Two-body contribution to the relaxation of collective excitations in cold finite Fermi systems

The two-body contribution to the relaxation of collective excitations in fi nite Fermi systems has been investigated. Special attention has been given to exploring the effect that the special features of equilibrium distributi on functions in such systems may exert on this contribution. The diffusenes s and oscillations of the equilibrium distribution function in momentum spa ce have been taken into account together with retardation effects in a coll ision integral. A potential of the Woods-Saxon form has been used for an eq uilibrium mean field. It has been shown that oscillations of the equilibriu m distribution function lead to a compensation of particle flows in an equi librium system and to a significant reduction of the relaxation rate becaus e of the diffuseness of the equilibrium distribution function. As a result, the widths of giant quadrupole resonances take values that are close to th ose that are obtained by taking into account retardation effects in the col lision integral and by using the distribution function in the Thomas-Fermi approximation.