Two-body relaxation times in heated nuclei

The retardation and temperature effects in two-body collisions are studied. The collision integral with retardation effects is obtained on the base of the Kadanoff-Baym equations for the Green functions in a form with allowance for reaching the local equilibrium system. The collisional relaxation times of collective vibrations are calculated using both the transport approach and the doorway state mechanism with hierarchy of particle-hole configurations in heated nuclei. The relaxation times of the kinetic method are rather weakly dependent on multipolarity of the Fermi surface distortion and on the mode of the collective motion. The dependence of the relaxation times on temperature as well as on frequency of collective vibrations is considered and compared. It is shown that variations of the in-medium two-body cross-sections with energy lead to non-quadratic dependence of the collisional relaxation time both on temperature and on the frequency of the collective motion. (authors)