Fission timescales in H.I. reactions

In a H.I. collision, after the rapid attainment of thermal equilibrium, the nucleus cools uniformly through evaporation, largely of neutrons for heavy systems. Fission occurs in competition with evaporation, and the number of neutrons emitted before scission gives a measure of the fission timescale. Recently, many pre-scission neutron multiplicity measurements have been made. Those for fusion-fission reactions indicate a long timescale, which implies a highly damped motion towards scission. It is argued that this can explain the observed independence of the timescale on the fissility. Measurements for fast- or quasi-fission reactions show a decreasing timescale for asymmetric mass-splits, and together with estimates of the reaction timescale from the deduced rotation angle of the composite system, give a clearer picture of the dynamics of fission