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BAND STRUCTURE IN 84Sr
A. Dewald, U. Kaup, W. Gast, A. Gelberg, H.-W. Schuh, K. Zell, Von Brentano
To cite this version:
A. Dewald, U. Kaup, W. Gast, A. Gelberg, H.-W. Schuh, et al.. BAND STRUCTURE IN 84Sr.
Journal de Physique Colloques, 1980, 41 (C10), pp.C10-163-C10-166. �10.1051/jphyscol:19801018�.
�jpa-00220637�
JOURNAL DE PHYSIQUE CoZZoque CIO, supple'ment au n012, Tome 42, ddcembre 1980, page C10- 163
BAND STRUCTURE I N 8 4 ~ ~
A. Dewald, U. Kaup, W. Gast, A. Gelberg, H.-W. Schuh, K.O. Zell and P. Von Brentano.
I n s t i t u t fiir Kemphysik der Universitttt zu KiiZn, 5000 KFZn 41, W.-Germany.
Abstract.- Excited levels in " ~ r have been populated in the reactions 7 6 ~ e ('*C, 4,) and " ~ r (6~i,3n).
Singles and coincidence gamma-ray spectra as well as angular distributions were measured. Lifetimes were measured by the recoil-distance method. The positive parity levels were sorted out into four bands ; the existence of at least two negative parity bands was established. The properties of ''~r were compared to those of neighbouring nuclei.
The s y s t e m a t i c s of S r i s o t o p e s l ) w i t h A I 88 shows a g r a d u a l t r a n s i t i o n from t h e
n e a r l y d o u b l e magic n u c l e u s " ~ r t o t h e ro- V)
t
t a t i o n a l l i k e 8 0 ~ r . The i n v e s t i g a t i o n of k Z 3 t h i s t r a n s i t i o n i s p a r t i c u l a r l y i n t e r e s t i n g m o?
s i n c e t h e p r o t o n number Z = 3 8 o r 4 0 beha- a W
n
v e s l i k e a magic one, a t l e a s t when t h e n e u t r o n number has t h e v a l u e N = 50 o r N = 56. While 8 6 ~ r i s a r a t h e r t y p i c a l s h e l l model n u c l e u s showing t h e two h o l e c o n f i g u r a t i o n ( v ~ ~ / ~ ) ' ~ , much l e s s i s known a b o u t and 8 2 ~ r , which s h o u l d d i s p l a y t h e o n s e t o f c o l l e c t i v i t y . The main aim of t h i s work h a s been t o i n v e s t i g a t e t h e e x c i - t e d l e v e l s of 8 4 ~ r by t h e methods of i n - beam gamma-ray s p e c t r o s c o p y .
E x c i t e d l e v e l s i n 8 4 ~ r were p o p u l a t e d i n t h e r e a c t i o n s 7 6 ~ e ( 1 2 ~ , 4 n ) and
' l ~ r ( ~ ~ i , 3 n ) ; t h e p a r t i c l e e n e r g i e s were 35-60 MeV f o r t h e 1 2 c beam and 19-32 MeV f o r t h e 6 ~ i beam, r e s p e c t i v e l y . S i n g l e s and c o i n c i d e n c e gamma s p e c t r a , a n g u l a r d i - s t r i b u t i o n s a s w e l l as e x c i t a t i o n f u n c t i o n s w e r e measured. L i f e t i m e measurements by t h e r e c o i l - d i s t a n c e method were c a r r i e d o u t u s i n g t h e 7 6 ~ e ( 1 2 ~ , 4 n ) r e a c t i o n . A 0.6 mg/
cm2 7 6 ~ e t a r g e t e v a p o r a t e d o n t o a 1.6 mg/cm2 Au f o i l and a 10 pm Ta s t o p p e r f o i l were used i n t h e measurement. The y-rays were de- t e c t e d by t h r e e G e ( L i ) d e t e c t o r s p o s i t i o n e d a t t h e a n g l e s
oO,
550, and 1'60°, r e s p e c t i v e - l y , t o t h e beam a x i s . The f l i g h t d i s t a n c e h a s been v a r i e d between 0 pm ( e l e c t r i c con- t a c t between t a r g e t and s t o p p e r ) and 6000vm. Fig. 1 : Decay c u r v e s of u n s h i f t e d peaks.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801018
C10-164 JOURNAL DE PHYSIQUE
The measured l i f e t i m e s and reduced E2- t r a n s i t i o n p r o b a b i l i t i e s a r e qiven i n Table I. Some t y p i c a l decay c u r v e s of u n s h i f t e d p e a k s a r e g i v e n i n f i g . 1.
The l e v e l scheme of 8 4 ~ r deduced from o u r gamma s p e c t r o s c o p i c e x p e r i m e n t s ( f i q . 2 ) a g r e e s r a t h e r w e l l w i t h t h o s e given p r e v i - o u s l y The o n l y s e r i o u s d i s c r e p a n c y c o n c e r n s t h e e n e r g i e s o f t h e 8+ and 10+ s t a -
t e s o f t h e y r a s t c a s c a d e . Our c o i n c i d e n c e measurements have shown t h a t t h e r e i s a 524 keV gamma t r a n s i t i o n l o c a t e d between t h e 11 16 keV and t h e 1040 keV ones; t h e 524 keV gamma h a s n o t been observed by Inamura e t a l . 2 ) . Moreover t h e decay c u r - v e s of a l l t r a n s i t i o n s s i t u a t e d below t h e 3332 keV 8+ s t a t e on f i g . 2 show a compo- n e n t w i t h T = 226 p s which i s n o t d i s p l a y e d by t h e 7116 keV t r a n s i t i o n s . Twenty-two new l e v e l s have been added t o t h o s e a l r e a d y known. The y r a s t c a s c a d e h a s been e x t e n d e d up t o 14+. S t a t e s i n t h e known n e g a t i v e pa- r i t y band have been measured up t o l l - . The p r e s e n c e of two 5- and 7- s t a t e s shows t h a t t h e r e i s more t h a n one n e g a t i v e p a r i t y band.
S i n c e B(E2) v a l u e s a r e a v a i l a b l e we can t r y t o group t h e s t a t e s i n t o bands;
such a t e n t a t i v e c l a s s i f i c a t i o n i s shown i n f i g . 3.
The ground s t a t e band ( q s b )
,
whose t r a n s i t i o n s a r e c h a r a c t e r i z e d by r a t h e r l a r g e c o l l e c t i v e B (E2) ' S can b e f o l l o w e d up t o t h e 2808 keV 6' s t a t e . The 524 keV, 8 + + 6+ t r a n s i t i o n h a s B ( E 2 ) = 4.2 Weiss- kopf u n i t s (NU) which p o i n t s o u t t o t h e t w o - p a r t i c l e n a t u r e of t h e 3332 keV, 8+s t a t e . We i n t e r p r e t t h i s s t a t e a s a two n e u t r o n h o l e (vgg/2 )
8:
c o n f i g u r a t i o n coup- l e d t o t h e 8 6 ~ r ground s t a t e . These( u g g / 2 ) e $ s t a t e s have been found i n most N = 46 and N = 48 i s o t o n e s . S i n c e
B(E2; 10+ + 8+)=6.8 W U i s a l s o r a t h e r s m a l l , we i n t e r p r e t t h e 4447 keV 10+ s t a t e a s be-
- 2
i n g t h e same, ( ~ g 9 / 2 ) ~ + s t a t e coupled t o t h e 2' s t a t e o f t h e 8 6 ~ r c o r e . The e n e r g y d i f f e - rence'E10+-8+ = 1116 keV i s v e r y c l o s e t o t h e c o r r e s p o n d i n g e n e r g y i n t e r v a l E Z + - ~ + = 1077 keV i n e 6 ~ r .
A second 8+ s t a t e h a s been found a t 3980 keV, which i s t o o low t o be a s c r i b e d t o e i t h e r t h e gsb or t o t h e quasi-gamma- band. We s h a l l t e n t a t i v e l v assume t h a t we
F i g . 2: Level scheme of 8 4 ~ r
2
d e a l w i t h a two p r o t o n ( ~ r l g ~ , ~ ) 8+ s t a t e coupled t o a 8 2 K r c o r e ; t h e e n e r g y o f t h e 10+ + 8+ t r a n s i t i o n i s 854 keV, which i s n o t f a r from = 776 keV i n 8 2 ~ r . I t i s t r u e t h a t t h e B(E2) o f t h e 8' + 6+, 854 keV t r a n s i t i o n i s r a t h e r l a r g e ; t h i s c v u l d b e e x p l a i n e d by t h e mixing t o t h e g s b 8' s t a t e . The weakly p o p u l a t e d s t a t e a t 4029 keV, f o r which we do n o t have a r e l i a b l e s p i n a s s i g n - ment i s v e r y l i k e l y t h e 8+ l e v e l o f t h e gsb.
I t s e n e r g y c o r r e s p o n d s t o what we c o u l d ex- p e c t by e x t r a p o l a t i n g t h e g s b e n e r g i e s ; moreover, it d e c a y s o n l v t o t h e g s b 6+.
R e g a r d l e s s o f t h e c o r r e c t n e s s o f t h e s e c o n f i g u r a t i on a s s i g n m e n t s , t h e l e v e l s i n t h e 3-4 . S MeV r a n g e d i s p l a y an i n t e r e s t i n g f e a t u r e , v i z . t h e mixing o f a t l e a s t t h r e e bands: t h e g s b and two bands b a s e d on a two-neutron-hole 8+ s t a t e and a two p r o t o n 8+ s t a t e , r e s p e c t i v e l y . Moreover, t h e lower t h r e e states o f a f o u r t h band w i t h l I r = 2'1 3+, and '5 have been o b s e r v e d a t 1454, 2056, and 2736 keV, r e s p e c t i v e l y . W e c o n s i - d e r it t o b e t h e quasi-gamma-band.
Turning now o u r a t t e n t i o n t o n e g a t i v e p a r i t y s t a t e s , we n o t i c e t h e p r e s e n c e o f a 3- s t a t e , t h e & p i n - p a r i t y a s s i g n m e n t b e i n g unambiguous 4 ) . We c o n s i d e r t h i s s t a t e a s
T a b l e I
L i f e t i m e s and r e d u c e d E2 t r a n s i t i o n p r o b a b i l i t i e s
h a v i n g a n o c t u p o l e nat.ure; i t i s p r o b a b l y made o u t o f p a r t i c l e - h o l e e x c i t a t i o n s s u c h
a s (g9/21 f;/2) 1
,
( g 9 1 2 , e t c . The5'
*
3- t r a n s i t i o n h a s c o l l e c t i v e s t r e n g t h , t h e r e f o r e w e c a n assume t h a t a c o l l e c t i v e band i s b u i l t upon t h e 3- s t a t e . The band i s i n t e r r u p t e d a t 7', s i n c e B (E2; 9' + 7-)= 5.2 WU, which i n d i c a t e s t h a t t h e 9- s t a t e h a s a d i f f e r e n t i n t r i n s i c s t r u c t u r e . We c a n i n t e r p r e t i t a s 3- m ( ~ g ~ / ~ ) ' ' .
I t i s i n t e r e s t i n g t o compare 8 4 ~ r t o t h e more c o l l e c t i v e 8 2 ~ r i s o t o p e . T h i s nuc- l e u s h a s b e e n r e c e n t l y i n v e s t i g a t e d by t h e same methods. 6 , ; l i f e t i m e s a r e n o t y e t a v a i l a b l e . T h r e e p o s i t i v e p a r i t y bands have been i d e n t i f i e d i n t h i s n u c l e u s : t h e g s b , a quasi-gamma-band which shows a more r e g u l a r b e h a v i o u r t h a n t h a t o f 8 4 ~ r and a ( v g g / 2 ) - 2 band which g i v e s r i s e t o t h e b a c k b e n d i n g o f t h e gamma-band. A n e g a t i v e p a r i t y band h a s a l s o b e e n i d e n t i f i e d .
The p o s i t i v e p a r i t y b a n d s h a v e been c a l c u l a t e d by means o f t h e p r o t o n - n e u t r o n I n t e r a c t i n g Boson Model ' l . Only two p a r a - meters have been v a r i e d compared t o t h e de- s c t i p t i o n o f t h e i s o t o n e 8 0 ~ r 8).
I n c o n c l u s i o n we c o n s i d e r t h a t t h e on- s e t o f c o l l e c t i v i t y d o e s o c c u r i n 8 4 ~ r . W e r e g a r d t h e t r a n s i t i o n from t h e s h e l l model n u c l e u s 8 8 ~ r t o a r a t h e r c o l l e c t i v e n u c l e u s e o ~ r a s v e r y t y p i c a l o f s i m i l a r t r a n s i t i o n s i n o t h e r c a s e s . As w e have shown i n t h e S r
c a s e , t h e e n e r g i e s of t h e q u a s i p a r t i c l e e x c i t a t i o n s remain r a t h e r c o n s t a n t w h e r e a s t h e e n e r g i e s o f t h e c o l l e c t i v e s t a t e s a r e l o w e r s d w i t h i n c r e a s i n g d e f o r m a t 1 on. I t a p p e a r s t h a t t h i s change i n d e f o r m a t i o n which l o w e r s t h e 8+ g s b l e v e l below t h e '8 t w o - q u a s i p a r t i c l e s t a t e w i l l a l l o w t h e ob- s e r v a t i o n of t h e c o l l e c t i v e b a n d s t o much h i g h e r s p i n s .
octupole
band 1 7 1
quasi y-band
9s. band
F i g . 3: Band s t r u c t u r e o f 8 4 ~ r .
JOURNAL DE PHYSIQUE
0
L - C'
g.s. band
0'
-2 bands
R e f e r e n c e s
P r o c e e d i n g = o + t h e I n t e r n a t i o n a l S c h o o l on N u c l e a r S t r u c t u r e , Varna 1979.
T. I n a m u r a , S. Nagamiya, A. Hashizume, and Y . Tendow, I n s t i t u t e o f P h y s i c a l and Chemical R e s e a r c h ( J a p a n ) , C y c l o t r o n P r o g r e s s R e p o r t No. 5 , 197 1 ( u n p u b l i s h e d ) p. 46
Nucl. Data S h e e t s
27,
339 ( 1 9 7 9 )J. B. B a l l , J. J. P i n a j i a n , J. S. L a r s e n , and A . C. R e s t e r , Phvs. Rev.
C
1438( 1 9 7 3 ) '
N . Yoshikawa, Y . S h i d a , 0. Hashimoto, and M. S a k a i , Nucl. Phys. A 327 477
( l Q 7 9 )
A. Dewald, U. Kaup, W. G a s t , A. G e l b e r g , K. 0. Z e l l , and P. von B r e n t a n o , C o n t r i - b u t i o n t o t h e I n t . Conf. on Nucl. P h v s . , B e r k e l e y , 1980.
T. O t s u k a , A. Arima, F. I a c h e l l o , and I. T a f m i , Phys. L e t t .
E
139 (1978) U. Kaup and A. G e l b e r g , Z. Phys. A 293 311 ( l Q 7 9 )Fiq. 4: Band s t r u c t u r e o f 8 2 ~ r .
