DECAY MODES OF HIGH SPIN STATES IN LIGHT NUCLEI

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DECAY MODES OF HIGH SPIN STATES IN LIGHT

NUCLEI

R. Daniel, P. Butler, A. Irving, P. Nolan, J.F. Sharpey-Schafer

To cite this version:

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JOURNAL DE PHYSIQUE CoZloque C I O , supple'ment

au

n O l 2, Tome 41, dhcembre 1 9 8 0 , page C10- 155

DECAY MODES OF H I G H S P I N STATES I N L I G H T N U C L E I

R. Daniel, P.A. B u t l e r , A.D. I r v i n g , P.J. Nolan and J . F . Sharpey-Schafer.

Department o f Physics, University of Liverpool, OZiver Lodge Laboratory, P.O. Box 147, Liverpool, L69 3dX, United Kingdom.

A b s t r a c t . - The e x c i t a t i o n energy i n "Ca populated i n t h e r e a c t i o n 2 8 ~ i ( 1 6 0 , ?py) f o r a beam energy of 40 ?rev has been measured by d e t e c t i n g both protons f r m t h e r e a c t i o n . The average ~ - m u l t i ~ l i c i t ~ and d e - e x c i t a t i o n p a t t e r n of Yrast s t a t e s was measured a s a f u n c t i o n of t o t a l e x c i t a t i o n energy. The d a t a suggest t h a t f o r s t a t e s of high a n g u l a r momentum (J > 10) a - p a r t i c l e emission i s t h e dominant mode of decay.

1 . INTRODUCTION

-

T h e o r e t i c a l s t u d i e s of by y emission) i n t h e nucleus of i n t e r e s t ; t h e r o t a t i n g n u c l e i s u g g e s t t h a t a t s u f f i c i e n t l y high o t h e r i s t h e Doppler broadening of t h e y-ray a n g u l a r momentum d r a s t i c changes i n n u c l e a r shape l i n e s h a p e s due t o r e c o i l from p a r t i c l e emission, o r s t r u c t u r e might occur under t h e i n f l u e n c e of which i s l a r g e f o r l i g h t n u c l e i . I n t h e

macroscopic c e n t r i f u g a l f o r c e s and q u a n t a l s h e l l p r e l i m i n a r y experiment d e s c r i b e d here, t h e two e f f e c t s

[l11

.

A t p r e s e n t most of t h e experimental p r o t o n s from t h e 2 8 ~ i ( 1 6 0 , 2 p y ) 4 2 ~ a r e a c t i o n a t s t u d i e s of t h e behaviour of n u c l e i a t high Spin EIG0 = 4C MeV were d e t e c t e d simultaneously w i t h have been c a r r i e d o u t f o r n u c l e a r masses around d e t e c t i o n of r h e y emission, e i t h e r i n a Ge(Li) 7 0 - 1 6 0 , although a s has been p o i n t e d o u t by f o r d i s c r e t e l i n e s t u d i e s , or i n a NaI f o r y-ray Klapdor t h e same behaviour c a n be seen i n m u l t i p l i c i t y measurements. By measuring t h e t o t a l l i g h t n u c l e i . For A 40 o b l a t e r o t a t i o n about energy of t h e p a r t i c l e s emitted t h e e n t r y l i n e i n a symmetry a x i s has been p r e d i c t e d t o occur a t 4 2 ~ a can be s t u d i e d , and by s u i t a b l e kinematic J 3 8 C311 and superdeformation should occur f o r s e l e c t i o n of t h e p a r t i c l e s the Doppler broadening

J s 24 C411

.

It would t h u s be e n l i g h t e n i n g t o can be minimised and d i s c r e t e y t r a n s i t i o n s s t u d i e d study t h e y-decay p r o p e r t i e s f o r s t a t e s with

J > 10 i n such n u c l e i . 2. METHOD AND RESULTS - The experimental set-up

The aim of t h e p r e s e n t work was t o determine how much a n g u l a r momentum i s a v a i l a b l e f o r y emission f o l l o w i n g a compound nucleus r e a c t i o n l e a d i n g t o 4 2 ~ a , where t h e Y r a s t s t a t e s a r e known up t o J = 10 a t 7.5 MeV L 5 3

.

There a r e two known l i m i t a t i o n s t o t h e o b s e r v a t i o n of d i s c r e t e y t r a n s i t i o n s ( a p a r t from e n t r a n c e channel e f f e c t s ) : one i s due t o t h e e x t e n t of t h e e n t r y l i n e ( t h e l o c u s of s t a t e s s p e c i f i e d by energy and a n g u l a r momentum populated

d i r e c t l y by evaporated p a r t i c l e s and depopulated

i s shown i n f i g u r e 1 . The t a r g e t c o n s i s t e d of a s e l f - s u p p o r t i n g f o i l of 300 pg/Cm2 2 8 ~ i , enriched t o >99%. The 1 6 0 beam was stopped i n a 22 mg/cm2 Au f o i l placed i n f r o n t of t h e p a r t i c l e t e l e s c o p e . The t e l e s c o p e , s i t u a t e d symmetricaily around

o0

t o t h e beam d i r e c t i o n , c o n s i s t e d of a 250 u m AE element and a 5 m E element and subtended a t o t a l s o l i d a n g l e of 0.7 s r t o t h e t a r g e t . Events were recorded which corresponded t o c o i n c i d e n c e s between t h e E and A E d e t e c t o r s and e i t h e r a Ge(Li) d e t e c t o r of 25% e f f i c i e n c y

( r e l a t i v e to 75

nun

x 75 m NaI) placed a t 0'

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C10- 156 JOURNAL DE PHYSIQUE

F i g . 1

-

Experimental set-up.

o r a 75 mm x 75 mm NaI placed a t go0. The p r o b a b i l i t y of simultaneous d e t e c t i o n of two protons was about 8% of s i n g l e p r o t o n d e t e c t i o n f o r t h i s geometry; t h e s e events were e a s i l y s e p a r a t e d from s i n g l e p , d o r a e v e n t s by a n a l y s i n g t h e r e l a t i v e e n e r g i e s d e p o s i t e d i n each of t h e d e t e c t o r s . Since t h e c r o s s s e c t i o n f o r e v a p o r a t i o n of 3 p a r t i c l e s i s small [ 6 3

,

i t was assumed t h a t t h e d e t e c t i o n of two p r o t o n s i s s u f f i c i e n t t o determine t h e f i n a l nucleus a s 4 2 ~ a . By measuring t h e t o t a l l a b o r a t o r y energy d e p o s i t e d by t h e two p r o t o n s , t h e r e s i d u a l e x c i t a t i o n energy i n , 4 2 ~ a a v a i l a b l e f o r y t r a n s i t i o n s was determined w i t h i n a range of I MeV f o r each e v e n t . The spectrum of e x c i t a - t i o n energy

i h

4 2 ~ a i s shown i n f i g u r e 2. The mean e x c i t a t i o n energy i s 13 MeV.

I n

t h e coincidence Ge(Li) spectrum, t h e Doppler broadening of t h e y-ray l i n e s h a p e s was

reduced t o 0.3%. This compares w i t h a v a l u e of 0.8% f o r t h e s i n g l e s spectrum. I n t h i s way f a s t (r < 1 p s ) t r a n s i t i o n s which might s t r o n g l y feed t h e known Yrast s t a t e s could be observed. No evidence f o r such t r a n s i t i o n s were s e e n i n t h i s spectrum which s u g g e s t s t h a t t h e e n t r y l i n e i n t h i s r e a c t i o n does n o t extend much beyond J = 10.

The behaviour of t h e observed d i s c r e t e

F i g . 2

-

4 2 ~ a e x c i t a t i o n energy spectrum.

t r a n s i t i o n s and t h e t o t a l y-ray m u l t i p l i c i t y ( < W )

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Ex curves for the discrete transitions hardly varies with their spin, and <M> is almost independent of Ex, which indicates that the entry line is nearly parallel to the angular momentum axis. This is con- sistent with the lack of states observed above 10 X together with the observed mean value of E,. Since the initial angular momentum of the compound nucleus

Fig. 3

-

(a) Intensity (normalised to particle singles) of 4 2 ~ a transitions (energy in keV) versus excitation energy (b) Total gamma-ray multiplicity versus excitation energy.

as deduced from total fusion cross section measurements [8] extends to 17 X, it is likely that inter- mediate states with J > 10 decay predominantly by a emission. Since a emission efficiently removes

angular momentum no higher spin states are observed in the residual nucleus ( 3 9 ~ in this case).

Statistical model calculations performed using the computer code GROG1 [g] indicate that the branching ratio from a compound nucleus state with J = 16 of the 2p channel relative to the mp channel is 10%. Our measurements indicate that this value is an over-estimate.

3.

CONCLUSION

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The present results indicate that the difficulty in observing discrete transitions from states in light nuclei

(A

?. 40) with

J > 10 originates from a premature termination of the entry line. This termination appears to be caused by competing a emission. It therefore seems unlikely that the predicted structural changes in light nuclei at high angular momentum can be studied by y-ray spectroscopy unless more sensitive te'chniques are developed.

ACKNOWLEDGEMENTS

-

This work was supported by the U.K. Science Research Council. P.A.B. acknow- ledges the receipt of a S.R.C. Advanced Fellowship and R.D., A.D.I. the receipt of S.R.C. studentships for the period of the work. We thank Joe Reynolds for providing the targets used in the experiments.

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