Energy-Dependence of Nucleus-Nucleus Potential and Friction Parameter in Fusion Reactions

摘要

Applying a macroscopic reduction procedure on the improved quantum moleculardynamics (ImQMD) model, the energy dependences of the nucleus-nucleuspotential, the friction parameter, and the random force characterizing aone-dimensional Langevin-type description of the heavy-ion fusion process areinvestigated. Systematic calculations with the ImQMD model show that thefluctuation-dissipation relation found in the symmetric head-on fusionreactions at energies just above the Coulomb barrier fades out when theincident energy increases. It turns out that this dynamical change withincreasing incident energy is caused by a specific behavior of the frictionparameter which directly depends on the microscopic dynamical process, i.e., onhow the collective energy of the relative motion is transferred into theintrinsic excitation energy. It is shown microscopically that the energydissipation in the fusion process is governed by two mechanisms: One is causedby the nucleon exchanges between two fusing nuclei, and the other is due to arearrangement of nucleons in the intrinsic system. The former mechanismmonotonically increases the dissipative energy and shows a weak dependence onthe incident energy, while the latter depends on both the relative distancebetween two fusing nuclei and the incident energy. It is shown that the lattermechanism is responsible for the energy dependence of the fusion potential andexplains the fading out of the fluctuation-dissipation relation.