Theoretical Investigation of the Angular-Momentum Dependence of the Mean Fission Lifetime of Excited Nuclei

Mean fission lifetimes of nuclei excited to energies of 80 to 400 MeV were recently measured at the GANIL accelerator by the crystal-blocking technique. Those experiments served as a motivation for us to perform systematic calculations of the time distributions of fission events and the mean fission lifetimes versus the angular momentum, the initial excitation energy, and the fissility of a primary excited nucleus. The mean fission lifetimes are given as a function of the angular momentum L. The calculations were performed within the refined version of the combined dynamical-statistical model. It turned out that, if the height of the fission barrier at L=0 is sizably greater than the neutron binding energy, the L dependence of the mean fission lifetimes has a resonance character. Such behavior of the mean fission lifetimes 〈tf〉 is obtained both from statistical calculations and from a dynamical simulation of the fission process with allowance for friction. It is shown that the maximum in the L dependence of 〉tf〉 is due to the fission of nuclei that lost a considerable part of the initial excitation energy through the emission of neutrons. The majority of the calculations were performed for 190Pt at an initial excitation energy of 150 MeV. It is shown that the resonance behavior disappears with increasing fissility, but that it survives over a broad range of initial excitation energies. Systematic experimental studies are required for confirming or disproving our theoretical predictions.