Zero- to first-sound transition for the giant dipole propagation in hot nuclei

We discuss the transition to first sound propagation of the isovector dipol e mode in heated nuclear matter. The presence of a damping of the isovector current leads to new features of the transition: i) the attenuation keeps a T-2 dependence, although with a change in the slope; ii) for large n - p friction the dipole mode can be fully damped before reaching the transition point. From a realistic estimation of relevant relaxation times we predict a transition around T = 3MeV in medium-heavy nuclei with detectable effect s on the GDR gamma-yield in the direction of a better agreement with experi ments.