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ALPHA EMISSION WITH 10 MeV/A 14N
PROJECTILES
G. Morrison, R. Bhowmik, E. Pollacco, J. England, N. Sanderson
To cite this version:
JOURNAL DE PHYSIQUE Colloque ClO, suppZdment au n012, Tome 41, de'cembre 1980, page
C10-243
ALPHA E M I S S I O N WITH 10 M ~ V / A 1 4 ~ PROJECTLLES
**
G.C.. Morrison, R.K. ~howrnik*, E.C. P o l l a c c o
,
J.B.A. England and N.E. Sanderson tDepartment o f P h y s i c s , U n i v e r s i t y o f Birmingham, Birmingham B15 2TT, England.
Beams o f 148 MeV "N from t h e V a r i a b l e Energy Cyclotron, AERE, Harwell were used t o bombard t a r g e t s o f 5 8 ~ i , 2 7 and ~ ~
12c
o f t h i c k n e s s e s 2.5,3.2 and 0.6 mg/cm2, r e s p e c t i v e l y . The heavy e j e c t i l e s (2=3-81 were detected a t OH 13O, c l o s e t o t h e g r a z i n g a n g l e f o r t h e r e a c t i o n "N + "Ni and a l s o a t 8H1=260. C o i n c i d e n t a l p h a p a r t i c l e s were detected i n c o p l a n a r geometry i n a second t e l e s c o p e moved i n t h e range O a = i 7 0 0 about t h e beam a x i s , where n e g a t i v e angles correspond t o t h e a-telescope on t h e same s i d e o f t h e beam as t h e HI-telescope. A l i m i t e d number o f out-of-plane measurements were a l s o performed. The measured q u a n t i t i e s were t h e a-HI c o r r e l a t i o n s
-
d40(EHI,~a, eHI,ea) as a f u n c t i o n o f , Z and t h e corresponding s i n g l e s c r o s s s e c t i o n s-
d 2 a ( ~ H 1 ; 8 H I ) and d2a(~,,8,). The c o n t r i b u t i o n s from carbon and oxygen i m p u r i t i e s have been care- f u l l y i n v e s t i g a t e d , and a r e e s t i m a t e d t o be < l o % f o r Z 2 6, i n c r e a s i n g t o i 3 0 % f o r t h e h e a v i e s t e j e c t i l es. F u r t h e r deta i l s have been g i v e n e l sewhere'.T y p i c a l two-dimensional e n e r g y - c o r r e l a t i o n
dent a l p h a and heavy e j e c t i l e s can be obtained. As an example Fig.2 shows t h e in-plane energy-integrated angular c o r r e l a t i o n s f o r a p a r t i c l e s i n coincidence w i t h d i f f e r e n t e j e c t i l e s f o r a l l t h r e e t a r g e t s . For a comparison between d i f f e r e n t t a r g e t s t h e c o i n c i d e n t c r o s s s e c t i o n s have been d i v i d e d by t h e
s p e c t r a f o r t h e r e a c t i o n 5 8 ~ i ("N,Hla) a t ElHI = 13O
Fig.1. EHI versus Ea energy c o r r e l a t i o n s f o r Li, Be 8, = 13' a r e shown i n F i g . I . From such b a s i c data
and eJect I es for the react ion 5 v N i ( l+N,H la) at
angular c o r r e l a t i o n s and energy s p e c t r a o f c o i n c i - O H I = 13O, 8, = 130.
'AI I1'N Hl z I lLB MeV
l d e g )
Fig.2: D i f f e r e n t i a l a m u l t i p l i c i t i e s f o r
"c,
"AI and "Ni t a r g e t s f o r t h e r e a c t i o n ( ~ ' N , H I ~ ) a t 148 MeV.ClO-244
JOURNAL D E P H Y S ~ Q U Ecorresponding s i n g l e s deep i n e l a s t i c cross s e c t i o n t o o b t a i n t h e d i f f e r e n t i a l a m u l t i p l i c i t i e s dMa/daa (Z,eHI 1. For each t a r g e t t h e sol i d curves give t h e shape o f t h e a s i n g l e s angular d i s t r i b u t i o n da,/dQ,. The angle i n t e g r a t e d a m u l t i c i p l i c i t i e s , approximately corrected f o r out-of-plane anistropy, are given i n Table I f o r a l l t h r e e t a r g e t s . These m u l t i p l i c i t i e s a r e q u i t e large, i n t h e range 0.4
-
0.8 f o r "Ni and 2 7 and 0.8 ~ ~-
1.6 f o r"c.
Table I I n t e g r a l a m u l t i c i ~ l i c i t i e s f o r the r e a c t i o n ("N,Hla) on
"c,
A l and 5 8 ~ i t a r g e t s f o r d i f f e r e n t0,.
Target 'A I 5 8 ~ i e j e c t i l e 13' 26 O 13O 13O 26 Oa
1.6 1.2 1 . I 0.8 0.8 L i 1.5 I . I 0.8 0.7 0.8 Be 1.4 0.7 0.6 0.5 0.5 B 1.3 0.8 0.5 0.4 0.6 C I .O 1.4 0.4 (0.4) 0.4 N 0.7 I .I 0.2 (0.15) 0.4 0 0.7 I. I 0.3For Z<6 e j e c t i les t h e angular c o r r e l a t i o n s are symmetric about t h e beam a x i s w i t h a narrow width s40° FWHM. The angular c o r r e l a t i o n s f o r f a s t cc p a r t i c l e s (Ea>30 MeV) are narrower b u t again have t h e same shape as t h e corresponding s i n g l e s
d i s t r i b u t i o n . For e j e c t i l e s w i t h 2 2 6, departures from these systematics are observed. The s i ngl es HI spectra show a l a r g e q u a s i - e l a s t i c component which i s n o t present i n t h e projected spectra. Perhaps o f more i n t e r e s t i s a tendency f o r t h e peak o f t h e angular c o r r e l a t i o n (Fig.2) t o s h i f t t o t h e opposite s i d e o f t h e beam a x i s from t h e HI telescope. This e f f e c t i s more noticeable f o r the 2 7 than ~ ~
5 8 ~ i t a r g e t , and i s very marked f o r t h e
'*c
t a r g e t . T h i s e f f e c t , which becomes more enhanced t h e heavier2 t h e e j e c t i l e , has been observed before
.
For t h e 5 8 ~ i t a r g e t much o f t h e information on the energy and angular c o r r e l a t i o n s i s already
and w i l l only be summarised here. For e j e c i i l e s w i t h Z<6 t h e systematics show:
(a) No appreciable c o r r e l a t i o n between HI energy and a p q r t i c t e ' e n e r g y .
( b ) The p r o j e c t e d energy spectra o f a p a r t i c l e s are independent o f HI, type o r HI angle and are s i m i l a r i t , shape.to t h e s i n g l e s energy spectra
a t t h e corresponding a-angle. The a - p a r t i c l e energies are h iqh w i t h average v e l o c i t i e s approach-
i ng the'beqm ve lo^ i t y a t forward angles.
( c ) The p r o j e c t e d energy spectra o f d i f f e r e n t HI types are independent o f a angle and are simi l a r t o t h e corresponding s i n g l e s H I spectra.
( d l The a-HI angular c o r r e l a t i o n s a r e roughly symmetric about t h e beam a x i s w i t h a f u l l width a t h a l f maximum s400, and have shapes s i m i l a r t o t h e s i n g l e s a-angular d i s t r i b u t i o n s .
The same systematics hold t o a large degree f o r t h e
2 7 and ~ ~ t a r g e t .
The apparent independence o f t h e emission processes f o r a - p a r t i c l e and HI suggests a f a c t o r i s - a t i o n o f t h e two-dimensional ,energy-correlation data according to:
~ ' U ( E ~ ~ , ~ ~ ~ , E ~ , ~ , ) = K . ~ ~ U ( E ~ ~ , ~ ~ ~ ) . ~ ~ U ( E , , ~ ~ )
where t h e terms on t h e r i g h t are t h e s i n g l e s cross sections f o r HI and a emission r e s p e c t i v e l y and K
i s a constant o f p r o p o r t i o n a l i t y .
The order t o compare t h e energy c o r r e l a t i o n s (EHI,E,) obtained from t h i s f a c t o r i s a t i o n procedure, t h e three-body kinematic v a r i a b l e s 93, EI2 and EZ3 have been computed from t h e above equation and compared w i t h t h e corresponding p r o j e c t ions from t h e data. E12. EZ3 are t h e r e l a t i v e energies i n t h e i r common c.m. frame; I 5 HI, 2 Z a and 3
-
r e c o i l i n g nucleus. Contributions from t h e k i n e m a t i c a l l y forbidden region (Q3>O) have been excluded; these a r e however small f o r 2<6. The r e s u l t s are showni n Fig.3 f o r t h e 5 8 ~ i t a r g e t and it can be seen t h a t t h e f a c t o r i s a t i o n procedure reproduces t h e data s a t i s f a c t o r i l y f o r e j e c t i les with 2<6.
~ h k
dashed curves f o r 2=6 are obtained by r e p l a c i n g t h e s i n g l e s cross sections by t h e p r o j e c t e d cross sections i n t h e expression given above. Although n o t shown t h e'
'
N
+ SBNi 118 MeVdata from t h e 2 7 and 12c targets, f o r ~ ~ Z<6, can a l s o be reasonably we1 l f i t t e d by t h i s procedure3.
The f a c t o r i s a t i o n a l s o implies t h a t the d i f f e r e n t i a l a m u l t i p l i c i t i e s are proportional t o t h e s i n g l e s a angular d i s t r i b u t i o n s and t h i s has already been seen i n Fig.2., again f o r Z<6. The s o l i d l i n e s are obtained w i t h normalisation f a c t o r s of K=O.5, K=0.6, and K*I .O f o r t h e " ~ i , 2 7 ~ 1 and
"C t a r g e t s . The f a c t o r K i s i n each case found t o be %//aR where oR i s the t o t a l r e a c t i o n cross s e c t i o n (barns).
The success o f t h e f a c t o r i s a t i o n procedure i n describing t h e coincidence data f o r e j e c t i l e s w i t h
Z<6 i n terms of t h e s i n g l e s cross sections implies t h a t the p r o b a b i l i t i e s and hence t h e mechanisms o f HI and
er
emission are, t o a large extent, independ- e n t o f each other. A model which could account f o r t h i s i s t h e emission o f an a - p a r t i c l e ( o r, 5
a - p a r t i c l e s ) i n a f a s t d i r e c t process a t an e a r l y stage o f t h e i n t e r a c t i o n . Fol lowing t h i s , t h e residue o f t h e p r o j e c t 1 le - t a r g e t combination then undergoes a f u r t h e r i n t e r a c t i o n which can be best described as a deep i n e l a s t i c c o l l i s i o n which leads t o t h e emission o f a range o f e j e c t i l e s which are observed i n coincidence w i t h t h e a - p a r t i c l e ( s ) . Due t o t h e s t a t i s t i c a l nature o f t h e energy relaxa- t i o n i n such an i n t e r a c t i o n t h e e j e c t i l e r e t a i n s
l i t t l e memory o f t h e mmentum t h a t was t r a n s f e r r e d t o t h e a - p a r t i c l e . Since t h e a - m u l t i p l i c i t i e s are close t o unity, t h i s implies t h a t a major f r a c t i o n o f t h e e j e c t i l e s ( a t l e a s t f o r Z<6) a r e formed by such a three-body process, and t h i s a l s o e x p l a i n s t h e s i m i l a r i t y between t h e HI s i n g l e s and projected coincidence spectra.
The residue o f t h e p r o j e c t i l e - t a r g e t combina- t i o n ( a f t e r a-emission) can a l s o undergo fusion, g l v i ng r i se t o the process observed i n recent years c a l l e d incomplete fusion. Indeed it has been shown4 f o r t h e system "C + . l 6 ' ~ d t h a t such r e a c t i o n s accompanied by one, two o r t h r e e a - p a r t i c l e emission can account f o r most o f t h e observed s i n g l e s a - p a r t l c l e cross sect ion. I t i s probable t h a t t h i s process a l s o accounts f o r t h e major f r a c t i o n o f s i ngles a-parti c l es observed i n t h i s work as o n l y about 10% ( f o r 5 8 ~ i and "AI t a r g e t s ) o f these a - p a r t i c l e s are found t o be i n coincidence w i t h outgoing e j e c t i l e s . Since t h e ?-emission mechanism would appear t o be common t o both deep i n e l a s t i c and i ncomplete fusion processes,
t h e s i m i l a r i t y between t h e s i n g l e s a-spectrum and coincidence a-spectrum observed f o r Z<6 can again be understood.
For Z
L
6 e j e c t i ie s t h e backward s h i f t i n t h e angular c o r r e l a t i o n s observed with a l l t a r g e t s necessari l y imp1 i e s a d i f f e r e n t r e a c t i o n mechanism f o r a p a r t i c l e production. However, i t should be noted t h a t experimental information on Z>6 e j e c t i l e s i s harder t o o b t a i n since t h e i r d e t e c t i o n (and i d e n t i f i c a t i o n ) i s more d i f f i c u l t and t h e r e l a t i v e f r a c t i o n o f 2 % ions from I l g h t i m p u r i t i e s increases f o r heavier t a r g e t s . For these reasons the data under consideration i s mainly f o r 12c and 2 7 ~ 1 ,targets, although simi l a r e f f e c t s have been
2
p r e v i o u s l y observed i n t h e "3 + 5 8 ~ i system
.
Angular c o r r e l a t i o n s f o r a p a r t i c l e s i n coincidence w i t h C, N and 0 e j e c t i l e s (EH1 75 MeV) are shown separately i n Fig.4. In a d d i t i o n t o t h e s y s t e m a t i c s h i f t towards l a r g e r angles w i t h increasing Z
p r e v i o u s l y noted, the peak a l s o broadens and s h i f t s towards larger angles f o r higher energy e j e c t i l e s .
Fig.4 Angular c o r r e l a t i o n s f o r a p a r t i c l e s i n coincidence w i t h high and low energy C, N and 0
e j e c t i l e s . The arrows include t h e d l r e c t i o n o f t h e t a r g e t - l i k e r e c o i l I n a binary reaction. The peak posi?ions roughly correspond t o t h e d i r e c t ion o f recoi l o f the target- l i ke f ragment i n a binary reaction, OR ( i n d i c a t e d by arrows). T h i s correspondence i s consistent with a process wherein t h e a - p a r t i c l e s c o i n c i d e n t w i t h Z26 e j e c t i l e s are mainly emitted from t h e r e c o i l i n g nucleus produced i n t h e reaction. OF course r e l a t i v e l y f a s t alphas may s t i l l be produced, corresponding t o pre- equl i ibrium alpha ernission5,
c10-246
JOURNAL DE PHYSIQUEt h e angular c o r r e l a t i o n p a t t e r n s can a l s o be understood from simple r e a c t i o n kinematics, t h e r e a c t i o n on t h e
12c
t a r g e t has been studied i n more d e t a i l . A M o n t e C a r l o s i m u l a t i o n o f t h e r e a c t i o n"c(
"N, "Nal has been performed assuming t h a t t h e'%
fragment i s emitted f i r s t and t h a t t h e r e c o i l - ing12c
fragment subsequently emits an a p a r t i c l e . Calculated angular c o r r e l a t i o n s are shown i n Fig.5.Fig.5 Calculated angular c o r r e l a t i o n s f o r t h e r e a c t i o n 1 2 ~ + 1 4 ~ f 4 ~ + 1 2 ~ * ; I2C*+3a f o r
e H f =
I3O and 26O.The width o f t h e c o r r e l a t i o n i s determined by ( a ) the temperature T o f t h e excited 12C and ( b ) t h e smearing o f OR because o f t h e continuous HI energy d i s t r i b u t i o n .
A Monte Carlo c a l c u l a t i o n has a l s o been performed assuming t h a t a compound nucleus 2 6 ~ 1 i s f i r s t formed from which t h r e e a p a r t i c l e s are successively evaporated6. S i nce t h e emitted a p a r t i c l e s are uncorrelated i n t h e c.m. frame, t h e r e c o i I i ng '+N r e s l due should have an average
laboratory v e l o c i t y
=
vcm. However, s e t t i n g energy and angular windows on t h e detected "Nk i n e m a t i c a l l y s e l e c t s those events I n which t h e combined alpha momentum i s non-zero. Thus, although t h e a p a r t i c l e s are emitted f i r s t , t h e a-HI c o r r e l a t i o n s again s h i f t towards t h e r e c o i l d i r e c t i o n (Fig.6). However, t h e shape o f t h e a angular c o r r e l a t i o n i s not very s e n s i t i v e t o t h e
Fig.6 Calculated angular c o r r e l a t i o n s f o r t h e r e a c t i o n ' 2 ~ + 1 4 N + 2 6 ~ l *+'"N+~CL f o r
eH
I = 13O.A f i n a l comment concerns t h e apparent non- observation o f 'evaporation' alphas i n coincidence w i t h 216 e j e c t i l e s . One may speculate t h a t such alphas, although present, are swamped by t h e f a s t d i r e c t alphas which dominate t h e coincidence spectra f o r Ze6. The model can a l s o e x p l a i n whyevaporation alphas a r e observed f o r Z L 6 : f o l l o w i n g d l r e c t alpha emission t h e p r o j e c t i l e involved i n t h e deep
i n e l a s t i c c o l l i s i ~ n h a s , a n , e f f e c t i v e Z%5 and so Z>4 e j e c t i l e k I i e on the t a i l o f the ; ' d i s t r i b u t i o n o f
deep i n e l a s t i c products. I t would be i n s t r u c t i v e t o t e s t t o what e x t e n t t h e backward s h i f t i s a general feature o f a-HI c o r r e l a t i o n data f o r heavy e j e c t i les I n o t h e r systems.
*K.V. I
.,
Gronlngen, The Netherlands.**
U n i v e r s i t y o f Malta, Msida, Maltatoaresbury Laboratory, SRC, Warri ngton, England. I. R.K. Bhowmik, E.C. Pollacco, N.E. Sandersbn, J .B.A. England and G.C.. Morrison, Phys.Rev.Lett. 43, -619 (19791.
-
2. R.K. Bhowmi k, E.C. Pol l acco, N.E. Sanderson, J.B.A. England and G.C. Morrison, Phys.Lett.8OJ, 41 (1978).
3. R.K. Bhowmik, E.C. Pollacco, N.E. Sanderson, J.B.A. England, D.A. Newton and G.C. Morrison,in Lecture Notes i n Physics 1 1 7 (Springer-Verlag pub l ishers, W.von ~ e r t z e n , e d i t o r ) , 194 ( 1980). 4. K.Siwek-Wliczynska, E.H. du Marchie van Voorthuysen, J.van Popta, R.H. Siemssen and J. Wylczynski, Phys.Rev.Lett.42, 1599 (1979).
.
.
--assumed r e a c t i o n mechanism and i s determined mainly 5. J.P. Wurm, t h i s conference.
bv m n t u m c o n s t r a i ~ n t s imposed , . by t h e r e a c t i o n ' 6. J. Gomez de Campo, R.G. Stokstad, J.A. kfnematlcs. Of course f o r t a r g e t s heavier than B i g g e r s t a f f , R.A. Stelson, Phys.Rev.C. ~ a ~ r a s ,
