Speaker
Description
Nuclear fission has been studied for many years since its discovery in 1938, yet many aspects of its dynamics, particularly the mechanism of particle emission during the fission process, remain unclear. The mass asymmetry of fission fragments is strongly influenced by the shell structure of the fissioning nucleus. Conventionally, it has been understood that at high excitation energies, the damping of shell effects leads to liquid-drop-like behavior, making mass-symmetric fission dominant.
However, recent experiments using multi-nucleon transfer reactions have demonstrated that the fission fragment mass distributions maintain a double-humped structure even at excitation energies as high as 50 MeV in the actinide region, such as $^{236}$U [1,2]. This observation can be explained by multi-chance fission, where the emission of neutrons during the fission process reduces the excitation energy, leading to the restoration of shell effects.
In this work, we developed a model that couples the 3D Langevin model with the statistical model to account for dynamic neutron emission accompanying the time evolution of the nuclear shape [3]. This approach allows us to incorporate the restoration of shell effects not only at the compound nucleus stage but also at any deformation stage, from the saddle point to the scission point.
Using this model, we analyzed isotopes ranging from uranium to plutonium and confirmed that neutron emission effectively lowers the nuclear temperature, thereby sustaining mass-asymmetric fission even at high excitation energies. Furthermore, the analysis revealed significant neutron emission not only near the ground-state shape but also at the saddle point and beyond (i.e., highly deformed configurations). This finding provides new insights into the origin and mechanism of pre-scission neutron emission.
Reference
[1] K. Hirose, et al., Phys. Rev. Lett. 119, 222501 (2017).
[2] M. J. Vermeulen, et al., Phys. Rev. C. 102, 054610 (2020).
[3] S. Takagi, et al., Phys. Rev. C, 112, 044607 (2025).