Supported by the National Science Foundation and Indiana Universit

Bacher, A.

Debevec, P.

Emery, G.

Gupta, O.P.

Jenkins, D.

Long, D.

Pickar, M.

English

IUScholarWorks

co r rec t ions  necessary t o  convert t he  y i e l d s  obtained 
using the  geometry of Fig. 1 i n t o  c ros s  s ec t ions .  
We concluded t h i s  experiment by making a prel imi-  
nary measurement of t he  analyzing power i n  t he  r eac t ion  
l ~B($,IT') 1 2 ~ ( g . s . ) ,  with t h e  pion having an energy of 
3.6 MeV i n  t he  center  of mass. The geometry used is 
i l l u s t r a t e d  i n  Fig. 6. It has an e f f i c i ency  response 
funct ion  which is  peaked f o r  pions emitted a t  about 
60' t o  t h e  r i g h t  i n  t h e  cen te r  of mass. The azimuthal 
angle is loca l i zed  t o  wi th in  30°, while t he  e f f i c i ency  
i n  t h e  polar  coordina te  i n  t he  cen te r  of mass is 
nonzero only i n  t h e  forward hemisphere, has i t s  
cent ro id  a t  about 53O, and a width of about 60'. I f  
cm 
one i n t e g r a t e s  t h e  t r u e  asymmetry over cos0 
I T '  
weighting i t  with t h e  appropr ia te  e f f i c i ency  funct ion ,  
one obta ins  an observed average asymmetry over t h a t  
region,  of 
<A(0)> = -0.58 + 0.22 
The r e s u l t s  from Expt. 18 a r e  being prepared f o r  
pybl ica t ion .  
+ 0 A s  a followup t o  t he  f i r s t  successfu l  ( p , ~  ) 
asymmetry measurement reported above, t h i s  group has 
proposed and been a l l oca t ed  time f o r  Expt. 127, 
"Determination of Analyzing Powers f o r  Neutral  Pion 
Production Near Threshold." I n  t h i s  experiment we 
Geometry Used to Measure <A (0)) in the A - 
Famrd -~emisphen for 
"B w,wa) =C (gas.) 
1: = 3.6 MeV 
g, An =I8 msr. Tm!) 
\ 
Aluminum 
Gbss 
H 
I0 cm. 
-f 0 Figure 6 .  Geometry used t o  measure ( p , ~  ) 
asymetry  for scattering angles near 60 . 
plan t o  complete our study of t he  r eac t ion  
-k 
~ B ( ~ , I T ' )  l  2~ (g. s. ) , and perhaps examine a few other  
t a r g e t s .  The b a s i c  approach w i l l  be t he  same a s  t h a t  
used i n  Expt. 18;  however, we plan t o  improve both 
our energy r e so lu t ion  and angular  r e so lu t ion  by moving 
our de t ec to r s  out  of t h e  plane. This w i l l  permit us 
t o  do measurements a t  energies  above the  thresholds  
of exc i ted  s t a t e s ,  t o  about 15 MeV above the  ground 
s t a t e  threshold ,  a s  well  a s  giving us t he  c a p a b i l i t y  
of making measurements on exci ted  s t a t e s  t h a t  a r e  
well  separated from neighboring s t a t e s .  We plan  t o  
have t h i s  experiment completed by t h e  f a l l  of 1980. 
STUDIES OF IT' PRODUCTION NEAR THRESHOLD 
D. Jenkins,  O.P. Gupta, and D. Long 
Virginia Polytechnic Ins t i tu te ,  Blacksburg, Virginia 24061 
A. Bacher, G .  Emery, and M. Pickar 
Indiana University Cyclotron Faci l i ty ,  Bloomington, Indiana 4 7405 
P. Debevec 
University of I l l i n o i s ,  Urbana, I l l i n o i s  61801 
We have begun a s e r i e s  of measurements which w i l l  beams. The energy and angle  of t he  T O  is in fe r r ed  
measure t he  cross  s ec t ion  f o r  ( p , ~ O )  r eac t ions  on a from i t s  decay gammas by a measurement of t h e  angle  
range of nuc l e i  with both unpolarized and polar ized  and energy of each gamma. 
To analyze t h e  (p,nO) experiments, we expand 
the  d i f f e r e n t i a l  c ros s  s ec t ion  i n  t h e  cen te r  of mass 
assuming only s and p waves i n  an unpolarized beam 
do 
- =  dn a+b cos 8 + c cos28 
The angular  d i s t r i b u t i o n  of gammas i n  t h e  l a b  can be 
r e l a t e d  t o  t h e  parameters f o r  t h i s  d i f f e r e n t i a l  c r o s s  
sec t ion .  By measuring t h e  angular  d i s t r i b u t i o n  of 
gammas i n  t h e  l a b  and f i t t i n g  t h e i r  d i s t r i b u t i o n  t o  
obta in  t h e  parameters a ,b ,  and c ,  we ob ta in  t h e  angular  
d i s t r i b u t i o n  of pions i n  t h e  cen te r  of mass. Since 
we a r e  not  recons t ruc t ing  each event  t o  ob ta in  t h e  no 
d i s t r i b u t i o n  i n  t h e  cen te r  of mass, an energy measure- 
ment is  not  required but  is use fu l  i n  d iscr iminat ing  
aga ins t  background events.  
The counter  arrangement is  shown i n  Fig. 1. Pions 
i n  t he  t a r g e t  decay t o  gammas which a r e  de tec ted  i n  
t h e  gamma counters.  The system measures t h e  angles  
and 82 r e l a t i v e  t o  t h e  d i r e c t i o n  of t h e  inc ident  
proton and t h e  opening angle  $2 between t h e  two gammas. 
To measure t h e  angle of t h e  gamma, we use  t he  technique 
described by ~ e u s c h l  and i l l u s t r a t e d  i n  Fig. 2. The 
y COUNTER n 
-------3 FARADAY CUP 
TARGET 
Figure I .  Counter arran ernent for ( p ,  no) reaction. 
Protons incident on the 92C target produce a no which 
subsequently decays i n t o  two gammas. The experiment 
measures the angles e l ,  82 and of  the g m a s .  
y COUNTER ASSEMBLY 
Figure 2. The incident g m a  ray i s  converted i n  the 
counter. The position of the conversion i s  found 
by extrapotating the electron tracks from the wire 
chambers. 
gamma is converted i n  a p iece  of lead  g l a s s  (AE counter)  
and t h e  t r a j e c t o r y  of t h e  converted e l ec t rons  i s  
measured with wire  chambers (Wl, W2, W3, W4 counters ) .  
The t r a c k  is  ext rapola ted  back t o  t h e  conver ter  t o  f i n d  
t h e  i n t e r a c t i o n  poin t .  The gamma angle  can be de t e r -  
mined once an i n t e r a c t i o n  poin t  i s  known r e l a t i v e  t o  
t h e  t a r g e t .  A sample d i s t r i b u t i o n  of opening angles  
is shown i n  Fig. 3 .  
Large lead  g l a s s  counters (E counter)  behind t h e  
wire  chambers measure t h e  remaining energy of t h e  con- 
ver ted  e l ec t rons  which can be added t o  t h e  energy 
recorded i n  t h e  converter  t o  ob ta in  a crude energy 
measurement and complete t h e  i d e n t i f i c a t i o n  of t h e  
gamma. The t o t a l  energy r e so lu t ion  is  about 40% and 
t h e  s o l i d  angle  is 0.38 sr. 
During t h e  pas t  year we had two runs which co l l ec t ed  
da t a  on beryll ium and oxygen a t  d i f f e r e n t  energies ;  
however r a t e s  i n  t h e  wire  chambers l imi t ed  t h e  beam 
i n t e n s i t y  t o  about 20 nA. We have completed our ana lys i s  
of t h e  144.1 MeV ' ~ e  da t a .  The da t a  (226 poin ts )  was 
subjected t o  a maximum l ike l ihood  f i t  which y ie lded  t h e  
fol lowing prel iminary va lues  f o r  t h e  parameters a , b ,  
and c respect ive ly :  0.978, - 0.505 + 0.176, 0.065 + 
0.045. 
During t he se  runs we not iced  t h a t  t h e  s i n g l e s  r a t e s  
i n  t h e  counters  made wide f l u c t u a t i o n s  over  long t ime 
i n t e r v a l s  even though t h e  cyc lo t ron  ope ra to r  maintained 
what was descr ibed  a s  a good tune.  To t e s t  t h e  beam 
tune  and a t tempt  t o  reduce t h e  s i n g l e s  r a t e s  i n  our  
counters ,  we made a s h o r t  run  i n  November 1979 wi th  
only one of t h e  gamma d e t e c t i n g  t e l e s copes .  Two s c i n t i l -  
l a t i o n  counters  were placed on oppos i t e  s i d e s  of t h e  
beam next  t o  t h e  beam p ipe  about  t h r e e  f e e t  upstream 
from t h e  t a r g e t  pos i t i on .  We found t h a t  t h e r e  were 
many s t r a y  p a r t i c l e s  pass ing  through t h e s e  counters  
when t h e  opera tor  had achieved a good tune.  When t h e  
beam tune  requirement included minimizing t h e  r a t e s  
i n  t h e s e  new monitor counters ,  we found t h a t  t h e  s i n g l e s  
r a t e s  i n  our  gamma t e l e scope  had decreased cons iderab ly  
and we were a b l e  t o  ope ra t e  a t  beam c u r r e n t s  over  100 nA. 
We expect  t h a t  f u r t h e r  beam s t u d i e s  should a l low our 
ope ra t i ng  a t  even h igher  i n t e n s i t i e s .  
During t h e  pa s t  year  we a l s o  examined t h e  p o s s i b i l i t y  
of us ing  NaI counters  t o  improve t h e  p r e c i s i o n  of 
our  energy measurement of t h e  gammas. We found t h a t  
Figure 3. Opening angle d is t r i -  
bution for the two coincident 
g m a s  from the g ~ e f p , n O )  OB data 
a t  144.1 MeV. The heavy l ine in- 
dicates the angular range pre- 
dicted by kinematics. 
ANGLE (DEGREES) 
f o r  a g iven  number of events ,  a more p r e c i s e  energy 
measurement can improve t h e  accuracy w i th  which 
t h e  angular  d i s t r i b u t i o n  parameters  a r e  determined 
s i n c e  an  ambiguity i n  determining t h e  no  d i r e c t i o n  
can be  removed. The ambiguity a r i s e s  when recons t ruc-  
t i n g  t h e  no  d i r e c t i o n  from t h e  d i r e c t i o n  of t h e  decay 
gammas because kinematics  a l lows  two d i r e c t i o n s  f o r  
t h e  no.  This  twofold ambiguity reduces t h e  p r ec i s i on  
wi th  which angular  d i s t r i b u t i o n  parameters  can be 
determined, bu t  can be removed by a measurement of t h e  
gamma ene rg i e s .  S ince  t h e  energy r e s o l u t i o n  of t h e  
lead-g lass  counters  i s  no t  s u f f i c i e n t  f o r  t h i s  purpose, 
we considered t h e  use  of NaI counters .  We found t h a t  
t h e  improvement i n  accuracy f o r  t h e  angular  d i s t r i -  
bu t i on  parameters  i s  approximately equiva len t  t o  t h e  
accuracy gained by doubling t h e  number of events  wi th  
t h e  lead-g lass  system. S ince  s i n g l e s  r a t e s  i n  t h e  
NaI counter  may l i m i t  t h e  beam i n t e n s i t y  which could 
be used i n  t h e  experiment, t h e  a c t u a l  running t ime 
could be about t h e  same f o r  a NaI-detector system a s  
compared t o  a lead-g lass  d e t e c t i o n  system which would 
handle higher r a t e s .  We concluded t h a t  a NaI de t ec to r  d a t a  c o l l e c t i n g  r a t e  by al lowing us  t o  run wi th  
system would be much more complicated than t h e  present  higher beam i n t e n s i t i e s .  We then plan t o  t ake  more 
system and most l i k e l y  not  j u s t i f i e d  a t  t h i s  time. da t a  on oxygen t o  improve t h e  s t a t i s t i c a l  accuracy 
We hope t o  t ake  a few more s h i f t s  i n  t h e  spr ing  of our measurement of t h e  angular  d i s t r i b u t i o n  para- 
t o  fu r the r  improve t h e  proton beam focus.  A modest meters. 
time i n  studying t h e  beam could g rea t ly  i nc rease  our 1)  C.A. Heusch, R.V. Kline,  and S.J. Ye l l i n ,  Nucl. I n s t .  
and Meth. 120, 237 (1974). 
MEASUREMENTS OF (p , IT-xn) ANB ( 3 ~ e ,  a-xn) TOTAL CROSS SECTIONS BY ACTIVATION TECHNIQUES 
M. Rickey, P. Singh, and J. Wiggins 
Indiana University CycZotron Facility,  BZoomington, Indiana 47405 
I n  t hese  measurements we hope t o  t ake  advantage of 7 ~ e  [ i .  e .  , 160 (p,  IT-) 17Ne; *0 (p,  IT-2,) 7 ~ e  ; 
t h e  d i s t i n c t i v e  p rope r t i e s  of delayed proton emitter 1 4 ~ ( 3 ~ e , ~ ' )  17Ne; and ~ ' N ( ~ H ~ , I T - ~ )  17Ne]. 
s p e c t r a  a s  a h ighly  s e l e c t i v e  i nd i ca to r  t h a t  t h e  It was hoped i n i t i a l l y  t h a t  counting on one 
r eac t ions  under s tudy have taken place.  t a r g e t  could proceed while t h e  next  was undergoing 
The i n i t i a l  r e a c t i o n  of i n t e r e s t  is bombardment. This proved t o  be no t  pos s ib l e  because 
2 4 ~ g ( p , n - ) 2 5 ~ i  4' 25~!?,* -+ 2 4 ~ g  + p of background probably a r i s i n g  from neutron induced 
with measurements of t h e  6-delayed proton spectrum reac t ions  i n  and near t h e  de t ec to r s .  A mechanical 
and t h e  l i f e t i m e s  of t h e  var ious  peaks being used t o  chopper was then designed and i n s t a l l e d  between t h e  
s e l e c t  events corresponding t o  t h e  decay of 25~i .  I f  pre- in jec tor  and the  i n j e c t o r  cyc lo t ron  t o  provide 
adequate signal- to-noise r a t i o s  and high counting e f -  convenient, c l ean  beam puls ing  on a time s c a l e  
f i c i e n c i e s  can be achieved,  c ross  s ec t ions  (which - > 10 m s ec .  The t a r g e t  wheel, i ts  d r i v e  motor, 
correspond t o  t h e  production of 2 5 ~ i  i n  any of i ts  and the  associa ted  d e t e c t o r s  a r e  p re sen t ly  being 
seven pa r t i c l e - s t ab l e  s t a t e s  below t h e  3.42 MeV proton i n s t a l l e d  f o r  runs i n  t h e  64" s c a t t e r i n g  chamber; and 
separa t ion  energy) w i l l  be measured a s  a funct ion  of t he  chopper motor, t a r g e t  motor, and counting system 
energy. These d a t a  a r e  intended t o  complement r a t h e r  c o n t r o l  pulses  a r e  provided by a 6800 microprocessor. 
than r ep l ace  spectrometer  da t a .  Using t h i s  t a r g e t ,  Two runs have provided d a t a  u se fu l  f o r  d i agnos t i c s  
i t  might a l s o  be poss ib l e  t o  observe t h e  production and evalua t ion  of t h e  p r a c t i c a l i t y  of t h e  technique.  
of 2 4 ~ i ,  produced by the  24#g(p ,r'n) 2 4 ~ i  reac t ion .  
Other t a r g e t s  and r eac t ions  i n  which the  pro- 
duction of 2 5 ~ i  would provide t h e  s igna tu re s  a r e  
2 5 ~ g ( p , ~ - n ) 2 5 ~ i ;  2 6 ~ g ( p , ~ - 2 n ) 2 5 ~ i ;  and 2 3 ~ a ( 3 ~ e , ~ ' n ) 2 5 ~ i .  
Among the  a d d i t i o n a l  p o s s i b i l i t i e s  f o r  use  of t he  
apparatus we a r e  developing a r e  s t u d i e s  of t h i s  family 
of r eac t ions  leading  t o  t h e  production of 
2 1 ~ g [ i . e . ,  1 9 ~ ( 3 ~ e , ~ ' n ) 2 1 ~ g ]  and of 
Figure 1 shows t y p i c a l  d a t a  summed over t h e  t i m e  
channels. The background present  comes from t h e  
comparatively l a r g e  amounts of lMg and 1 7 ~ e  produced, 
together  with t h a t  a r i s i n g  from continuum low energy 
protons and from t h e  energy l o s s  of t h e  more ene rge t i c  
9~ delayed protons passing through the  500 urn S i  de- 
t e c t o r  (which l o s e  from 3 t o  8 ?lev i n  t he  d e t e c t o r ) .  
This background obscures t h e  region  i n  which t h e  


Studies of Pi° Production Near Threshold

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Studies of Pi° Production Near Threshold

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