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

高木 慎弥*; 原田 壮太*; 有友 嘉浩*; 廣瀬 健太郎  ; 西尾 勝久   

Takagi, Shinya*; Harada, Sota*; Aritomo, Yoshihiro*; Hirose, Kentaro; Nishio, Katsuhisa

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