Neutron emission during fission and its impact on fission-fragment mass distribution studied by a Langevin model

Actinide nuclei exhibit mass-asymmetric fission at low energy due to shell structure. The fission-fragment mass distributions produced at high energy tend to have a symmetric shape due to smearing of shell effects. On the other hand, the distribution can be changed by neutron emission occurring before fission, as this decreases the excitation energy of the fissioning nucleus, and thus revives the shell structure. In so-called multichance fission, neutron emission is considered prior to fission at the initial nuclear shape, and competition between fission and neutron emission is determined with the framework of the statistical model. In the present work, we describe fission in the Langevin equations, and neutron emission is treated throughout the fission process. The calculation reproduces experimentally observed mass distributions, for a wide range of initial compound-nucleus excitation energy up to 60 MeV. The results show that, while neutron emission dominates at the ground-state shape, it occurs along the shape evolution path to the scission point.