Fission timescales of excited nuclei
Pre-scission neutron multiplicities have been measured for fusion-fission, quasi-fission and fast-fission. With certain assumptions, these data give information on the characteristic dynamical reaction timescales of each process. Times ranging from up to 10−19 s for fusion-fission down to 3 × 10−21 s for mass-asymmetric fast-fission are deduced. Most of the excitation energy of highly excited composite systems is removed by the evaporation of light particles prior to scission, thus the fission fragments are rather cold at scission. Fusion-fission timescales are found to show little dependence on the compound nucleus fissility. The long timescales deduced indicate that the motion of the nucleus towards scission is highly overdamped. The results for quasi-fission reactions reveal important new information about this reaction.