Determination of the Transient Time for Fission from a New Experimental Approach

Fission induced by peripheral heavy-ion collisions at relativistic energies is a powerful experimental approach to determine the dynamical delay of fission (transient time) due to dissipation. The fissioning nuclei produced by this method have small shape distortions, low angular momentum and high excitation energies at which this dynamical delay can be observed. These conditions allow for applying the model of Grangé and Weidenmüller to obtain a quantitative value of the transient time. Such approach was followed at GSI where the total and partial fission cross-sections and the widths of the charge distributions of the fission residues from a large number of nuclei were measured. The analysis of these data in the frame of an abrasion-evaporation code has lead to a transient time of 2.1⋅10s when using a step function to describe the time dependence of the fission width. This result implies that excitation energies larger than 100 MeV are needed to observe this transient time.