Semiclassical origin of asymmetric nuclear fission: nascent-fragment shell effect in periodic-orbit theory

The origin of the asymmetry in the fragment mass distribution of low-energy nuclear fission is considered from the semiclassical point of view. Using the semiclassical periodic-orbit theory, one can define and quantify the shell effect associated with spatially localized nascent-fragment (prefragment) part of the potential. We investigate the roles of prefragments in the deformed shell effect using a simple cavity potential model, but with realistic shape degrees of freedom for describing the fission processes. The results suggest that the prefragment magic numbers play essential roles in determining the shapes at the fission saddles, which should have a close relation to the fragment mass distribution. (paper)