New results on dissipation in fission

Fission after very peripheral heavy-ion collisions at relativistic energies is a very powerful tool for the investigation of dissipation. Such approach has been followed at GSI where the total and partial fission cross sections of some stable and a large number of unstable nuclei have been measured. These data are sensitive to the description used for the time dependent fission decay width, Γf(t), and to other model parameters like the fission barriers, Bf, and the ratio of the level-density parameters, af/an. The partial fission cross sections are not reproduced when an exponential in-grow function is used to describe Γf(t) in the theoretical code. Moreover, a further preliminary analysis of the experimental data leads to a confirmation of the theoretical values predicted for Bf and af/an, and to a dissipation coefficient β=2 x 1021s-1 that corresponds to a transient time τf≅ 2 x 10-21s. (orig.)