Mechanisms of very heavy-ion collisions: The 209Bi + 136Xe reaction at ELab = 1130 MeV

Abstract

The mechanisms of kinetic-energy dissipation and nucleon exchange operating in damped heavy-ion collisions, and the corresponding time scales are investigated. As an example of a very heavy system, the reaction 209Bi + 136Xe at E Lab = 1130 MeV is choosen for a comparison with various reaction models. The experimental data include atomic number, energy and angular distributions of the projectile-like fragments and the correlations between these experimental observables. The integrated fragment Z distribution is found to be broad, but centered at the charge of the projectile. The kinetic energy distribution extends from quasi-elastic down to Coulomb energies of highly deformed fragments. The angular distribution of the reaction products is focussed into a narrow angular range a few degrees forward of the quarter-point angle. Examining the correlations of experimental observables with fragment charge and energy loss in detail, it is concluded that the kinetic-energy loss is a fundamental parameter indicating the stage of evolution of the reaction, i.e., the interaction time. The energy loss is inferred to decrease linearly with angular momentum, and this relation is used to construct an average experimental deflection function. The latter is compared to the predictions of classical dynamical models employing various potentials and frictional forces. These calculations do not provide a consistent description of the experimental results. Employing a simple phenomenological reaction model, l-dependent interaction times are deduced which are used to evaluate proton-number diffusion coefficients. A rather model-independent comparison of the time dependence of the mechanisms of nucleon exchange and kinetic energy dissipation is achieved by using the microscopic time scale provided by the nucleon-exchange mechanism. Energy dissipation is found to be of the one-body type, nucleon exchange contributing about 30% to the total energy loss. A consistent description of the observed reaction phenomena has been achieved in terms of a statistical mechanism of damped reactions evolving in time as nucleons are exchanged and energy is dissipated. It connects in a continuous fashion the domains of quasi-elastic scattering and strongly damped collisions.