t h e Pd i s o t o p e s r e p r e s e n t t h e n e x t s t e p t o w a r d s more complex s t a t e s . Such a s t e p i s i m p o r t a n t s i n c e i t may show w h e t h e r , when g o i n g away from c l o s e d s h e l l s , t h e i n t r u d e r p i c t u r e w i l l g e t o b s c u r e d by t h e i n c r e a s i n g number o f t h e p a r t i c l e d e g r e e s o f f r e e d o m . We have s t u d i e d t h e l o w - s p i n s t a t e s i n 102,104p(j u s i n g (p,2n) a n d (ρ,ρ') r e a c t i o n s , a n d Coulomb e x c i t a t i o n w i t h ^ 0 i o n s [LUO85]. Conversione l e c t r o n s p e c t r o s c o p y w i t h t h e i n s t r u m e n t shown i n F i g . 1 , a s w e l l a s c o n v e n t i o n a l gamma-ray a n d gamma-gamma c o i n c i d e n c e s p e c t r o s c o p y ( i n c l u d i n g d e l a y e d c o i n c i d e n c e s ) , were employed i n o b t a i n i n g t h e r e s u l t s s u m m a r i z e d i n F i g . 2. 1
EO TRANSITIONS ç x10 2
E2 TRANSITIONS
B(E2»)in Wu.
3
Fig.
2. P a r t i a l
level
EO TRANSITION çxlO'
Ε 2 TRANSITIONS
B(E2»)inW.u.
schemes o f 102,10Hpd. 1 0 2
102
Some f e a t u r e s i n t h e l e v e l schemes o f b o t h P d and *Pd resemble t h o s e o f t h e s i m p l e q u a d r u p o l e phonon p i c t u r e ; t h e two-phonon t r i p l e t s w o u l d c o n s i s t o f t h e 0^, 2 j , a n d 4{ s t a t e s i n P d a n d o f t h e O j , 2^, a n d states i n ^Pd. However, a g l a n c e a t F i g . 2 i m m e d i a t e l y r e v e a l s s e r i o u s d i s c r e p a n c i e s from t h e s i m p l e phonon m o d e l . The i n f o r m a t i o n o n t h e 0^ a n d 20 l e v e l s i n P d i s s c a r c e , except t h a t they both a r e connected w i t h t h e 21 s t a t e v i a E2 t r a n s i t i o n s h a v i n g p r o b a b l y m o d e r a t e s p e e d s [LU085, NDS]. I n 1 Û P d , h o w e v e r , t h e r e i s a v e r y c o n s p i c u o u s f e a t u r e i n t h a t t h e i s o m e r i c 0£ s t a t e l i e s below t h e more p h o n o n - l i k e O3 s t a t e , a n d o n l y a t a d i s t a n c e o f 65 keV f r o m i t . A l t h o u g h t h e r e i s no d i r e c t e v i d e n c e f o r t h e m i x i n g o f t h e s e two s t a t e s ( e . g . , t h e i n t e r c o n n e c t i n g EO t r a n s i t i o n has n o t been o b s e r v e d ) , t h e s m a l l e n e r g y i n t e r v a l and t h e f a c t t h a t t h e s e s t a t e s a r e c o n n e c t e d t o c o m p l e t e l y d i f f e r e n t l e v e l s ( c f . F i g . 1) seem t o s u g g e s t a v e r y weak m i x i n g o f t h e two 0 states. Another p e c u l i a r f e a t u r e i s t h e e x t r e m e l y s t r o n g l y h i n d e r e d E 2 ( 0 ^ 2 f ) t r a n s i t i o n w i t h B ( E 2 ) JJ00. As compared w i t h t h e Cd ( 112,11 ^Cd) a n d S n i s o t o p e s ( 1 l M l 6 , 1 l 8 s ) [JUL80, BAC81 ] , t h e m i x i n g between t h e 0£ a n d 0$ s t a t e s seems t o be s t r o n g e r i n t h e s e n u c l e i , a s i m p l i e d , e . g . , by E0 t r a n s i t i o n s between t h e e x c i t e d 0 s t a t e s which a r e e s p e c i a l l y s t r o n g i n t h e Sn nuclei. The 0$-*2t t r a n s i t i o n i s a l s o l e s s h i n d e r e d i n Cd a n d S n ( a g a i n , t h i s 1 0 2
1 0
1 Q i ,
2
+
_1
2
n
+
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
33.
223
EO Transitions and Intruder States
KANTELE ET AL.
t r a n s i t i o n i s strongest i n Sn). That the 0£ and 0^ s t a t e s are more mixed i n S n than i n C d i s a l s o suggested by the (3He,n) r e s u l t s , although not f u l l y c o n c l u s i v e l y [FIE77]. The decay c h a r a c t e r i s t i c s of the 0J l e v e l i n P d are very s i m i l a r t o those of the 0^ s t a t e s i n 112,11^cd. Therefore, s i m i l a r e x c i t a t i o n mecha nisms could be responsible f o r these s t a t e s . According to both experimental and t h e o r e t i c a l i n v e s t i g a t i o n s [BR078, BAC8M, APR84, H E Y 8 3 ] , the 2p-2h s t a t e s i n the Sn n u c l e i and the 2p-4h s t a t e s i n the Cd n u c l e i l i e lowest i n the mid-shell r e g i o n , i . e . , near N=66. This i s demonstrated i n a transparent way by Aprahamian et a l . [APR84] who a l s o suggest that the r a t i o R=B(E2;0$-2£)/B(E2;0$-2f) i s l a r g e s t f o r N=66 f o r the Cd n u c l e i . In keeping with t h i s p i c t u r e i s the l o w - l y i n g s t r o n g l y c o l l e c t i v e band observed by Hasselgren et a l . [ H A S 8 5 ] i n ΠΟρ^ with the 0£ bandhead at 9^6.7 keV (see the systematics of low-spin s t a t e s i n the Pd n u c l e i i n F i g . 3 based mainly on [ N D S ] ) . However, the decay c h a r a c t e r i s t i c s of the s t a t e i n !02pc[ and those of the 0£ s t a t e i n °Pd are conspicuously 1 l 6
1 1 2
1 0 2
1 1
Ε (MeV)
LOW-LYING 0*AND 2* STATES IN Pd (Z = A6)
