This work was supported by National Science Foundation Grants PHY 76-84033A01, PHY 78-22774, and Indiana Universit

Debevec, P.T.

Gutay, L.J.

Moake, G.L.

Quin, P.A.

Scharenberg, R.P.

English

IUScholarWorks

A STUDY OF ONE-PION EXCHANGE I N  (p,n)  REACTIONS 
USING PCAC AND THE ELEMENTARY PARTICLE MODEL 
G.L. Moake*, P.T. Debevec**, L . J .  Gutsy***, P.A. Quin****, R.P. Scharenberg*** 
It is  i n t e r e s t i n g  common f e a t u r e  of t h e  r e a c t i o n  
p + 6 ~ i  -t n + 6 ~ e  (1)  
p  + 1 2 ~  -t n + 1 2 ~  (2) 
p + 1 4 ~  -t n + 140 (3)  
t h a t  t h e  i s o s p i n  (T), angula r  momentum ( J ) ,  and p a r i t y  
(P) change of t h e  t a r g e t  n u c l e i  a r e  A J  = AT = 1, AP = 0. 
These quantum number changes a r e  l i k e  those  i n  Gamow- 
T e l l e r  b e t a  decay,  and they could be caused by one-pion 
The e f f i c i e n c y  of t h e  d e t e c t o r  was determined by 
comparing i t s  y i e l d  t o  t h a t  of a  5" diameter  by 5" deep 
l i q u i d  s c i n t i l l a t o r  d e t e c t o r  (5x5). This  5x5 was then  
c a l i b r a t e d  f o r  130 MeV neut rons  i n  a  s p e a r a t e  experiment 
us ing  a  tagged-neutron beam. The experiment was done 
w i t h  t h e  64" s c a t t e r i n g  chamber us ing  t h e  r e a c t i o n  
p + 7 ~ i  -t n + 7 ~ e .  (4) 
Since 7 ~ e  has  no e x c i t e d  s t a t e s ,  t h i s  r e s u l t e d  i n  a  
exchange. We t h e r e f o r e  con jec tured  t h a t  t h e  r e a c t i o n s  monoenergetic neu t ron  beam. A AE-E-veto t e l e s c o p e  was 
(1)-(3) a r e  dominated by one-pion exchange f o r  144 MeV p laced  i n s i d e  of t h e  chamber a t  56' i n  o r d e r  t o  d e t e c t  
p ro tons  and s m a l l  s c a t t e r i n g  angles .  Then, accep t ing  t h e  r e c o i l i n g  7 ~ e .  The s o l i d  a n g l e  of t h e  t e l e s c o p e  
t h e  sugges t ion  of K i m  and Primakoff t h a t  t h e  n u c l e i  be was def ined  by two s e t s  of h o r i z o n t a l  and v e r t i c a l  
1 1 
t r e a t e d  a s  elementary p a r t i c l e s ,  t h e  i n t e r a c t i o n s  slits.  The 5x5 was p laced  o u t s i d e  of t h e  chamber a t  
were descr ibed  i n  terms of a  pion-nucleus coupling con- 60' and was p o s i t i o n e d  such t h a t  i f  a  7 ~ e  e n t e r e d  t h e  
s t a n t  i n  t h e  framework of t h e  Absorpt ion Model. 2 ,3)  t e l e s c o p e ,  then  t h e  corresponding neu t ron  would p a s s  
The advantage of t h i s  procedure is t h a t  PCAC, b e t a  de- through a 3 m i l  t h i c k  kapton window and i n t o  t h e  5x5. 
cay,  and i n e l a s t i c  e l e c t r o n  s c a t t e r i n g  can be  used t o  I n  o r d e r  t o  i n s u r e  t h a t  t h e  5x5 was p o s i t i o n e d  pro- 
p r e d i c t  bo th  t h e  magnitude of t h e  coupling c o n s t a n t  and p e r l y ,  t h e  p ro tons  corresponding t o  t h e  r e a c t i o n  
i t s  dependence on momentum t r a n s f e r .  4  p + 7Li -t p + 7Li (5)  
Angular d i s t r i b u t i o n s  f o r  r e a c t i o n s  (1)- (3) were were d e t e c t e d  i n  co inc idence  w i t h  t h e  r e c o i l i n g  7 ~ i ,  
measured w i t h  t h e  ad hoc Neutron Time-of-flight F a c i l -  
i t y 5 )  a t  144 MeV. The d e t e c t o r  c o n s i s t e d  of a  charged 
p a r t i c l e  i d e n t i f i e r  fol lowed by two p l a s t i c  s c i n t i l l a t o r  
t iming r o d s  ( i n  p a r a l l e l )  and a l i q u i d  s c i n t i l l a t o r  v a t ,  
which was used t o  s e t  t h e  energy t h r e s h o l d .  A v a l i d  
neu t ron  event  c o n s i s t e d  of a  co inc idence  between one 
of t h e  rods  and t h e  v a t ,  w i t h  no s i g n a l  i n  t h e  charged 
p a r t i c l e  i d e n t i f i e r .  Phototubes were p laced  on each 
end of t h e  rods ,  and t h e  t ime d i f f e r e n c e  between each 
phototube p u l s e  and t h e  subsequent  c y c l o t f o n  RF peak 
p u l s e  was s t o r e d .  The neu t ron  time of f l i g h t  spectrum 
f o r  each rod was then  ob ta ined  by adding t h e  t ime d i f f e t -  
and t h e  c e n t e r  of t h e i r  cone was found. The c e n t e r  of 
t h e  neu t ron  cone could then  be  found from k inemat ics  
( they were v e r y  c l o s e . )  The d a t a  was recorded i n  even t  
mode, s o  t h a t  t h e  e f f i c i e n c y  could be  determined f o r  
many t h r e s h o l d  s e t t i n g s .  The normalized c r o s s  s e c t i o n  
f o r  r e a c t i o n s  (1)-(3) and f o r  
p  + 1 4 ~  -t n + 140 (7 .78 MeV) (6) 
a r e  shown i n  F igure  1. 
The t h e o r e t i c a l  c a l c u l a t i o n s  were done w i t h  t h e  
Absorpt ion Model. 2 9 3 )  This  rakes  t h e  Born h e l i c i t y  
ampli tudes f o r  one-meson exchange (given i n  momentum 
space)  and i n s e r t s  d i s t o r t i o n s  through t h e  e l a s t i c  
ences from each of i t s  ends. P u l s e  s e l e c t i o n  of 1:4 s c a t t e r i n g  S matr ix .  The model was o r i g i n a l l y  used i n  
was u s u a l l y  employed. Elementary P a r t i c l e  Phys ics ,  b u t  i t  h a s  been modified 
to apply to the nucleus. The nucleus was treated as an that these reactions are being dominated by one-pion 
elementary particle, and the relationship between the exchange. On the other hand, why is the 1 4 ~  case so 
weak axial Vector Current and the pion field given by different? Part of the discrepancy is probably due to 
PCAC was expJoited to give the magnitude of the pion- the neglect of higher order terms in the determination 
nucleus coupling constant . l) This relationship was of the coupling constant. If the coupling constant is 
also used in conjunction with inelastic electron scat- determined from the Impulse Approximation, then the 
tering to obtain the momentum dependence of the coupling are low by a factor 
constant . 4, Consequently, the calculations contained three 9 which be due to the uncertainty of the 
(a>- (c). the formfactor used was incorrect because of terms 
The agreement between the theory and experiment neglected in its derivation. Consequently, it is unclear 
6 as to whether the 1 4 ~  case could be explained by for Li and 12c is remarkable. This is strong evidence 
Figure I .  The measured center of mass cross sections (dots) and the theoretical predictions (curves) calculated 
assuming one-pion exchange dominance. 
one-pion exchange dominance, or whether other inter- 
actions are involved. 
*Purdue University, c/o IUCF 
**University of Illinois, Champaign, Illinois 61801 
***Purdue University, West Lafayette, Indiana 47907 
****University of Wisconsin, Madison, Wisconsin 53706 
1) C.W. Kim and H. Primakoff', Phys, Rev. 139B, 1447 
(1965) 
2) K. Gottfried and J.D. Jackson, Nuovo Cimento 34, 
5783 (1964) 
3) L. Durand and Y.T. Chiu, Phys. Rev. 139, 646 (1965). 
4) C.W. Kim, Ref. Del Nuove Cimento 5, 189 (1974). 
5) IUCF Technical and Scientific Report, 1975-1977, 
p. 61. 
6) B. Goulard et al., Phys. Rev. C 16, 1999 (1977). 


A Study of One-Pion Exchange in (p,n) Reactions Using PCAC and the Elementary Particle Model

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A Study of One-Pion Exchange in (p,n) Reactions Using PCAC and the Elementary Particle Model

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