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

Cassagnou, Y.

Dayras, R.

Kwiatkowski, K.

Legrain, R.

Norbeck, E.

Pollacco, E.C.

Viola, V.E.

Volant, C.

Wile, J.L.

Yennello, S.J.

Yoder, N.R.

English

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MULTIFRAGMENT EMISSION IN THE 3He + natHg REACTION 
AT 0.90 AND 3.6 GeV* 
S. J. Yennelloa, K. Kwiatkowski, N. R. Yoder, J. L. wileb and V. E. Viola 
Ind iana  Univers i ty  Cyclotron Facility, B loomington ,  Ind iana  47408 
E. C. Pollacco, C. Volant, R. Dayras, R. Legrain and Y. Cassagnou 
D P h N / S E P N  C E N  Saclay, France 
E. Norbeck 
Univers i ty  of Iowa Ci ty ,  IA 
We report experimental data from multifold coincidence studies of IMFs emitted in 
the bombardment of natAg nuclei with 0.90 and 3.6 GeV 3 ~ e  ions at the Laboratoire 
Nationale Saturne. High purity, self-supporting natAg targets of thickness 900 ,ug/cm2 
and area 20 cm2 were bombarded with 3He ions of energies 0.90 and 3.6 GeV. An array of 
36 particle-identification telescopes that covered 8% of the total solid angle was employed 
in these studies. 
At both energies the evidence for multiple fragment events is clear. Even with the 
relatively low solid-angle coverage, events with multiplicity  MY&^ up to four are observed. 
In addition there is a significant increase in probability for multifragment emission at the 
higher energy. For example, the cross section for M ? & ~  = 3 events is over 40 times larger 
at 3.6 GeV than at 0.90 MeV. 
In Fig. 1 we show the summed IMF charge distributions and the summed IMF kinetic 
energies for  MY&^ 2 2 events. Light-charged particles are not included in these sums. At 
3.6 GeV bombarding energy events are detected which account for up to 75% of the total 
available charge-even with only 8% of total solid-angle coverage. Fig. 1 (a) demonstrates 
that the observed events contain significant contributions from higher-Z fragments and are 
not exclusively due to light elements such as Li and Be. In addition, the total observed 
kinetic energy per event extends up to 400 MeV; light-charged particles are excluded from 
these sums. The solid line in Fig. l(b) is the result of a calculation based upon sequential 
statistical emission of IMFs from an equilibrated target-like source with normal nuclear 
matter density, initially at a temperature of 8 MeV, and corrected for detector accep- 
tance. The simulation assumed as input experimental values from the fragment charge 
distributions1 and reconstructed multiplicity results. In order to assess the upper limit for 
fragment kinetic energies, cooling by H and He emission was blocked in this calculation. 
Even for this extreme assumption, the calculated tot a1 kinetic energies fail to approach the 
experimental distribution, suggesting that the fragment acceleration mechanism is more 
complex than simple Coulomb repulsion from a thermal source. 
The most striking result is found in the systematic evolution of the fragment kinetic 
energy distributions with increasing multiplicity and bombarding energy. In Fig. 2 the 
kinetic energy spectra for carbon fragments measured at each of the three angles are shown 
as a function of observed multiplicity,  MY&^, for both bombarding energies. Similar results 
are observed for all IMF Z values. Each frame presents spectra gated on ~ y $ ~  = 1, 2 
and 3, respectively, from the entire detector array. When the spectra are examined as 
0 10 20 30 40 0 100 200 300 400 
Z t o t a ~  Total EMF (MeV) 
Figure 1. Top: Distributions of summed IMF charge for IMFs with M ; & ~  > 2 for the 3.6 
(top) and 0.90 (bottom) GeV 3 ~ e  + natAg reactions. Bottom: Distributions of summed 
detected kinetic energy per event for IMFs for M ? & ~  > 2 for 3.6 (top) and 0.90 (bottom) 
GeV. The solid line is the result of a simulation based on sequential statistical emission, 
normalized to the peak of the 3.6 GeV data. 
a function of MY&*, one observes marked changes in which the spectral shapes become 
increasingly flat with increasing multiplicity. This is most apparent in the backward-angle 
data. A simple Maxwellian fit to the O = 117O spectra at 3.6 GeV gives slope parameters 
of T 2 13, 15, and 19 MeV for MY&* = 1, 2, and 3, respectively. 
The fragment angular distributions become increasingly isotropic with increasing mul- 
tiplicity, which can be deduced from examination of Fig. 2. The effect is most noticeable 
for the 900 MeV data, where the spectra for high M ; & ~  events are found to be much flat- 
ter and exhibit Coulomb peaks which extend to much lower energies than for the singles 
observations. This is consistent with an interpretation that at an energy of 0.90 MeV, the 
IMF yield is dominated by binary nonequilibrium emission or st atistical decay. At 3.6 GeV 
the probability for multifragment emission is greatly enhanced, thus masking the spectra 
of simpler binary-like processes and producing a more isotropic emission pattern. 
900 MeV 3600 MeV 
Figure 2. Energy spectra of carbon fragments for 900 MeV (left column) and 3.6 GeV 
(right column) observed at 35" (top row), 63" (middle row) and 117" (bottom row). In 
each frame top spectrum is for M;EF = 1, middle is for = 2 and bottom is for 
M;kF = 3. Errors are statistical, based on total counts. 
Finally, comparison of the spectra at the two bombarding energies shows that at 
3.6 GeV the Coulomb peaks are systematically broadened toward lower fragment energies 
and the spectral tails are much harder. This behavior suggests significant modifications 
of the Coulomb field of the emitting system, consistent with the predictions of the ex- 
panding, evaporating source model of Friedman2 which can also successfully reproduce the 
multiplicity data. 
* Supported by DPhN/SEPN CEN Saclay, the U.S. Department of Energy and National 
Science Foundation. Experiment performed at the Laboratoire National Saturne. 
a Present address: NSCL, Michigan State University, East Lansing, MI 48824. 
Present address: Indiana Academy, Ball State University, Muncie, IN 47306. 
1. S. J. Yennello, e t  al., Phys. Lett. 246, 26 (1990). 
2. W. A. Friedman, Phys. Rev. C42, 667 (1990). 


Multifragment Emission in the 3-He + nat-Hg Reaction at 0.90 and 3.6 GeV

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Multifragment Emission in the 3-He + nat-Hg Reaction at 0.90 and 3.6 GeV

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