Fission dynamics at low excitation energy

TL;DR

This paper clarifies the origin of mass asymmetry in low-energy uranium fission through Langevin equation analysis, highlighting the roles of shell effects and shape fluctuations in fragment mass distribution.

Contribution

It introduces a trajectory analysis approach revealing the importance of shape fluctuations and random vibrations in fission dynamics, offering a new perspective on the splitting mechanism.

Findings

Mass distribution peaks are determined by shell correction energy.

Width of peaks results from shape fluctuations at scission.

Oblate shape vibrations are crucial for fission process.

Abstract

The origin of mass asymmetry in the fission of uranium at a low excitation energy is clarified by a trajectory analysis of the Langevin equation. The positions of the peaks in the mass distribution of fission fragments are mainly determined by fission saddle points originating from the shell correction energy. The widths of the peaks, on the other hand, result from a shape fluctuation around the scission point caused by the random force in the Langevin equation. We found that a random vibration in the oblate direction of fissioning fragments is essential for the fission process. According to this picture, fission does not occur with continuous stretching in the prolate direction, similarly to that observed in starch syrup. This is expected to lead to a new viewpoint of fission dynamics and the splitting mechanism.