Seminars

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.

Then the following fundamental questions will be addressed:

— What governs the fission paths?

— Why do neighboring fissioning nuclei exhibit such differences in charge and mass yields?

— How can the fine details (peaks and dips) in primary fission yields be explained?

— Which shell effects are relevant, and what are their properties?

We will show that shell effects in krypton isotopes play a particularly significant role, consistent with experimental results, while also offering additional point of view on the fission mechanism in this region.

[1] P. Morfouace et al, An asymmetric fission island driven by shell effects in light fragments. Nature, 641:339–344, 2025