Entrance-channel dependence of fission transients

Abstract

Fission transients describe the fission rate as it evolves towards the quasistationary value given by Kramers' formula. The nature of fission transients is dependent on the assumed initial distribution of the compound nuclei along the fission coordinate. Although the standard initial assumption of a near-spherical object leads to a transient suppression of the fission rate (fission delay), a moderate initial fissionlike deformation can reduce the magnitude of this suppression. For still larger initial deformations, transient fission enhancements are possible. Examples of this behavior are illustrated via a one-dimensional Langevin simulation. It is argued that the initial conditions are determined by the fusion dynamics and thus are entrance-channel dependent. Transient fission may be considered intermediate between statistical fission and quasifission as the rapid time scale of transient fission may not lead to an equilibrium of the angular and mass-asymmetry coordinates. The relationship between the mean first passage time and the transients are discussed. For temperatures much smaller than the fission barrier, the mean first passage time is independent of the nature of the fission transients if there is no strong competition from evaporation. Thus, fission transients are most important when the evaporation time scale is smaller than, or of the order of, the transient time.