EXPERIMENTAL EVIDENCE AND PHYSICAL IMPLICATIONS OF THE TIME EVOLUTION ALONG THE MASS ASYMMETRY MODE IN HEAVY ION REACTIONS

Abstract

The complex experimental features associated with the mass or charge distributions, and with the angular distributions as a function of fragment mass or charge, are interpreted as evidence of an intermediate structure, or intermediate complex, evolving in time along the mass asymmetry mode. Strong circumstantial evidence suggests that this time evolution is diffusive in nature and can be described in terms of the Master Equation or the Fokker-Planck Equation. The experimental evidence of broad mass distributions for large ratios E/B, where E is the center of mass energy and B is the interaction barrier, and narrow mass distributions peaked at the projectile and target mass for small ratios E/B, is interpreted as due to an increasing lifetime of the complex with energy. For short lifetimes, the system has little time to evolve in mass asymmetry and gives rise to rather narrow distributions centered about the target and projectile mass. For long lifetimes the system undergoes extensive relaxation in mass asymmetry and gives rise to very broad mass distributions. Similarly the angular distributions seem to evolve from side peaked to forward peaked with increasing E/B. This is interpreted as due to a transition from a short lifetime-slow angular velocity regime whichmore » does not allow for orbiting beyond 0/sup 0/, to a long lifetime-large angular velocity regime which produces orbiting past 0/sup 0/. The evolution from side peaking to forward peaking in the same reaction as one moves away in Z from the projectile is interpreted as due to the time lag introduced by diffusion in the population of fragments farther removed in Z from the projectile. The variation of charge and angular distribution with the fragment kinetic energy allows one to connect the energy relaxation to the mass asymmetry relaxation. Theoretical calculations based on diffusion models allow one to fit mass and angular distributions as well as to extract transition probabilities and Fokker-Planck coefficients.« less