  A semiphenomenological approach to the nucleon self-energy in nuclear matter
at finite temperatures is followed. It combines elements of Thermo Field
Dynamics for the treatment of finite temperature with a model for the
self-energy, which evaluates the second order diagrams taking the needed
dynamics of the NN interaction from experiment. The approach proved to be
accurate at zero temperature to reproduce Im(Sigma) and other properties of
nucleons in matter. In the present case we apply it to determine Im(Sigma) at
finite temperatures. An effective NN cross section is deduced which can be
easily used in analyses of heavy ion reactions.
