This work was supported by National Science Foundation Grant PHY 75-00289 and Indiana Universit

Anderson, B.D.

Bainum, D.

Bertrand, F.E.

Foster, C.C.

Goodman, C.D.

Greenfield, M.B.

Knudson, J.N.

Madey, R.

Rapaport, J.

Witten, T.R.

English

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THE ISOSPIN MAKEUP OF THE GIANT RESONANCES FROM (p,n) 
REACTION STUDIES AT INTERMEDIATE ENERGIES? 
B.D. Anderson*, J.N. Knudson*, R. Madey*, T.R. Witten*, 
D. Bainum** and J. Rapaport**, M.B. Greenfield***, 
C.C. Fos ter ,  F.E. Bertrand' and C.D. ~oodman' 
Neutron spec t r a  above about 20 MeV have been 
m e a ~ u r e d l , ~ )  a t  r eac t ion  angles from 7' t o  22' 
from 61.9 MeV proton bombardment of t a r g e t s  of 
'Be, 12c, and 28~i.  Time-of-f l i g h t  measurements 
were made by suppressing 5 out  of 6 beam b u r s t s  
a t  a  frequency of 32.29 MHz. Timing from t h e  
cyclotron r. f .  s i g n a l  r e su l t ed  i n  an o v e r a l l  
time r e so lu t ion  of about 1 .3  nsec (fwhm). The 
5" diam x 4" t h i ck  NE-102 pl .ast ic  s c i n t i l l a t i o n  
counters have an i n t r i n s i c  time r e so lu t ion  of 
l e s s  than 1.0 nsec (fwhm) 3). The energy 
r e so lu t ion  was about 0 .3  MeV f o r  35-MeV neutrons 
t ravers ing  the  f l i g h t  path of 29 meters. 
The spectrum from each t a r g e t  is  dominated 
by a s i n g l e  sharp peak a s  can be seen i n  Fig. 1 
f o r  t he  1 2 ~ ( p , n ) 1 2 ~  r eac t ion  a t  an angle of  
10'. This sharp peak is the  g i an t  M l  resonance 
which is exci ted  he re  by a charge-exchange 
t r a n s i t i o n  t h a t  a l s o  f l i p s  the  sp in  of a  nu- 
cleon. This s t a t e  occurs a t  0-MeV e x c i t a t i o n  
6 1 . 9  M E V  
I 
2 2 15 i 0 5 0 
E X C  I T R T  I O N  ENERGY.  T( M E V  1 
Figure 1. 
i n  1 2 ~ ,  and a t  2.1 MeV i n  2%, corresponding t o  t he  
analogs of t he  15.1-MeV s t a t e  i n  12c, and t h e  11.4- 
MeV s t a t e  i n  28~i ,  respect ive ly .  I n  t he  1 2 c ( p , n ) 1 2 ~  
and 28~ i  (p ,n) 2 8 ~  r eac t ions ,  a  broader concentrat ion 
of s t r eng th  appears i n  each spectrum a t  h igher  exci- 
t a t i o n  energies ,  presumably represent ing  a c l u s t e r  
of par t ic le -hole  s t a t e s .  A t  s t i l l  higher e x c i t a t i o n ,  
another concentrat ion of s t r eng th  might be t h e  analog 
of t he  g i an t  E l  resonance. 
Addit ional  measurements w i l l  be made a t  62 and 
136 MeV on self-conjugate t a r g e t  nucle i .  We w i l l  
compare our  measurements of (p,n) spec t r a  wi th  o the r  
measurements of  (p,  p ' )  and (a,  a ' )  spec t r a  t o  study 
t h e  i sosp in  makeup of g i an t  resonances. The (p, p ' )  
r eac t ion  on t a r g e t  n u c l e i  of  i sospin  zero permits  
e x c i t a t i o n  of mult ipole resonances i n  both i sosp in  
T = 1 and T = 0 s t a t e s  of t he  r e s idua l  nucleus;  
however, (p,n) r eac t ions  on t a r g e t  nuc l e i  of zero 
i sospin  can e x c i t e  only T = 1 s t a t e s ,  and (a, a ' )  
r eac t ions  on such t a r g e t s  can e x c i t e  only T = 0 
s t a t e s .  The r a t i o  of  t he  (p,n) t o  (p,  p ' )  cross  
s ec t ions  is  two f o r  exc i t i ng  a T = 1 s t a t e  from a 
self-conj  ugate t a r g e t  nucleus. Thus, t he  cross  
s ec t ion  f o r  a  T = 0 s t a t e  exc i ted  by t h e  (p,  p ' )  
r eac t ion  can be deduced by sub t r ac t ing  one-half of  
t he  cross  s ec t ion  f o r  a T = 1 state i n  a (p,n)  
r eac t ion  from the  t o t a l  (p, p ' )  c ross  s ec t ion  
which exc i t e s  both i sosp in  s t a t e s .  
t Work supported i n  p a r t  by NSF and USERDA 
* Kent S t a t e  Universi ty,  Kent, Ohio 44242 
** Ohio Universi ty,  Athens Ohio 45701 
*** Flor ida  A & M Univers i ty  , Tallahassee,  F lo r ida  32307 
+ Oak Ridge National  Laboratory, Oak Ridge, 
Tennessee 37830 
1 )  B. Anderson, A. Baldwin, J. Knudson, 
T. Witten, R. Madey, C.D. Goodman, 
D.E. Bainum, J. Rapaport, C.C. Fos ter  and 
M.B. Greenfield,  Bull.  Am. Phys. Soc. 21, 
978 (1976) . 
2) C.D. Goodman and F.E. Bertrand, Oak Ridge 
National  Lab ; R. Madey , B. Anderson, 
A. Baldwin, J. Knudson, and T. Witten, 
J. Rapaport and D. Bainum, M.B. Greenfield 
and C.C. Fos ter ,  Bull.  Am. Phys. Soc. 22, 
(Apr i l  1977), t o  be published. 
3) T.R. Witten, B.D. Anderson, J . N .  Knudson, 
and R. Madey, Bull .  Am. Phys. Soc. 22, 
208 (1977). 


The Isospin Makeup of the Giant Resonances from (p,n) Reaction Studies at Intermediate Energies

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The Isospin Makeup of the Giant Resonances from (p,n) Reaction Studies at Intermediate Energies

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