Fission Dynamics with the "neutron Clock" Technique Using Demon Neutron Array at the Louvain-La Cyclotron Facility.

In this work, the particle evaporation "clock" method has been applied with success to the interpretation of pre- and postscission neutron and light charged particle multiplicities extracted from the fusion-fission reactions; 8 to 16 MeV/A ²⁰Ne+¹⁵⁹Tb and ²⁰Ne+¹⁶⁹Tm. The neutron properties were established with the DEMON neutron array. The experimental observables were compared with statistical model calculations using a new version of GEMINI Monte-Carlo code. These simulations, incorporating simple aspects of the fission dynamics (time-dependent fission width and deformation-dependent transmission coefficients) predicted by the HICOL dynamical code, have enabled us to estimate the ranges of conventional-fission and fast-fission timescales at different compound-nucleus excitation energies. The additional fission dynamical delay times which were required to reproduce our data were found to decrease as the initial excitation energy of the fissioning nucleus increases from 108 to 254 MeV (or for nuclear temperature (T) from 1.3 to 2.3 MeV). This method is shown in simulations, to give times closer to the median lifetime (halflife). The time distributions have extremely long tails covering more than SEVEN orders of magnitude, consistent with previous results obtained with other techniques (crystal blocking, KX-rays and GDR γ-rays). The decrease of median fission lifetimes and subsequent fission transient times with increasing the excitation energies may suggest a decrease in nuclear viscosity above T ∼= 2MeV , consistent with the onset of two-body dissipation mechanism.