This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Fields, D.E.

Korteling, R.G.

Kwiatkowski, K.

Morley, K.B.

Renshaw, E.

Viola, V.E.

Wile, J.L.

Yennello, S.J.

English

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COMPLEX FRAGMENT EMISSION 
IN COINCIDENCE WITH ANGLE-CORRELATED FISSION FRAGMENTS 
IN THE 270 MeV 3He + 232Th SYSTEM 
D.E. Fields, K. Kwiatkowski, K.B. Morley, E. Renshaw, 
J.L. Wilea, S.J. ~ennello" and V.E. Viola 
Indiana University Cyclotron Facility, Bloomington, Indiana 474 08 
R.G. Korteling 
Simon Fraser University, Burnaby, BC, V5A IS6 CANADA 
A description of the sources of complex fragments formed in nuclear reactions well 
above the Fermi energy is critical for understanding the decay of hot nuclear matter. 
Earlier with 3He and I4N projectiles from E/A = 20 MeV/A to 100 
MeV/A on heavy target nuclei have shown that the sources of intermediate mass frag- 
ment (IMF = 3 5 Z 5 15 ) production can be classified into three distinct categories: 
(1) a projectile-like source whose angular distribution peaks strongly at forward angles 
and whose energy spectra peak near the beam velocity; (2) emission from an equilibrated 
compound-nucleus-like source resulting in an isotropic angular distribution and steeply 
sloping Maxwellian energy spectra; and (3) non-equilibrated fast emission from a target- 
like source with a forward peaked angular distribution and Maxwellian energy spectra that 
have flatter slopes than for equilibrated emission. In order to classify IMFs according to 
their production mechanism, exclusive experiments have been conducted both at IUCF 
and el~ewhere.~ These experiments have confirmed the existence of two target-like mecha- 
nisms and indicate that the non-equilibrium mechanism becomes more important at higher 
bombarding energies. Missing from these experiments, however, was a complete picture of 
the reaction dynamics including excitation energy in the emitting system. 
In order to fill this gap in the experimental data, an experiment has been performed 
at the Indiana University Cyclotron Facility in which detected IMFs have been tagged 
with linear momentum and excitation energy transfer to the target. Beams of 270 MeV 
3He were used to bombard a self-supporting 678 g/cm2 232Th target in the IUCF 162- 
cm scattering chamber. IMF detectors consisting of gas ion chambers followed by silicon 
detectors and CsI (or BgO) crystals were placed at eight angles around the target (see 
Fig. 1) and delivered excellent charge resolution up to Z = 15 and good mass resolution for 
isotopes up to mass 12. In the same plane with the IMF detectors were two arrays, each 
consisting of four silicon strip detectors placed on either side of the beam. The two arrays 
were used to detect energy, position, and time-of-flight of coincident fission fragments. 
This information can give us the direction and velocity of the recoiling heavy residue in 
coincidence with an emitted IMF. Also, since the total missing mass can be related to 
the number of emitted neutrons, the excitation energy deposited in the system can be 
determined given a reasonable mass resolution of the fission fragment detection system. 
Figure 2 shows both the geometry of the fission fragment detector and a diagram of the 
resistive network for position detection and subsequent electronics. Signals proportional to 
(L-X)/L are extracted from the front surface of the detector through the resistive chain. 
The entire back of the detector is covered with an aluminized layer, and the energy and 
Fzgure 1. Setup for experiment E304. 
2.0 rnrn 0.1 rnrn 
T Pick-off PreArnp Energy 
'- -=- 
Figure 2. Geometry and electronics diagram of the fission fragment detector. 
time signals are taken from there. Figure 3 shows strip position as a function of fragment 
energy. The top portion of the figure shows the curvature of the position lines due to 
offsets in the electronics whereas in the bottom portion the function 
P - E  
Position = -E 
has been replaced by 
PeE-Po 
Position = E - Eo 
thereby straightening the position lines and making analysis easier. 
A check on the accuracy of the angular calibrations of the fission fragment detectors 
was performed by using data from a thin 252Cf fission source taken immediately after the 
production run. The folding angle between coincident fission fragments from the source 
NUMBER OF COUNTS VS. FOLDING ANGLE 
L ' ' ' ' ~ ' ' ' ~ ' ' ' ' ' ' ' ' ' ' ' i ' I  
Figure 3. Top: Position = (P . 
E)/E, Bottom: Position = ((P 
E) - Po)/(E-Eo). 
NUMBER OF COUNTS VS.  FOLDING ANGLE 
1 1 ' ' ' 1 ' 1 ' ' 1 " '  1 '  
1500 - 
lo00 - 
500 - 
0 
120 140 
Figure 4.  Folding angle distribu- 
tions for (top) 252Cf source and 
(bottom) fission fragments from 
the 270 MeV 3He + 232Th reaction 
in coincidence with IMFs at 120". 
should be 180' with a width dominated by neutron emission of approximately 4'. This 
was checked and found to be correct to within 0.05' as seen in the top portion of Fig. 4. In 
addition, the method of momentum transfer determination was checked by gating fission 
fragment folding angles from the 232Th target on coincident IMFs at a particular angle and 
noting the corresponding shift in the centroid of the folding angle distribution. The lower 
portion of Fig. 4 shows a folding angle distribution for fission fragments in coincidence 
with IMFs at 120'. The data has a centroid shifted and spread in agreement with Monte 
Carlo simulations of the kinematics involved. 
In addition to position and folding angle calibrations for the fission fragments, all 
charge and mass gating for all IMF telescopes were performed and compressed data was 
written to tape in the first pass through the data. As a result, singles cross-sections for 
all isotopes were derived; preliminary inclusive IMF angular distributions are presented in 
Fig. 5. 
a Present address: Indiana Academy, Ball Sate University, Muncie, IN 
Present address: Michigan State University, East Lansing, MI 
1. K. Kwiatkowski, e t  al., Phys. Lett. B171, 41 (1986). 
2. M. Fatyga, e t  al., Phys. Lett. B185, 321 (1987). 
3. D.E. Fields, e t  al., Phys. Lett. B220, 356 (1989). 
4. M. Fatyga, Phys. Rev. Lett. 58, 2527 (1987). 
Figure  5. Preliminary inclusive angular distribution for Z = 4-7 for the 270 MeV 3He + 
232Th reaction. 


Complex Fragment Emission in Coincidence with Angle-Correlated Fission Fragments in the 270 MeV 3-He + 232-Th System

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Complex Fragment Emission in Coincidence with Angle-Correlated Fission Fragments in the 270 MeV 3-He + 232-Th System

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