Experimental Measures of Fission Time Scales

Abstract

The nuclear fission process involves a drastic rearrangement of the nuclear matter of the fissioning system as it undergoes dramatic shape changes from a single, compact nucleus to two separated fission fragments. Much research, both theoretical and experimental, has focused on understanding the dynamics of this process using a number of different approaches. Soon after the discovery of fission in 1938-39, Bohr and Wheeler proposed the transition state model in which the fission decay width / lifetime was obtained by the statistical counting of transition states at the fission barrier. Subsequently, Kramers suggested that the friction or viscosity of the nuclear matter can play a substantial role in slowing down the decay rate by large factors. Since then, many theoretical works have addressed this issue, most intensely during recent decades, but consensus on the dynamical description of fission has not yet been reached. In light of this situation, it is important to consider whether experiments can shed some light on the issue. Several different methods have been used to obtain experimental information on the dynamics of fission and the associated time scales for the process. In general, the approach is to compare the fission time to some other process that is believed to be better understood. Although the divergence of results obtained by the different methods has been known for years, these discrepancies have not yet been resolved and they have recently received renewed attention. In this talk, I will review some of the experimental measurements of fission time scales and discuss the discrepancies between the different methods.

The role of angular momentum in fission

“…1 as a function of c. The relaxation times are compared with t fiss , the time needed for the fissioning system to evolve from the first appearance of the proto-fragments to the neck rupture. Because t fiss is difficult to measure experimentally, it is not well known [21,22]. We assume that t fiss is between one to a few 10 −21 s. As seen in Fig.…”