Nuclear fission: The "onset of dissipation" from a microscopic point of view

Semianalytical expressions are suggested for the temperature dependence of those combinations of transport coefficients that govern the fission process. This is based on experience with numerical calculations within the linear response approach and the locally harmonic approximation. A reduced version of the latter is seen to comply with Kramers's simplified picture of fission. It is argued that for variable inertia his formula has to be generalized, as already required by the need that for overdamped motion the inertia must not appear at all. This situation may already occur above T≈2 MeV, where the rate is determined by the Smoluchowski equation. Consequently, comparison with experimental results does not give information on the effective damping rate, as often claimed, but on a special combination of local stiffnesses and the friction coefficient calculated at the barrier.