RETARDATION EFFECTS IN DAMPING OF NUCLEAR COLLECTIVE EXCITATIONS .1. RELAXATION-TIME

We expand the nonmarkovian collision integral in terms of multipolarit ies of the distortion of the Fermi surface. It is shown that damping o f zero-sound is determined by all multipolarities of the Fermi-surface deformation. For large zero-sound velocity with respect to the Fermi velocity the relaxation time is related to the quadrupole deformation of the Fermi surface. The contributions of collisions to the total wid ths of the giant multipole resonances are calculated in a semiclassica l macroscopic nuclear model.