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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DECAY OF HIGHLY EXCITED NUCLEI 
EMISSION OF INTERMEDIATE MASS FRAGMENTS NORMAL TO 
THE FISSION AXIS IN HOT HEAVY NUCLEI 
D.E. Fields, J.L. Wile, K. Kwiatkowski, K.B. Morley, E. Renshaw, 
S .J. Yennello, and V.E. Viola 
Indiana Universi ty  Cyc  lotron Facility, Bloomington, Indiana 4 74 08 
R. G. Korteling 
S i m o n  Fraser University, Burnab y, BC Canada 
In a recent experiment designed to study the source properties of complex fragments 
emitted in the 270-MeV 3He + 232Th reaction, an unusual component was observed in 
the fragment spectra at backward angles. The experiment was carried out at the Indiana 
University Cyclotron Facility using a beam of 270-MeV 3He ions to bombard a 232Th target. 
The experimental apparatus consisted of two major components: (1) two fission fragment 
detector arrays placed asymmetrically on opposite sides of the beam, and (2) several low- 
threshold, large dynamic range detector telescopes placed 25-cm from the target at angles 
between 20" and 160" for detection of coincident Z = 1-10 ejectiles. Here we will focus 
only on the backward hemisphere detectors, placed at 100°, -130" and f 160". Each fission 
fragment array consisted of four, 28-strip passivated silicon detectors' of area 4 x 6 cm2 
placed 30 cm from the target. The strip detectors provided fragment energy, position, and 
timing information with an angular coverage of 46.7" for each array and A0 = 0.41°/strip. 
The centroid of one array was at 64.0" and that of the second was at -96.0". 
The intermediate mass fragment (IMF) detector telescopes consisted of a gas- 
ionization chamber operated at 32 Torr of CF4, a 500-pm passivated silicon detector of 
area 5 cm x 5 cm and a CsI (Tl) scintillator of thickness 2 cm. These devices provide 
charge resolution for Z = 1-15 fragments, with low-energy thresholds of approximately 
0.5 MeV/nucleon. In addition, the timing signal from the silicon detector and that from 
the cyclotron RF provided time-of-flight mass identification with a timing resolution of 
about 300 ps. Both Z and A identification were obtained for Z 5 4 fragments; for Z 2 5 
only charge identification was possible, in part due to low statistics. 
In Fig. 1 we show spectra for ''Be fragments observed at +160°, -160°, +100°, 
and -130" in coincidence with binary fission events; i.e., triple coincidences. The solid 
angle acceptance of both f 160" IMF detectors is identical. The +160° detector is oriented 
approximately midway between the two detector arrays, i.e., normal to the fission fragment 
separation axis (with corrections for the kinematic folding angle). Correspondingly, the 
Figure 1. Spectra of ''Be fragments at angles of f 160°, 100°, and -130°, in coincidence 
with angle-correlat ed fission fragments. 
- 160" detector is at an angle aligned 40" closer to the fission axis. It is observed that the 
+160° spectrum of ''Be fragments contains a pronounced low-energy component. Similar 
events are essentially absent from the same spectrum in the -160" detector, as well as for 
detectors at -130" and +lOOO. These latter three spectra are all nearly identical in both 
counts and the location of peaks in the spectra, which occur near 35-40 MeV as expected 
for Coulomb repulsion of ''Be fragments from a 232Th-like source. 
Because this effect is observed in only one detector, it would be desirable to make it 
appear in the -160" detector. Unfortunately, there is no combination of fission fragment 
detectors that allow us to examine the -160" detector in an orientation normal to the 
fission-fragment axis. However, there does exist a narrow angular range in the fission- 
fragment array (25 strips) that is symmetric about the beam axis. In this case both f 160" 
detectors are equally distant (20") from the normal to the fission axis; hence the spectra 
should be identical. In Fig. 2 the symmetric configuration is compared for all Z = 3-6 
fragments with the results of the full fission array test. Since, for lo Be alone, the statistical 
significance is low, the spectra here are summed over the range of Z = 3-6. The difference 
spectrum shows a net of 5 counts, whereas for the full array a net of 518 counts is found 
- 
- J 
I - 
- 
- 
- @+10 - 
- ,@++ 
Net = 518 - 
- 
- 
- cp 
- 
- 
- 
- 01 a + 
- 
- 
I g+***  
- 
w + 
Em 
I I I I  1 1 1 1  I I I I  1 1 1 1  
- 
Net = 5 
Figure 2. Top frame: Left-Sum spectrum of Z = 3-6 fragments in coincidence with angle- 
correlated fission fragments from full array; open squares are +160° data, closed triangles 
are - 160' data. Right-Difference between +160° spectrum and - 160' spectrum. Bottom 
frame: Left-Sum spectra as above for symmetric subset of fission array. Symbols and right 
frame are same as above. 
in the difference spectrum. Thus, while the effect cannot be made to appear in the -160" 
detector due to detector geometry, it can be made to disappear in the +160° detector by 
selecting fission fragment angles well-removed from the normal. 
In order to examine this angular focussing in more detail we have gated on the +160° 
detector and examined the angular correlation relative to the fission array. In Fig. 3 this 
correlation is shown, where the angle f i M F - ~ ~  is the angle between the IMF detector and 
the angle in the left-hand fission array. The normal to the fission axis for the average fission 
event is approximately 98' for the fragments, as measured previously by Fatyga et  a1.l The 
high-energy component is found to be nearly isotropic across the fission array, whereas the 
low-energy component is sharply peaked near 97O, with a full-width-at-half-maximum of 
about 12'. The deflection of the IMF is seen to be greater for fission fragments with 
A > ACn/2 than for those with A < ACn/2. This angulax correlation behavior resembles 
the strong focussing of light-charged particles normal to the separation axis in low-energy 
fission-a phenomenon commonly associated with emission from the neck region during the 
fission process.2j3 In both cases the most probable kinetic energy for the focussed fragments 
is significantly lower than the isotropic component of the fragment spectra. Integrating 
t = High Energy = High Energy  I t 0 = Low Energy  1 0 = Low Energy  1 
@ IMF - FF light (deg) 
F i g u r e  3. Fission-IMF correlation angles for high energy IMFs (solid squares) and low en- 
ergy fragments (open triangles). Left frame is gated on light fission fragments (A < Acn 12) 
and right frame is for heavy fragments (A > Acn/2). 
over all angles, the low-energy component appears to be about 20% of the ' O B ~  yield 
originating from equilibrium-like processes. 
The linear momentum transfer measured for both high- and low-energy ''Be events 
is about pII/pleam M 0.80. Thus, all events reported here are consistent with large 
momentum transfers, corresponding to excitation energies in excess of 150 MeV. 
Finally, we observe this same low-energy component in all IMF spectra at +160° (and 
none at -160°), although the statistics are poor for Z 2 8. The charge distributions for the 
high and low energy components have been fit with a power law of the form a ( z )  oc z-". 
The charge distribution for the high energy component is in good agreement with previous 
~ tud i e s ,~  i.e. T Z 4 for IMFs emitted at back angles in these light-ion bombardments. In 
the low-energy component, a value of T = 2.8 is found, indicating a much flatter charge 
distribution for this process. 
1. M. Fatyga e t  al., Phys. Lett. B 185, 321 (1987). 
2. Z. Frankel, Phys. Rev. 156, 1283 (1967). 
3. C. Wagemans, The Nuclear Fission Process (CRC Press, Boca Raton, FL) p. 5458 
(1991). 
4. K. Kwiatkowski e t  al., Phys. Lett. B 171, 41 (1986). 


Emission of Intermediate Mass Fragments Normal to the Fission Axis in Hot Heavy Nuclei

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Emission of Intermediate Mass Fragments Normal to the Fission Axis in Hot Heavy Nuclei

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