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

TL;DR

This study investigates how the nucleus-nucleus potential and friction parameter in fusion reactions depend on incident energy, revealing that energy dissipation mechanisms and fluctuation-dissipation relations change with energy levels.

Contribution

It introduces a microscopic analysis of energy-dependent dissipation mechanisms and their impact on the fusion potential and fluctuation-dissipation relation in heavy-ion fusion.

Findings

Fluctuation-dissipation relation fades at higher energies.

Two dissipation mechanisms: nucleon exchange and intrinsic rearrangement.

Energy dependence explained by the intrinsic nucleon rearrangement mechanism.

Abstract

Applying a macroscopic reduction procedure on the improved quantum molecular dynamics (ImQMD) model, the energy dependences of the nucleus-nucleus potential, the friction parameter, and the random force characterizing a one-dimensional Langevin-type description of the heavy-ion fusion process are investigated. Systematic calculations with the ImQMD model show that the fluctuation-dissipation relation found in the symmetric head-on fusion reactions at energies just above the Coulomb barrier fades out when the incident energy increases. It turns out that this dynamical change with increasing incident energy is caused by a specific behavior of the friction parameter which directly depends on the microscopic dynamical process, i.e., on how the collective energy of the relative motion is transferred into the intrinsic excitation energy. It is shown microscopically that the energy dissipation in the fusion process is governed by two mechanisms: One is caused by the nucleon exchanges between two fusing nuclei, and the other is due to a rearrangement of nucleons in the intrinsic system. The former mechanism monotonically increases the dissipative energy and shows a weak dependence on the incident energy, while the latter depends on both the relative distance between two fusing nuclei and the incident energy. It is shown that the latter mechanism is responsible for the energy dependence of the fusion potential and explains the fading out of the fluctuation-dissipation relation.