Dissipative diabatic dynamics: A shell-model approach to large-amplitude collective nuclear motion

Abstract

Starting from the discussion of isoscalar giant vibrations as small-amplitude diabatic modes, a time-dependent shell-model approach is introduced which simultaneously treats coherent and incoherent aspects of large-amplitude collective nuclear motion. The coherent coupling of the single-particle motion to the time-dependent mean field is treated in a diabatic approximation. The incoherent aspect is represented by residual two-body collisions which lead to statistical equilibrium within the intrinsic degrees of freedom (local equilibrium). An important consequence of the combined effects from diabatic single-particle motion and dissipative two-body collisions is a significant retardation of the friction force for realistic values of the mean free path of nucleons. The retarded-friction term describes an elastic response of the nucleus for perturbations fast compared to the local equilibration time, and a dissipative response for slow perturbations. This elastoplastic behaviour represents an interesting dynamical property of nuclear matter. A diabatic two-center shell model is introduced and results on diabatic single-particle levels and potentials are given and compared with corresponding quantities from TDHF calculations. Effects on cross-sections of nucleus-nucleus collisions (deeply inelastic reactions, extra-push for capture and fusion) are discussed. A specific finger print for the diabatic single-particle motion is the preequilibrium emission of neutrons from unbound diabatic levels in central nucleus-nucleus collisions.