42 Dipole Collectivity in Nuclei and the Cluster Model Moshe Gai
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch042
A. W. Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06511
Measurements on enhanced E1 deexcitation transitions in light and heavy nuclei are discussed. In light nuclei these appear only from core-excited cluster c o l l e c t i v e states that have large alpha particle widths (Ө α) as well as large B(E2) deexcitation widths. They suggest the presence of an α+ C eluster dipole degree of freedom as suggested by Iachello and Jackson. In heavy nuclei the enhanced B(E1) are found in the light Ra-Th isotopes where small alpha particle hindrance factors for low-lying J =1 states and large ground state reduced alpha particle widths are also found. In addition, the B(E1) show a smooth dependence on J and on isotopic number N. A phenomenon reminiscent of backbending is observed for E1 deexcitations in Ra. The data suggest the presence of a new low-lying collective dipole mode in both heavy and light nuclei. In heavy nuclei this may reflect either cluster states or stable octupole. Analysis of the data on Ra isotopes in terms of the Vibron cluster model yields a resonable fit with only one variable parameter. 2
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18 The nucleus 0 i s crucial f o r understanding of the T=l e f f e c t i v e n u c l e o n - n u c l e o n i n t e r a c t i o n as w e l l as f o r t h e s t u d y o f coexistence of simple configurations of the n u c l e a r many-body s y s t e m , s u c h as t h e ohu) s h e l l m o d e l c o n f i g u r a t i o n s [ 1 ] and c o r e excited collective cluster configurations [2,3.4,5]. We h a v e c a r r i e d out a complete s p e c t r o s c o p i c study of 0 [6] w i t h s p e cial a t t e n t i o n t o m e a s u r i n g weak d e c a y b r a n c h e s t h a t , however, c o r r e s p o n d to l a r g e decay m a t r i x e l e m e n t s . The bound s t a t e s o f ^ 0 'were p o p u l a t e d by a l p h a t r a n s f e r r e a c t i o n s and t h e i r deexci tation was s t u d i e d in a coincidence experiment using the Ç£ L i , t Y ) 0 reaction. Quasi-bound r e s o n a n t s t a t e s above the a+ C t h r e s h o l d a t ^ . 2 M e V ^ n d b e l o w 8.5 MeV o f e x c i t a t i o n , were studied v i a the 0(α,γ) 0 r a d i a t i v e capture reaction. The gamma r a y t r a n s i t i o n s among t h e 16 l o w e s t states of 0 were e x a m i n e d w i t h t h e r e s u l t s shown i n F i g . 1. A surprising result i s t h a t o n l y v e r y few o f t h e E l d e e x c i t a t i o n s i n 0 a p p e a r t o be enhanced, and t h e e n h a n c e d ones link only s t a t e s , which have large alpha p a r t i c l e w i d t h s and w h i c h a r e u n d e r s t o o d t h e o r e t i cally t o be o f c l u s t e r s t r u c t u r e . These states also exhibit e n h a n c e d E2 d e e x c i t a t i o n s t o l o w e r c o l l e c t i v e s t a t e s . Moreover, 0097-6156/ 86/ 0324-0278506.00/ 0 © 1986 American Chemical Society
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
42.
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Dipole Collectivity in Nuclei
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t h e e n h a n c e d E l s a p p e a r to be v e r y s e l e c t i v e . For example, the c o l l e c t i v e 3 " " s t a t e a t 8.29 MeV in 0 with a large alpha parti cle width ( θ =20%) shows E l d e e x c i t a t i o n o n l y t o the c o l l e c t i v e third 2 s t a t e a t 5.26 MeV. The e n e r g e t i c a l l y f a v o r e d E l decays t o the t w o - n e u t r o n 2 s t a t e s at 1.98 and 3.92 MeV were n o t ob served, requiring smaller r e d u c e d m a t r i x e l e m e n t s by a t l e a s t a f a c t o r o f 100. The core-excited 1~ s t a t e a t 4.45 MeV exhibits e n h a n c e d E l d e e x c i t a t i o n t o the c o l l e c t i v e 4p-2h 0 s t a t e [2,3,4] but n o t to the g r o u n d s t a t e o f * 0 . The r e g u l a r b e h a v i o r of the enhanced E l d e e x c i t a t i o n i n 0, t h e c o r r e l a t i o n o f e n h a n c e d E l s and E2s ,^ * l a r g e c l u s t e r widths suggest that these enhanced Els in 0 reflect c l u s t e r i n g as r e c e n t l y p r o p o s e d by Iachello and J a c k s o n [7] i n h e a v y n u c l e i . The enhanced E l s c o r r e s p o n d to l a r g e f r a c t i o n s o f the m o l e c u l a r E l sum r u l e [8] as e x p e c t e d f o r c l u s t e r s t a t e s and the d a t a on 0 p r o v i d e the b e s t e x a m p l e , t h u s f a r , of a l o w - l y i n g c l u s t e r d i p o l e band i n n u c l e i . 1 8
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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.ch042
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