PREFACE
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THE
F U N D A M E N T A L C O N S T I T U E N T S O F N U C L E I , protons a n d neutrons, a n d
their interactions w i t h each other a n d w i t h particles o f their o w n k i n d w i t h i n the atomic nucleus have been studied intensively f o r more than h a l f a century. In attempts to understand a n d e x p l a i n the structure o f the nucleus, scientists have b o r r o w e d proven concepts f r o m a t o m i c chemistry and physics a n d used these as points o f departure. T h u s , n u c l e i , e x t r a o r d i narily s m a l l entities o f enormous density, have been characterized i n terms of shells, as analogous to the p e r i o d i c i t y o f atoms, a n d as h a v i n g p h y s i c a l properties such as shape a n d surface tension. E l e m e n t a r y m o d e s o f c o l l e c t i v e e x c i t a t i o n , rotations a n d v i b r a t i o n s , have been suggested as a n a l o g o u s to m o l e c u l a r systems. T h e s e concepts have served the n u c l e a r scientist w e l l ; l i k e b o r r o w e d clothes, they tend to be f a m i l i a r , e c o n o m i c a l , and w e l l - w o r n . U n f o r t u n a t e l y , these concepts do not fit n u c l e i quite as w e l l as they fit their o r i g i n a l owners. A m a j o r focus o f this b o o k is the recent e x c i t i n g progress t o w a r d f i n d i n g a new set o f " c l o t h e s " f o r the nucleus. T h e s y m p o s i u m upon w h i c h this b o o k is based w a s o r i g i n a l l y p l a n n e d as a t i m e l y snapshot o f an i n t e r d i s c i p l i n a r y field, one that has t r a d i t i o n a l l y i n v o l v e d both chemists and physicists. M a n y see o u r d i s c i p l i n e at a c r i t i c a l j u n c t u r e ; one path c o u l d lead to an e x c i t i n g future for a somewhat neglected discipline; the other c o u l d lead to l o w e r expectations. T h e nuclear structure research c o m m u n i t y as a w h o l e l o o k s t o w a r d a n e x c i t i n g future a n d w a s anxious to gather together f o r a c o m p r e h e n s i v e presentation o f recent research. U s u a l l y , a t w o - d a y s y m p o s i u m w o u l d have sufficed, but as w o r d spread a n d interest g r e w , the s y m p o s i u m expanded to four a n d o n e - h a l f days a n d n u m b e r e d nearly one h u n d r e d presentations. P a r t i c i p a n t s agreed that a very h i g h level o f excitement i n nuclear structure research has been brought about b y recent developments i n both theory and experiment. In o r g a n i z i n g this b o o k , w e d e c i d e d to arrange presentations i n four b r o a d categories: Section I, M o t i v a t i o n f o r F u r t h e r E x p l o r a t i o n ; Section II, E x p e r i m e n t a l E x p l o r a t i o n o f C u r r e n t Issues; Section III, N e w F a c i l i t i e s a n d Techniques; and Section I V : Survey o f C u r r e n t Research. Section I provides a r i c h but focused s u m m a r y o f m o t i v a t i o n s f o r future w o r k i n the field. R e v o l u t i o n a r y advances i n theory a n d s p a r k l i n g achievements i n e x p e r i mentation describe the state o f nuclear structure research. T h e introduction o f symmetries a n d supersymmetries has e n c o u r a g e d experimenters to c o n f i r m some o f the n e w predictions that f l o w f r o m boson models. R e e x a m i n a t i o n o f the e x i s t i n g n u c l e a r structure data base has brought to light s i m p l i f y i n g correlations that i n turn promise to i m p r o v e o u r ability to accurately extrapolate nuclear parameters into regions not yet accessible to experimental exploration. In this regard, the region o f n e u t r o n - r i c h isotopes xiii
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is especially significant because o f its i m p o r t a n c e to astrophysics a n d reactor technology. S e c t i o n II provides an e x a m i n a t i o n o f the current status o f n u c l e a r structure research a n d presents but a s a m p l e o f the m a n y e x c i t i n g a n d c l e v e r experiments u n d e r w a y or c o m p l e t e d recently. F o u r topics are h i g h l i g h t e d : intruder states i n n u c l e i , octupole modes i n n u c l e i , h i g h - s p i n state investigations using heavy ions, a n d nuclear moments. T h i s selection reflects the o p i n i o n o f independent reviewers and our belief that these four
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areas provide a comprehensive set o f new data for testing new theories. In m a n y instances, c r i t i c a l testing o f new theories w i l l be possible only i f new facilities a n d techniques are f o r t h c o m i n g . S e c t i o n III contains discussions o f m a j o r new f a c i l i t i e s — s o m e under c o n s t r u c t i o n , some i n intensive design stages, and s o m e still i n the idea stage. These and other m a j o r facilities are c r i t i c a l to m a i n t a i n i n g the r a p i d p a c e o f a d v a n c e m e n t i n n u c l e a r structure research a n d represent, for the most part, a history o f m u l t i n a t i o n a l p a r t i c i p a t i o n . T h e c o o r d i n a t i o n a n d phased development o f new facilities a v o i d d u p l i c a t i o n o f effort a n d enhance o v e r a l l productivity o f the w o r l d w i d e research c o m m u n i t y . Section III also l o o k s at a selection o f new techniques a n d s m a l l e r facilities that p r o v i d e a s a m p l e o f i m a g i n a t i v e a p p r o a c h e s to c u r r e n t a n d future experiments. T h r e e very different approaches to the study o f a p a r t i c u l a r nucleus, S m , are i n c l u d e d to illustrate the c o m p l e m e n t a r i t y o f different facilities a n d techniques i n a p p r o a c h i n g the study o f specific cases. F i n a l l y , the c a l l for papers for the s y m p o s i u m p r o v i d e d us w i t h a b r o a d s e l e c t i o n o f v e r y fine m a n u s c r i p t s . U n f o r t u n a t e l y , because o f space requirements we c o u l d p u b l i s h i n full o n l y a portion o f those presentations. Abstracts o f the other papers are i n c l u d e d i n Section I V . These papers c a n be obtained by w r i t i n g to R i c h a r d A . M e y e r or D a e g S. Brenner. M a n y i n d i v i d u a l s c o n t r i b u t e d to the success o f the s y m p o s i u m upon w h i c h this b o o k is based. W e especially thank R i c h a r d W . Hoff, C h a i r , and J o s e p h R . P e t e r s o n , T r e a s u r e r , o f the D i v i s i o n o f N u c l e a r C h e m i s t r y a n d T e c h n o l o g y o f the A m e r i c a n C h e m i c a l Society for their valuable assistance and t r o u b l e - s h o o t i n g . W e also gratefully a c k n o w l e d g e f i n a n c i a l support f r o m the N a t i o n a l Science F o u n d a t i o n , the D i v i s i o n o f N u c l e a r C h e m i s t r y and T e c h n o l o g y , a n d A p t e c N u c l e a r , Inc., i n presenting the s y m p o s i u m . T h e i r support e n h a n c e d s i g n i f i c a n t l y the q u a l i t y a n d scope o f the s y m posium. 1 3 8
RICHARD A. MEYER
DAEG S. BRENNER
Division of Nuclear Chemistry Lawrence Livermore National Laboratory Livermore, C A 94550
Office of Academic Affairs Clark University 950 Main Street Worcester, M A 01610
August 25, 1986 xiv
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INTRODUCTION
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By Richard A . Meyer and Daeg S. Brenner
^N^UCH R E S E A R C H O N N U C L E A R S T R U C T U R E and d y n a m i c s i n recent years has investigated the behavior of nuclear matter under extreme conditions. F o r e x a m p l e , the study o f r a p i d l y rotating n u c l e i has l e d to a s i m p l e understanding o f h o w a nucleus adjusts its structure i n order to carry h i g h angular m o m e n t u m . B y e x a m i n i n g the deexcitation process, scientists have g a i n e d detailed k n o w l e d g e o f shape changes that a c c o m p a n y a loss o f e x c i t a t i o n energy. A n o t h e r w a y o f stressing n u c l e i is to produce them i n such a w a y as to create an unstable i m b a l a n c e i n v o l v i n g constituent protons and neutrons. Studies o f decay processes o f these f a r - f r o m - s t a b i l i t y nuclides have r e v e a l e d i m p o r t a n t subtleties i n n u c l e a r structure. " I n t r u d e r " states, nuclear configurations that arise f r o m an exchange of particles (holes) w i t h an adjacent shell or subshell, are one such discovery. T h e coexistence o f n o r m a l and intruder states i m p l i e s a coexistence o f different shapes w i t h i n a given nucleus. F o r m a n y years researchers have k n o w n that nuclei can be excited into v i b r a t i o n a l modes o f m o t i o n that are not reflection symmetric. T h e simplest o f these a s y m m e t r i c modes, the octupole v i b r a t i o n , has been charted extensively. O n l y recently, however, has new evidence suggested that some nuclei have reflection a s y m m e t r i c , or pear-shaped, ground-state c o n f i g u r a tions. A l t h o u g h there is disagreement as to whether these n u c l e i are pearshaped or p i m p l e d , it is b e c o m i n g clear that a m o r e detailed m a p p i n g o f the nuclear surface is necessary to e x p l a i n both the spectroscopic properties a n d the masses o f heavy elements. T h e field o f nuclear structure and d y n a m i c s has developed for p r a c t i c a l as w e l l as esoteric reasons. T w o o f our most i m p o r t a n t customers for nuclear k n o w l e d g e , nuclear engineers and astrophysicists, have longstand i n g interests i n nuclear data and suggestions for nuclear spectroscopists. O n the p r a c t i c a l side, we often study properties o f nuclides of p a r t i c u l a r interest to the n u c l e a r p o w e r industry. U n f o r t u n a t e l y , o u r w a y o f studying these n u c l e i does not often lead to results useful to scientists c o n c e r n e d w i t h i m p r o v i n g our k n o w l e d g e o f the reactor source t e r m , w h i c h predicts the b u i l d u p o f heat i n a reactor f o l l o w i n g a loss-of-coolant accident. A s t r o p h y s i c i s t s are also a v i d customers for nuclear i n f o r m a t i o n a n d theories. C a l c u l a t i o n s o f e l e m e n t a l abundances resulting f r o m r-process nucleosynthesis a n d related m e c h a n i s m s depend o n the systematics o f xv
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Downloaded by 80.82.77.83 on May 28, 2018 | https://pubs.acs.org Publication Date: November 14, 1986 | doi: 10.1021/bk-1986-0324.pr001
n u c l e a r properties. N u c l e a r properties such as masses, h a l f - l i v e s , a n d d e l a y e d neutron e m i s s i o n p r o b a b i l i t i e s affect these predictions i n c o m p l i cated ways. In the absence o f data very far f r o m stability, models are used to extrapolate relevant parameters. T h i s uncomfortable situation w o u l d be eased i f more data were available for n e u t r o n - r i c h nuclides. T h u s , we have two very important reasons—one c o s m i c , one p r a c t i c a l — t o pursue studies o f nuclear properties to the frontier of spontaneous nuclear disintegration. In our o p i n i o n , however, the o v e r r i d i n g reasons for renewed excitement in the nuclear structure c o m m u n i t y are the successes i n theory arising f r o m formulations o f nuclear interactions i n terms o f pairs o f interacting bosons, or, i n the case o f o d d - A n u c l e i , b o s o n - f e r m i o n interactions. These new a p p r o a c h e s to u n d e r s t a n d i n g a n d c a l c u l a t i n g n u c l e a r structures a n d b e h a v i o r have b o r r o w e d the concepts o f s y m m e t r y f r o m h i g h - e n e r g y physics and f r o m crystallography. A l t h o u g h these algebraic models provide less o f a sense o f the tangible than older geometric models, they have t w o very important advantages: they are readily e m p l o y e d by experimenters as w e l l as theorists, and they c a n be applied to regions o f shape transition i n a natural w a y . T h e second advantage is extremely i m p o r t a n t because the interacting boson m o d e l ( I B M ) is the only m o d e l o f nuclear structure that provides a g l o b a l f o r m a l i s m for c a l c u l a t i n g nuclear structure. T h e c o m p u t a t i o n a l successes o f the m o d e l have s t i m u l a t e d a r e e x a m i n a t i o n o f the systematic patterns o f n u c l e a r structure. E x a m i n a t i o n o f the trends o f a variety o f n u c l e a r data as a function o f the product o f the n u m b e r o f valence neutrons and protons appears to provide an i m p r o v e d f r a m e w o r k for r e l i a b l e e x t r a p o l a t i o n o f n u c l e a r properties into regions far f r o m stability. F i n a l l y , because models such as the I B M consider o n l y valence p a r t i c l e s — t h o s e b e y o n d the nearest closed shell or, i n some instances, s u b s h e l l — r e s e a r c h e r s are interested i n m a p p i n g , e x p e r i m e n t a l l y , the locations o f shells and subshells into regions far f r o m stability. A n i m p r o v e d k n o w l e d g e o f shell a n d subshell gaps at the extremes o f n u c l e a r stability w i l l provide important b e n c h m a r k s for testing nuclear models.
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