Supported by the National Science Foundation and Indiana Universit

Bent, R.D.

Green, M.C.

Jacobs, W.W.

Meyer, H.O.

Pile, P.H.

Pollock, R.E.

Sjoreen, T.P.

Soga, F.

English

IUScholarWorks

POSITIVE PION PRODUCTION FROM THE BOMBARDMENT OF llB, 12C AND 4 0 ~ a  BY 146-159 MeV POLARIZED PROTONS 
P.H. Pile, T.P. Sjoreen, R.E. Pollock, R.D. Bent, M.C. Green, W.W. Jacobs, H.O. Meyer, and F. Soga 
Indiana University CycZotron Faci Zity, BZoomington, Indiana 4 7405 
As part of the continuing program to study the The asymmmetries for the 10~($,n+) l l ~ ,  l2C(;,n+) 13C, 
+ -+ + A(p,n ){A+l} reaction near threshold, polarization and 40~a(p,n )41~a reactions were measured at polarized 
asymmetries of this reaction for transitions to several proton bombarding energies of 154, 159 and 146 MeV, 
discrete final states in I ~ B ,  13C and 4 1 ~ a  have been respectively. The outgoing pions, which ranged in 
measured. One of the motivations for and an advantage energy from 5-12 MeV in the center-of-mass, were 
of studying the reaction near threshold is that the detected with the D: pion spectrometer.' The 1°B and 
outgoing pions should be dominated by s and p waves. 4 0 ~ a  targets were chosen to complement a set of 
This assumption is important because it limits the angular measurements2 for transitions to the llB and 
number of experimental points needed to determine the 4 1 ~ a  ground states. The 12c target was chosen 
angular distribution of the analyzing power to 3 or 4 because of the different structures of the 13c ground 
+ 
angles, when the corresponding differential cross is state (112-) and 3.09 MeV state (112 ) ,  and because 
well established. 
Figure I .  Results of the 
lO~($,n+)l lB asymmetry mea- 
surements. The curves i n  
t h i s  figure and the succeeding 
figures are poZynomiaZ f i t s  
which are discussed i n  the  
t e x t .  
Fi w e  2 Results of the 
-1 3~ asymmetry 
measurements. 
the 5=1/2 spins of these states allow a complete 
reaction analysis to be made using a method described 
by Pollock, er a1. Furthermore, the asymmetry mea- 
+ 
surements of the 1 2 ~ ( p , ~  ) 13c reaction at 200 MeV 4 
allow an interesting comparison to be made of the 
analyzing powers at two different energies. 
The results of the present measurements are 
shown in Fig. 1-3, where the differential cross 
sections as well as the asymmetries have been plotted 
as a function of the c.m. angle 8.  Figure 1 contains 
the results for the llB ground state and the 2.12 
and 4.44 MeV excited states. The llB ground state 
involves primarily the lp neutron orbital, while 312 
the excited states have more complex configurations. 
Figure 2 shows the results for the 13c ground and 
3.09 MeV states, both of which are good single particle 
states involving the lp and 2s orbitals, res- 
112 11 2 
pectively. Information was also obtained for the 
3.68-3.85 MeV doublet in 13c. This doublet was not 
resolved in the present experiment; the asymmetries 
of the doublet were observed to be negative, but not 
as large as those for the lower lying states. Figure 
3 contains the results for the transition to 4 1 ~ a  
ground state, which is a good single particle state 
involvingthelf orbital. 
7 / 2 
Several features can be seen in the present re- 
sults: 1) the observed asymmmetries are generally 
negative; 2) the large negative asvmmetries observed 
Figme 3. Results of the 40~ai$,n+)41~a asymmetry 
measurements. 
+ + by Auld, et alB4 for the 12c(~,n )13c reaction at 
200 MeV are also seen to persist at 159 YeV, indica- 
ting that the asymmetries are not sensitive to the 
pion energies near threshold (T c'm' - < 49 PleV) ; and 
3) the asymmetries exhibit some state dependence. 
The curves drawn through the differential cross 
section data points in Figs. 1-3 are polynomial fits 
to order cos38. With these fits, curves for the 
angular distribution of the analyzying power A ( 8 )  
were determined by fitting (da/dR)A(@)/sin 0 to 
order cos28. These polynomial fits will be a valid 
description if s and p wave outgoing pions dominate 
with the d wave included only through its interfer- 
ence with the s and p waves. The validity of this 
approximation has been tested3 on a number of light 
nuclei in this pion energy region. The angular distri- 
bution for the analyzing power of the 4 1 ~ a  ground 
state is not well defined at forward angles and is 
denoted by a dotted line. 
Recently there have been two different theoretical 
-F + 
attempts to explain the observed ( p , ~  ) asymmetries. 
~san~arides~ has calculated polarization asymmetries 
within the framework of the stripping model including 
the effects of both proton and pion distortions. While 
the sign of the analyzing power is given correctly for 
+ + 
the ~OB(~,IT ) I1B(g.s.) and 1 2 ~ ( p , ~  ) 13~(g.s.) reactions, 
the calculations are extremely sensitive to the form of 
the pion production operator {(static (A=O) or Galilean 
invariant (X=l)l and to both proton and pion distortions. 
+ 
For the 40~a(p,n )41~a(g.s.) reaction, X=l gives a 
reasonably good fit to the differential cross section 
data but the wrong sign for the analyzing power, where- 
as X=O gives the correct sign for the analyzing power 
but fails to reproduce its angular variation and the 
differential cross section data. 
6 Gibbs has taken a different approach. He assumes 
that the pion originates in the nucleus and is knocked- 
out by the incoming proton. Preliminary calculations 
give negative asymmetries which are not inconsistent 
with  the  experimental  r e s u l t s .  
1 )  P.H. P i l e  and R.E. Pollock,  Nucl. Instrum. Methods 
165, 209 (1979). 
-
2) P.H. P i l e ,  R.D. Bent, R.E. Pol lock ,  P.T. Debevec, 
R.E. Marrs, M . C .  Green, T.P. Sjoreen and F. Soga, 
Phys. Rev. L e t t .  42, 1461 (1979). 
3) R.E. Pollock,  P.H. P i l e ,  H.O.  Meyer and G.T. Emery, 
Bul l .  Am. Phys. Soc. 24, 614 (1979). 
4) E.G. Auld, A. Haynes, R.R. Johnson, G .  Jones ,  T. 
Masterson, E.L. Mathie, D. O t t ewe l l ,  P. Walden, and 
B. T a t i s c h e f f ,  Phys. Rev. L e t t .  41, 462 (1978). 
5 )  M.C. Tsangarides,  t h e s i s ,  Indiana Un ive r s i t y ,  1979, 
IUCF I n t e r n a l  Report #79-4. 
6) W. Gibbs, I nv i t ed  Paper ,  LAMPF Workshop on Nuclear 
S t r u c t u r e  wi th  In te rmedia te  Energy Probes,  Los 
Alamos, 1980 (unpublished).  
HIGH RESOLUTION STUDIES OF THE (p,n+) REACTION ON l p  SHELL NUCLEI AT Ep=200 MeV 
F. Soga, R.D. Bent, P.H. P i l e ,  T.P. S joreen ,  and M.C. Green 
Indiana univers i ty  Cyclotron Faci l i ty ,  Bloomington, Indiana 47405 
It was shown by Dahlgren e t  a l .  i n  one of t h e  t i o n  and have t h e  l a r g e s t  s i n g l e - p a r t i c l e  spec t ro scop i c  
e a r l y  Uppsala papers  on nuc l ea r  p ion  product ion  near  f a c t o r s .  The 2.14(1/2-) s t a t e ,  which is  forb idden  by 
+ 
t h r e sho ld  t h a t  t h e  1 2 ~ ( p ,  IT ) 3~ r e a c t i o n  produces f i n a l  
s t a t e s  of complicated s t r u c t u r e  j u s t  a s  s t r on g ly  a s  
s t a t e s  t h a t  can be  reached by t h e  d i r e c t  t r a n s f e r  of a  
neutron.  This  is  i l l u s t r a t e d  by t h e  upper spectrum 
shown i n  Fig. 1, which was taken  r e c e n t l y  a t  IUCF wi th  
an  o v e r a l l  energy r e s o l u t i o n  of 170 keV. The f a c t  t h a t  
t h e  1plI2, 2slI2 and ldgI2  s i n g l e - p a r t i c l e  s t a t e s  a t  
0.0, 3.09 and 3.85 MeV, r e spec t i ve ly ,  and t h e  two 
pa r t i c l e -one  h o l e  s t a t e s  a t  3.68, 6.86 and 9.50 MeV a r e  
populated about  equa l ly  provided e a r l y  evidence i n  sup- 
p o r t  of a  two-nucleon2 o r  m u l t i s t e p  r e a c t i o n  mechanism. 
+ Recent h igh  r e s o u l t i o n  s t u d i e s  of t h e  ( p , ~  ) re -  
a c t i o n  on o the r  t a r g e t s  i n  t h e  p-she l l  i n d i c a t e  t h a t  
s t r ong  product ion  of core-exc i ted  s t a t e s  may be  t h e  
except ion  r a t h e r  than  t h e  r u l e .  F igure  2 ( top)  shows a 
+ 
OB (p,  IT ) l~ spectrum taken  a t  IUCF wi th  an  o v e r a l l  
energy r e s o l u t i o n  of 210 keV. The f i v e  low-lying s t a t e s  
of odd-pari ty i n  1 1 ~  a r e  be l i eved  t o  a r i s e  mainly from 
con f igu ra t i ons  comprised of pjI2 and pIl2 nucleons and 
a r e  w e l l  descr ibed  by t h e  s h e l l  model based on t h e  i n t e r -  
mediate  coupling ~ c h e m e . ~  The s t a t e s  a t  0.00(3/2-) and Excitation Energy (MeV) 
+ 4.44(5/2-) MeV which a r e  s t r o n g l y  exc i t ed  i n  t h e  ( p , ~  ) Fi ure 1 Pion energy spectra from the  reactions 
3C and 3 ~ ( p ,  a+) 14c a t  T =200 MeV and 
r e a c t i o n  a l s o  a r e  populated s t r ong ly  i n  t h e  (d,p) reac-  OIT(labl = 25'. P 


Positive Pion Production from the Bombardment of 11-B, 12-C, and 40-Ca by 146-159 MeV Polarized Protons

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Positive Pion Production from the Bombardment of 11-B, 12-C, and 40-Ca by 146-159 MeV Polarized Protons

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