The production of intermediate mass fragments ͑IMF's͒ from the four reactions 55A MeV 124,136 Xe ϩ 112,124 Sn is studied with an experimental apparatus which is highly efficient for the detection of both charged particles and neutrons. The IMF's are more localized in the midvelocity region than are the light charged particles, and the detected multiplicity of IMF's depends linearly on the charge lost from the projectile and increases with the neutron excess of the system. Remnants of the projectile, with very little velocity reduction, are found for most of the reaction cross section. Isotopic and isobaric fragment yields in the projectile-velocity region indicate that charge-to-mass ratio neutralization is generally not achieved but is approached when little remains of the projectile. For all systems, the fragments found in the midvelocity region are substantially more neutron rich than those found in the velocity region dominated by the emission from the projectile. This observation can be accounted for if the midvelocity source ͑or sources͒ is either more neutron rich or smaller, with the same neutron-to-proton ratio, than the source with the velocity of the projectile. Taken together, the observations of this work suggest that the intermediate mass fragments are, to a large extent, formed by multiple neck rupture of the overlap material, a process which might enhance the neutron-to-proton ratio of the primary source material and/or limit the size of the sources. This scenario is reminiscent of low-energy ternary fission and one predicted by Boltzmann-Uehling-Uhlenbeck calculations. However, these calculations predict too much velocity damping of the projectile remnant. The calculations improve, in this regard, when the in-medium nucleon-nucleon cross sections and the cost of creating low density material are reduced.

The occurrence of orbiting and fusion-fission processes observed experimentally in some light and medium-light heavy-ion collisions at incident energies well above the Coulomb barrier is discussed in the framework of a number of available open channels calculation. The fusion-fission mechanism appears to be less competitive in systems for which the available phase space for the highest partial waves is restricted to a few exit channels where dinuclear configurations can survive through orbiting trajectories. The coexistence of quasimolecular resonances, orbiting mechanisms, and the fusionfission process for medium-light dinuclei is also brie6y discussed.

Based on measured correlations between experimental observables in the 209 Bi 1 136 Xe reaction at E͞A 28 MeV, it is shown that multiple intermediate-mass fragment (IMF) production is a dynamical process driven by the energy of relative motion of projectilelike and targetlike fragments. This kinetic energy is converted into thermal energy of the system, until a certain "saturation" value of approximately 3 MeV͞nucleon is reached. From this point on, this "conventional" dissipation mechanism is replaced by dynamical IMF production, constituting a new mode of energy dissipation. [S0031-9007(96)

No abstract