Abstract: Fission is the nuclear process by which a nucleus splits into fragments. Although this reaction was discovered nearly 90 years ago, it remains a major fundamental challenge, both experimentally, as data are difficult to obtain and interpret, and theoretically, as no existing model can yet simultaneously reproduce experimental results, provide reliable predictive power, and offer a consistent physical interpretation of the process.

The theoretical modeling of fission relies primarily on describing the evolution of the system from the initial configuration of the fissioning nucleus to the formation of the final fragments. This is achieved through potential energy surfaces (PES), generated by a set of theoretical calculations representing the atomic nucleus in various relevant states. Calculating these energy surfaces is an essential step, as it allows one to characterize the deformed configurations of the nucleus, identify fission valleys and modes, their associated saddle points, and the shape of the fission barriers that govern the dynamics of the process. These results provide a microscopic foundation for determining key observables such as fragment mass and charge distributions or their kinetic energies.

From the experimental perspective, several SOFIA (Studies On Fission with Aladin) campaigns have been conducted in recent years by the CEA DAM (Bruyères-le-Châtel) to measure fission yields of various fissioning nuclei. During the most recent campaign, around one hundred nuclei were measured, ranging from iridium (Z = 77) to thorium (Z = 90) [1]. These measurements revealed, on one hand, the existence of an asymmetric fission island, for which most fissioning systems produce fragments of unequal mass and charge, and on the other hand, a notable overproduction of krypton isotopes.

In this presentation, these experimental results will be interpreted using a mean-field theoretical framework, specifically the Hartree–Fock–Bogoliubov (HFB) approach under constraints. Within this framework, fission paths are analyzed in terms of shell effects, i.e., quantities related to the sensitivity of the system to its local energy level density. During the process, two types of shell effects can be distinguished: those intrinsic to the fissioning nucleus, and those belonging to the nascent fission fragments formed at large deformations, close to scission.

Speaker Bio: Rémi N. Bernard is a staff scientist at the Laboratory of Physics at the IRESNE CEA insitute in Cadarache. His main work focuses on the microscopic description of the fission process employing self-consistent mean-field methods and energy density functionals.