Light Charged Particles and Intermediate Mass Fragments from the Reactions 486, 550, 640, and 730 Mev KRYPTON86 + COPPER63
Abstract
A detailed study has been made of the reaction ^{86}Kr + ^ {63}Cu at incident energies of 486, 550, 640, and 730 MeV. Measurements include cross sections, angular distributions, and energy spectra for light charged particles (^{1,2,3}H and ^4He), intermediate mass fragments (IMF) (4 <= Z <= 17), and heavy fragments (Z >= 18). Coincidences between light charged particles and between particles and fragments have also been measured to obtain exclusive cross sections, energy spectra, and angular distributions. Statistical model analysis of the energy spectra for ^1 H and ^4He detected in coincidence with the fragments has allowed estimation of ^1 H and ^4He multiplicities associated with the evaporation residues, fragments, and composite nuclei prior to scission. In particular, the light charged particle multiplicities for the IMF's have allowed for the derivation of their primary masses. This in turn has permitted refined measurements of the kinetic energies of the primary IMF's. The ^{86}Kr bombarding energies were selected so that the excitation energies of the composite nuclei (^{149} Tb*) were matched to those of other entrance channel reactions that produce the same composite nuclei. A close comparison of cross sections, energy spectra, angular distributions, and particle multiplicities for these matched entrance channels has provided the means for a detailed test of the Bohr Independence Hypothesis. Results of this comparison indicate extensive shape and thermal equilibration of the composite nuclei over the excitation energy range of 128 to 231 MeV. This conclusion is reached even for nuclear systems whose decay lifetimes are expected to be similar to their relaxation times. For the 640 MeV ^{86} Kr + ^{63}Cu reaction, cross sections were measured for IMF's of 4 <= Z <= 17 in singles and in coincidence with heavy fragments. Three sources for IMF production have been identified: (1) asymmetric binary fission, (2) sequential binary fission, and (3) simultaneous three (or more) body breakup. Asymmetric binary fission is found to comprise most of the IMF cross section. The IMF angular distributions follow a 1/sin theta distribution in the center of mass, and their average c.m. energies are nearly independent of angle and excitation energy. Barriers to asymmetric fission and average emitter spins have been extracted from a statistical model analysis of the IMF excitation functions. The extracted barriers are more than 30% larger than those calculated by the Yukawa plus exponential finite range nuclear (YEFRN) model. However, the average total kinetic energies (TKE) are intermediate between values calculated by the YEFRN model for the upper (or saddle point) limit and the lower (or scission point) limit. This suggests that this model may overestimate nuclear stabilization due to the range of the nuclear force. On the other hand, one might infer that a dynamical model may be required for these fissionlike reactions in place of the statistical model.
 Publication:

Ph.D. Thesis
 Pub Date:
 January 1992
 Bibcode:
 1992PhDT.......147B
 Keywords:

 KRYPTON86;
 COPPER63;
 Chemistry: Nuclear; Physics: Nuclear; Physics: Atomic