LIMITS TO COMPLETE FUSION AND THE EVOLUTION OF NUCLEAR REACTION MECHANISMS IN THE E/A = 10-100 MEV ENERGY REGIME

With the advent of new intermediate-energy accelerators which produce beams of complex projectiles with energies in the range of E/A = 20-100 MeV, a new area of research in heavy-ion physics has opened. This energy region is viewed as one in which nucleon-nucleon scattering becomes an increasingly important factor in the dynamics of a nuclear reaction. Two contradictory effects must be considered as the energy increases across this energy regime: the decreasing nucleon-nucleon cross section, and the rapid opening of phase space due to suppression of Pauli blocking at incident energies per nucleon well above Fermi energy. If the decrease in nucleon-nucleon cross section is a dominant factor in the reaction, one expects that the ability of the nuclear target to capture and equilibrate the full beam energy in central collisions will decrease. If, on the other hand, the suppression of Pauli blocking dominates the reaction, one expects a high probability for rapid energy equilibration in nuclear collisions with small impact parameters. Multifragmentation of the projectile-target composite system is expected if energy equilibration occurs with high probability in intermediate incident energy central collisions.;Three major experimental efforts are reported in this work: (1) Inclusive measurements of linear momentum transfer in the ('14)N + ('238)U and p + ('238)U reactions were done, using the technique of angular correlations between fission fragments. (2) Coincidences between light charged particles and binary fission fragments in the 150 MeV ('6)Li + ('238)U and 60,135,200,270 MeV ('3)He + ('238)U reactions were measured to investigate major mechanisms of energy loss in the initial non-equilibrium phase of the reaction. (3) Coincidences between intermediate mass fragments (3 (LESSTHEQ) Z (LESSTHEQ) 7) and fission fragments were measured in the 270 MeV ('3)He + ('232)Th reaction. The mechanism of target fragmentation and its role in non-equilibrium energy loss in the reaction was investigated with this technique.;It was found that for the systems and energies investigated in this work, multifragmentation of the composite system does not deminate the reaction mechanism. The decrease in the probability for complete fusion, which is observed at incident energies of 30-50 MeV per nucleon, seems to be caused by the increase in the probability of non-equilibrium light particle emission in central collisions.;A description of the dominant reaction mechanisms in intermediate energy central collisions has been the main goal of this research dissertation.