This research was sponsored by the National Science Foundation Grant NSF PHy 87-1440

Byrd, R.

Fatyga, M.

Karwowski, H.J.

Komisarcik, K.

Kwiatkowski, K.

Nadkarni, D.M.

Sinha, A.

van Heerden, I.

English

IUScholarWorks

DECAY AND FISSION OF EXCITED STATES 
STUDY OF THE STRUCTURE OF COLD, HIGH-SPIN NUCLEI USING NEUTRON-MULTIPLICITY FILTER 
H.J. Karwowski+, A. Sinh.a, R. Byrd, M. Fatyga ,  I. van ~ e e r d e n s ,  K. Komisarcik,  and K. Kwiatkowski 
I n d i a n a  Un ive r s i t y  Cyclo t ron  F a c i l i t y ,  Bloomington, I nd i ana  47405 
D.M. Nadkarni 
Nuclear Phys i c s  D iv i s i on ,  Bhabha Atomic Research Cent re ,  Bombay 400085, I n d i a  
Much of t h e  c u r r e n t  i n t e r e s t  i n  t h e  s t r u c t u r e  of  
n u c l e i  a t  h igh  s p i n  stems from t h e  e x p e c t a t i o n  of 
s i g n i f i c a n t  shape changes w i th  t h e  a d d i t i o n  of angu l a r  
momentum. At t h e  s i m p l e s t  l e v e l ,  t h e  e v o l u t i o n  i n  
shape  can  be p r e d i c t e d  on t he  b a s i s  of  t h e  c l a s s i c a l  
r o t a t i ng - l i qu id -d rop  model (RLDM). The macroscopic 
RLDM p r e d i c t i o n s  f o r  high-spin s t r u c t u r e  a r e  expec ted  
t o  be s i g n i f i c a n t l y  modi f ied  i n  a r e a l  nuc leus  by t h e  
i n c l u s i o n  of  s h e l l  and p a i r i n g  c o r r e c t i o n s  t o  t h e  
p o t e n t i a l  energy s u r f a c e ,  which themselves depend on 
de fo rma t ion  and sp in .  
Nucle i  a t  h igh  s p i n s  a r e  produced i n  f u s i o n  
r e a c t i o n s  induced by heavy i o n s ,  i n  which a  l a r g e  
amount of  e x c i t a t i o n  energy  and angu l a r  momentum a r e  
brought  i n t o  t h e  compound system. The nucleus  t hen  
c o o l s  mainly by t h e  emi s s ion  of Low-L neu t rons  (and,  
w i t h  much s m a l l e r  p r o b a b i l i t y ,  charged p a r t i c l e s ) ,  t hus  
l e a d i n g  t o  t h e  fo rma t ion  of high--spin evapo ra t i on  
r e s i d u e s .  F i s s i o n  decay provide:; an  a l t e r n a t i v e  mode 
of  d e - e x c i t a t i o n  which i s  p a r t i c u l a r l y  impor t an t  f o r  
n u c l e i  wi th  A > 200. From o b s e r v a t i o n s  which a r e  
i n t e g r a t e d  over  t h e  decay c h a i n  one may e x t r a c t  
i n f o r m a t i o n  about  b a r r i e r  he igh t  Bfiss(J) and t he  
s-wave p a r t i c l e  s e p a r a t i o n  energ:Les B,,(J) on ly  by a  
s t a t i s t i c a l  model un fo ld ing  of t h e  compe t i t i on  among 
many open decay channels .  
* 
The compe t i t i on ,  de sc r i bed  t o  t h e  f i r s t  
app rox ima t ion  by t h e  r a t i o  of  f i s s i o n  and neut ron  
emi s s ion  widths  rf/rn, i s  s e n s i t ] - v e  t o  many a s p e c t s  of  
t h e  s t r u c t u r e  of  r o t a t i n g  nucleus  which cannot  be 
ob t a ined  by any o t h e r  method, i n c l u d i n g  y-spectroscopy.  
I n  r e c e n t  y e a r s  we have i n v e s t i g a t e d  t h e  decay of  
compound n u c l e i  formed a t  h igh  e x c i t a t i o n  and angu l a r  
momentum i n  6~i-bombardment of  l ~ a ,  lg4,  l g 8 p t ,  7 ~ u  
and 2 0 8 ~ b  i n  a  cohe reu t  program of  exper iments  and 
s t a t i s t i c a l  model a n a l y s e s ,  which have been c a r r i e d  ou t  
u s ing  t h e  code MBEGAT. The most impor t an t  conc lu s ion  
from t h i s  work1 i s  t h a t  t h e  measurements of  g r o s s  decay 
p r o p e r t i e s  can  be unders tood q u i t e  w e l l  u s ing  
s t a t i s t i c a l  model c a l c u l a t i o n s  i n  which a l l  n u c l e a r  
s t r u c t u r e  parameters  a r e  f i x e d  t o  v a l u e s  c o n s i s t e n t  
w i th  t h e  XLDM and non - in t e r ac t i ng  Fermi g a s  (NIFG) 
models. 
The succe s s  i n  t h i s  e a r l i e r  work of t h e  
s t a t i s t i c a l  model a n a l y s i s  based on RLDM-NIFG models 
i n d i c a t e s  t h a t  t h e  o v e r a l l  decay p r o p e r t i e s  of  hot 
f u s i o n  p roduc t s  a r e  no t  ve ry  s e n s i t i v e  t o  microscoyic  
s t r u c t u r e  c o r r e c t i o n s ,  bu t  a t  t h e  same t ime seems t o  
p rov ide  a  c r e d i b l e  s i g n a t u r e  f o r  s h e l l  and p a i r i n g  
e f f e c t s  i n  more s e l e c t i v e  second-genera t ion  
e x p e r i u e n t s .  I f  one develops  expe r imen t a l  t e chn iques  
f o r  enhancing t h e  decay c o n t r i b u t i o n s  from co ld  n u c l e i ,  
one  would then  expec t  microscopic  s t r u c t u r e  c o r r e c t i o n s  
t o  i n t r o d u c e  s i g n i f i c a n t  d e v i a t i o n s  of  measured t a r g e t  
dependences from those  p r e d i c t e d  by t h e  s t a t i s t i c a l -  
model-RLDM-NIFG c a l c u l a t i o n s ,  which succeeded s o  w e l l  
a t  h igh  t empe ra tu r e s .  
We t h e r e f o r e  proposed a  method t o  i n v e s t i g a t e  t h e  
decay of co ld ,  h igh-sp in  n u c l e i ,  which s t r e s s e s  
measurements w i th  enhanced s e n s i t i v i t y  t o  high-chance 
c o n t r i b u t i o n s  t o  a f i s s  i n  the  decay of compound systems 
formed i n i t i a l l y  a t  high temperature.  The instrument 
which helps  t o  provide t h i s  d i r e c t  experimental  
informat ion on the  f i s s i o n  of cold high-spin nuc le i  i s  
a p r e f i s s i o n  "neutron m u l t i p l i c i t y  f i l t e r . "  This 
device  allows us t o  emphasize f i s s i o n  from 
p rogres s ive ly  lower temperatures by ga t ing  on higher- 
f o l d  neutron coincidences,  j u s t  a s  i n  a y -mul t ip l i c i ty  
experiment one can emphasize cascades from 
p rogres s ive ly  higher  spin  by ga t ing  on higher-fold 
y-even t s . 
The neutron m u l t i p l i c i t y  f i l t e r  makes use of t he  
f a c t  t h a t  neutrons emit ted  a f t e r  f i s s i o n  come almost 
e n t i r e l y  from f u l l y  acce le ra t ed ,  r ap id ly  moving 
fragments and a r e  the re fo re  d i r e c t i o n a l l y  c o r r e l a t e d  
wi th  these  fragments. The p r e f i s s i o n  neutrons ,  on the  
o t h e r  hand, should show no such c o r r e l a t i o n  and a r e  
approximately i s o t r o p i c  i n  the  l a b  system. They w i l l  
have, on the  average, sma l l e r  l a b  ene rg ie s  than post- 
f i s s i o n  neutrons ,  decreas ing sha rp ly  with the  
temperature of t he  emi t t ing  nucleus.  
The experimental  set-up c o n s i s t s  of the  neutron 
m u l t i p l i c i t y  f i l t e r  (NMF) and l a r g e  s o l i d  angle  wire  
chambers f o r  de t ec t ing  f i s s i o n  fragments. The NMF 
i nc ludes  two a r r a y s  of 9 large-volume l i q u i d  
s c i n t i l l a t o r  neutron d e t e c t o r s  placed i n  and out of t he  
r e a c t i o n  plane. Each neutron d e t e c t o r  is a r ec t angu la r  
c e l l  of 265 cm3 volume, f i l l e d  with NE-213 
s c i n t i l l a t o r .  Neutrons a r e  separa ted from abundant 
y-rays with the  help of home-built pulse  shape 
d i sc r imina t ion  boxes, and a d d i t i o n a l  s epa ra t ion  is 
obta ined from the  t ime-of-fl ight measurements. The 
f i s s i o n  fragments a r e  de tec t ed  i n  two x-y p o s i t i o n  
s e n s i t i v e  wire-chambers, each of 15 x 15 cm2 a c t i v e  
a r e a ,  one covering 71.5'-116.5' angular  b i t e  and the  
second one covering 165'-175' region. 
In  the  f i r s t  run l a s t  summer we t e s t e d  a l l  t he  
components of hardware and measured the  neutron 
background i n  the  t a r g e t  room using 100 MeV 6 ~ i  beam. 
Figure  1 shows the  p i c t u r e  of experimental  set-up. 
Only one neutron a r r a y  (9  c e l l s )  was used s i n c e  the  run 
was not meant a s  a production run. We c o l l e c t e d  
neutron m u l t i p l i c i t y - f i s s i o n  fragment coincidences  with 
l g 7 ~ u  and 2 3 2 ~ h  t a r g e t  a t  two p o s i t i o n s  of neutron 
a r r a y  r e l a t i v e  t o  the  forward wire chamber. P o s i t i o n  
one, c a l l e d  the  90' pos i t i on ,  has a r e l a t i v e  angle  
between f i s s i o n  fragments and r e g i s t e r e d  neutrons i n  
the  v i c i n i t y  of 90°, p o s i t i o n  2, c a l l e d  the  0' 
pos i t i on ,  has a r e l a t i v e  angle  between fragment 
r e g i s t e r e d  i n  the  forward wire chamber and de tec t ed  
neutron i n  the  v i c i n i t y  of 0". We a l s o  c o l l e c t e d  
of £- l ine  coincidence da ta  with 2 5 2 ~ f  source i n  both 
p o s i t i o n s  of neutron de tec to r .  l g 7 ~ u  i s  the  f i r s t  
t a r g e t  we in tend t o  i n v e s t i g a t e  i n  the  experiment,  
a t tempt ing t o  s o r t  out  pos t - f i s s ion  neutrons  emit ted  
from f u l l y  acce le ra t ed  f i s s i o n  fragments using the  
p r i n c i p l e  of kinematic focusing of pos t - f i s s ion  
neutrons.  Data with 2 3 2 ~ h  t a r g e t  provide us with 
numerical e s t ima te  of t he  kinematic focusing e f f e c t ,  
s i n c e  the re  a r e  v i r t u a l l y  no pre-f iss ion neutrons  
emit ted  i n  the  r e a c t i o n  of 6 ~ i  with thorium. Off- l ine  
coincidence d a t a  with 2 5 2 ~ f  source a l low us t o  
c a l i b r a t e  our equipment, due t o  the  f a c t  t h a t  we a r e  
a b l e  t o  reproduce angular  d i s t r i b u t i o n s  of neutrons  
wi th  r e spec t  t o  the  f i s s i o n  fragment ( f  f )  d i r e c t i o n  
what can be compared with e x i s t i n g  data2 on f f -neutron 
angular  c o r r e l a t i o n s  i n  5 2 ~ f  decay. 
Data a n a l y s i s  is  s t i l l  i n  the  very pre l iminary  
s t age .  Figure  2 shows angular  c o r r e l a t i o n s  between 
f i s s i o n  fragments and neutrons i n  2 5 2 ~ f  decay deduced 
from our o f f - l i ne  data .  Crosses present  our 
measurements from a l l  t he  neutron c e l l s ,  whi le  c i r c l e s  
F i g u r e  1. P i c t u r e  of  expe r imen t a l  set-up.  S i n g l e  a r row p o i n t s  t o  neu t ron  a r r a y  i n  t h e  0' p o s i t i o n ,  doub l e  a r rows 
p o i n t  t o  w i r e  chambers. 
show t h e  d a t a  of  Bowman e t  a1.2 The improvement i n  
e f f i c i e n c y  c o r r e c t i o n s  t o  t h e  neu t ron  s p e c t r a  should  
f u r t h e r  improve t h e  agreement bet:ween both  s e t s  of  
d a t a .  
S e p a r a t i o n  of neu t rons  and v-rays emi t t ed  i n  t h e  
r e a c t i o n  is  a b s o l u t e l y  c r u c i a l  f o r  t h e  succe s s  of  t h e  
exper iment .  Hence, a  l o t  of  expe r imen t a l  e f f o r t  was 
p u t  i n t o  a ch i ev ing  t h e  b e s t  sepa1:ation pos s ib l e .  We 
used  NE213 l i q u i d  s c i n t i l l a t o r  i11 our  neu t ron  c e l l s ,  
w i t h  Pu l s e  Shape D i sc r im ina t i on  (PSD) boxes b u i l t  a t  
IUCF s p e c i f i c a l l y  f o r  t h i s  exper iment .  F i g u r e  3 
p r e s e n t s  a t y p i c a l  o f f - l i n e  PSD v s  Pu l s e  Height  
spec t rum taken  wi th  t h e  r a d i o a c t i v e  source .  The 
o f f - l i n e  s e p a r a t i o n  between neu t rons  and y ' s  a t  low 
p u l s e  h e i g h t s  is n o t  ve ry  good, bu t  i n  t h e  a c t u a l  run  
i t  is g r e a t l y  enhanced by t h e  t ime of  f l i g h t  (TOF) 
i n fo rma t ion .  Good background c o n d i t i o n s  i n  t h e  t a r g e t  
room a l l o w  u s  t o  o b t a i n  q u i t e  c l e a n  TOF s p e c t r a  (F ig .  
4) .  We a r e  a l s o  a b l e  t o  o b t a i n  v a l u a b l e  t ime-of - f l igh t  
i n fo rma t ion  i n  t h e  o f f - l i n e  2 5 2 ~ f  d a t a  by measuring 
9 6 
difference between TOF of neutrons or y's and TOF of 
fission fragments. Figure 4 shows on-line TOF with 
232~h target and off-line TOF with 252~f. 
FF- N ANGLE [DEG.] 
I U C F -  1665 
Figure 2. Angular correlations of neutrons with 
respect to fission fragment direction. Full points 
show data of Bowman et a1.,2 while crosses show our 
data. Our data were normalized to the data of Ref. 2 
at one point for one cell only. 
Figure 3. The spectrum of pulse shape discrimination 
signal vs. pulse height. 
Finally, we compared rates of neutron-fission 
fragments coincidences at O0 and 90° positions of 
neutron array. We calculated the ratio of two-fold 
coincidences at O0 and 90°, obtaining 3.7 as a result. 
This indicates relatively strong influence of kinematic 
focusing on observed yields. 
Data analyzed so far indicate that the experiment 
is feasible in the present experimental arrangement. 
More data analysis is necessary, before the experiment 
can be continued. 
I T i m e  ~f f l i g h t  ( ~ s .  R F )  
(gamma and neutron T O F )  - ( FF T O E )  
Figure 4. Typical time-of-flight spectra 
a) On line spectrum with gold target 
b) Off line spectrum taken with 252~f source. The 
difference beiween time-of-flight of detected y or 
neutron and time-of-flight of the coincident fission 
fragment . 
+current address: Physics Department, University of 
North Carolina, Chapel Hill, NC 27514 
$current address: University of the Western Cape, 
Bellville, South Africa 
1) S.E.  Vigdor et al., Phys. Rev. C 26, 1035 (1982). 
2) H.R. Bowmann et al., Phys. Rev. 126 G, 2120 (1962). 


Study of the Structure of Cold, High-spin Nuclei Using Neutron-Multiplicity Filter

https://scholarworks.iu.edu/dspace/bitstream/2022/1109/1/iucf_1984_94-97.pdf

Study of the Structure of Cold, High-spin Nuclei Using Neutron-Multiplicity Filter

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