Effective mass in nuclei and the level density parameter

Abstract

The influence of momentum and frequency dependence of the nuclear mean field Σ on the level density parameter a is studied. By introducing a local quasiparticle effective mass we determine the volume, surface and curvature coefficients of a, expressed as a = a V A + a S A 2 3 + a c A 1 3 . It is found that the magnitude of a c is significant due to the very nature of the surface. A purely momentum dependent field Σ determined from the nonlocality of the optical potential gives a level density close to a self-consistent Thomas-Fermi description with Skyrme III forces. An explicit frequency dependence of Σ mainly influences a s, enhancing it considerably for medium and heavy nuclei.