Evidence from Capture Reactions

In document Alpha capture to the giant dipole and quadrupole resonances (Page 151-154)

0 2 EXPERI MENTAL EVIDENCE FOR A GQR

6.2.2 Evidence from Capture Reactions

9 4.

gamma-ray t r a n s i t i o n s t r e n g t h o b s e rv e d by Watson e t a l.( W a 73) in t h e

3GA r ( a , y ) 40Ca r e a c t i o n i n t h e r e g i o n o f 14 MeV e x c i t a t i o n e x h a u s te d

0

20% o f t h e e n e rg y w e ig h te d sum r u l e . S in c e i t was u n l i k e l y t h a t

r a = r t o t a l f o r t h e s e s t a t e s , t h e E2 gamma-ray t r a n s i t i o n s t r e n g t h

was u n d o u b te d ly much l a r g e r . B ra n fo rd t h e r e f o r e c o n c lu d e d t h a t a

p l a u s i b l e e x p l a n a t i o n o f t h e s e r e s u l t s was t h a t t h e GQR was e x c i t e d .

However o t h e r a t t e m p t s by Hanna e t a l . (Ha 73a) u s i n g t h e 89Y (p ,y ) 90Zr

0

r e a c t i o n and V e n tu ra e t a l . (Ve 73) u s i n g t h e 20N e ( a ,y ) 24Mg r e a c t i o n

0

showed t h a t t h e E2 gamma-ray t r a n s i t i o n s t r e n g t h was s m a l l .

A n g u lar d i s t r i b u t i o n s have b een m easured f o r t h e 24M g(a,y ) 28S i

0

r e a c t i o n by M e y e r - S c h u tz m e is t e r e t a l . (Me 68) b u t were n o t e x p l i c i t l y

a n a l y z e d i n te rm s o f El and E2 gamma-ray t r a n s i t i o n s t r e n g t h s . However

Legendre P o ly n o m ial f i t s t o t h e d i s t r i b u t i o n s showed n e g a t i v e A4 c o ­

e f f i c i e n t s , i n d i c a t i n g t h e p r e s e n c e o f an E2 com ponent. To d e t e r m i n e

e x p l i c i t l y t h e s t r e n g t h o f t h e El and E2 m u l t i p o l a r i t i e s t h e p u b l i s h e d

d i s t r i b u t i o n s were r e a n a l y z e d u s i n g e q u a t i o n 3 . 7 . The r e s u l t s o f t h i s

a n a l y s i s showed E2 s t r e n g t h o v e r t h e whole r e g i o n o f e x c i t a t i o n f o r which

a n g u l a r d i s t r i b u t i o n s were m e a su re d . The e n e r g y w e ig h te d i n t e g r a t e d c r o s s s e c t i o n s e x h a u s te d 12% o f t h e EWSR s u g g e s t i n g t h a t t h e E2 s t r e n g t h was a s s o c i a t e d w i t h t h e GQR. The a l p h a c a p t u r e r e a c t i o n s d e s c r i b e d i n C h a p t e r s 2-5 showed t h a t E2 r a d i a t i o n was e v i d e n t a l t h o u g h t h e i n t e n s i t y was n o t w e l l d e t e r m in e d b e c a u s e o f t h e d o m inant El c o n t r i b u t i o n t o t h e c r o s s s e c t i o n . In t h e

c a s e o f t h e 38A r ( a , y ) 42Ca r e a c t i o n , t h e i n t e g r a t e d E2 s t r e n g t h was

0

s i m i l a r t o t h a t o b s e r v e d f o r 38A r ( a , y ) 40Ca im p ly in g t h a t t h e GQR was 0

a l s o e x c i t e d by t h i s r e a c t i o n .

The a l p h a c a p t u r e r e a c t i o n h a s t h e r e f o r e p ro v ed i t s e l f t o be a

s e n s i t i v e p ro b e o f E2 s t r e n g t h . T h is i s due t o t h e f a c t t h a t t h e El

and E2 c o n t r i b u t i o n s t o t h e c r o s s s e c t i o n can be d e t e r m in e d u n am b ig u o u sly

9 5.

m easurem ents a r c p e rfo rm e d on s e l f - c o n j u g a t e n u c l e i f o r which t h e ( a , y )

0

r e a c t i o n th r o u g h 1 s t a t e s i s i s o s p i n f o r b i d d e n , t h u s r e d u c i n g t h e El

c o m p o n en t.

The H a u s e r - F e s h b a c h c a l c u l a t i o n s i n C h a p te r 5 show t h a t t h e

d e g r e e o f i s o s p i n m ix in g i n 32S a t 12-18 MeV e x c i t a t i o n e n e r g y was l e s s

t h a n t h a t i n 40Ca and i n 28S i . The 28S i ( a , y ) 32S r e a c t i o n t h e r e f o r e

0 a p p e a r s t o be s u i t a b l e f o r o b s e r v a t i o n o f t h e GQR s i n c e t h e E2 gamma-ray t r a n s i t i o n s t r e n g t h s h o u ld be a r e a s o n a b l y l a r g e f r a c t i o n o f t h e t o t a l . To t e s t t h i s a rg u m e n t, a n g u l a r d i s t r i b u t i o n s were m easured f o r t h i s r e a c t i o n a s d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n . 6 . 5 EXPERIMENTAL METHOD A n g u la r d i s t r i b u t i o n s were m easured f o r t h e 28S i ( a , y ) 32S r e a c t i o n

a t 12 a l p h a p a r t i c l e bom barding e n e r g i e s i n t h e e n e rg y r a n g e 6 - 1 1 .5 MeV.

The t a r g e t s , which were p l a c e d a t 45° t o beam d i r e c t i o n , w ere 400 ± 20

2 2

ygm/cmz and 570 ± 30 ygm/eni f i l m s o f 99% e n r i c h e d 28SiO e v a p o r a te d on to

1/2 mm t h i c k g o ld b a c k i n g s . The t a r g e t t h i c k n e s s e s , which were d e te r m in e d

a s d e s c r i b e d i n s e c t i o n 2 . 5 . 2 , p ro d u ced an a v e r a g e e n e r g y l o s s o f

^ 400 keV. The gamma r a y s were d e t e c t e d by t h e Nal s p e c t r o m e t e r d e s c r i b e d

i n C h a p te r 2, u s i n g c o l l i m a t o r 1 w ith t h e f r o n t f a c e o f t h e c r y s t a l 54 ems

from t h e t a r g e t . In a t y p i c a l r u n 1500 yC o f c h a r g e was c o l l e c t e d . S in c e

t h e f i r s t e x c i t e d s t a t e o f 32S l i e s 2 .2 4 MeV above t h e g round s t a t e and

y was t h e r e f o r e w e l l r e s o l v e d , i t was u n n e c e s s a r y t o u s e t h e l i n e s h a p e

o

f i t t i n g p r o c e d u r e d e s c r i b e d i n s e c t i o n 2 . 9 . I n s t e a d , t h e y i e l d was d e ­

te r m in e d by summing o v e r t h e p h o to p e a k and s u b t r a c t i n g a f l a t b ack g ro u n d

d e t e r m in e d from t h e number o f c o u n t s p e r c h a n n e l above t h e y p e a k . The

0

y i e l d a t 0 ° , 30° and 55° was c o r r e c t e d by a p p r o x i m a t e l y 7% f o r a b s o r p t i o n

i n t h e g o ld b a c k i n g . The d i f f e r e n t i a l c r o s s s e c t i o n s were c a l c u l a t e d

96.

The results of the angular distribution measurements are plotted in figure 6.1 and show that there is a significant E2 contribution to the cross sections. The angular distributions measured at 6.7 and 7.21 MeV are particularly noteworthy since they show predominantly E2 dis­ tributions .

In document Alpha capture to the giant dipole and quadrupole resonances (Page 151-154)