STUDIES OF REACTION MECHANISMS FOR SYSTEMS FORMING A = 56 COMPOUND NUCLEI AT E(LAB)/A = 6-9 MEV (HEAVY-ION FUSION)

Two experiments have been performed, one to study incomplete fusion processes and the other damped binary reactions, in the energy region at which these reaction mechanisms begin to compete significantly with complete fusion in light heavy-ion systems. The first experiment involved the measurement of velocity distributions of evaporation residues from reactions of ('32)S + ('12)C, ('24)Mg, ('27)Al, ('28)Si, and ('40)Ca, as well as evaporation residue cross sections for the ('32)S + ('24)Mg system, at bombarding energies of 194, 239, and 278 MeV. A comparison of the velocity distributions with the predictions of Monte Carlo calculations revealed conclusive evidence of contributions from incomplete fusion, in the form of shifts in the velocity centroids, only for the ('32)S + ('12)C system. A slight shift in the centroid and some broadening of the distribution was also observed for the ('32)S + ('24)Mg residues at the highest energies. The ('32)S + ('12)C velocity shifts are examined within the framework of a model which relates the mass emitted prior to fusion to the degree of overlap of the compound nucleus' Fermi velocity distribution with those of the projectile and target nuclei. A statistical yrast limitation and competition with the fission decay channel are presented as two possible mechanisms of the observed limitation of the ('32)S + ('24)Mg fusion-evaporation cross section at higher energies.;In the second experiment the two massive products of damped binary reactions of 250 MeV ('28)Si + ('28)Si were detected in coincidence, one in a time-of-flight arm and the other in a large-solid-angle ionization counter. Two components were observed in the measured distributions of exit channel mass asymmetry and total kinetic energy and in the center-of-mass angular distributions. One component, which appears to correspond to a broad distribution of mass asymmetries and a 1/sin (theta) angular distribution, exhibits exit channel kinetic energies which are consistent with complete damping of the entrance channel kinetic energy into internal excitation. The second component, which involves less energy damping, seems to correspond to mass-symmetric exit channels and forward-peaked angular distributions. The production cross section for these components, plus the fusion-evaporation cross section, exhausts the reaction cross section.