On the origin of asymmetric fission of actinides

TL;DR

This paper explains the asymmetric fission of actinide nuclei by showing that octupole (pear-shaped) deformations, stabilized by shell effects at Z=52-56, play a key role in determining the mass asymmetry of fission fragments.

Contribution

The study demonstrates that octupole deformations, stabilized by shell effects at Z=52-56, are crucial in understanding asymmetric fission, extending beyond the traditional focus on spherical shell closures.

Findings

Heavy fission fragments are mainly produced with Z=52-56.

Octupole deformations favor asymmetric fission.

Spherical magic nuclei resist octupole deformation, hindering their fission fragment formation.

Abstract

Nuclear fission of heavy (actinide) nuclei results predominantly in asymmetric mass-splits. Without quantum shells, which can give extra binding energy to these mass-asymmetric shapes, the nuclei would fission symmetrically. The strongest shell effects are in spherical nuclei, so naturally the spherical "doubly-magic" ${^{132}}$Sn nucleus (${Z=50}$ protons), was expected to play a major role. However, a systematic study of fission has shown that the heavy fragments are distributed around ${Z=52}$ to 56, indicating that ${^{132}}$Sn is not the only driver. Reconciling the strong spherical shell effects at ${Z=50}$ with the different ${Z}$ values of fission fragments observed in nature has been a longstanding puzzle. Here, we show that the final mass asymmetry of the fragments is also determined by the extra stability of octupole (pear-shaped) deformations which have been recently confirmed experimentally around $^{144}$Ba (${Z=56}$), one of very few nuclei with shell-stabilized octupole deformation. Using a modern quantum many-body model of superfluid fission dynamics, we found that heavy fission fragments are produced predominantly with ${52-56}$ protons, associated with significant octupole deformation acquired on the way to fission. These octupole shapes favouring asymmetric fission are induced by deformed shells at ${Z=52}$ and 56. In contrast, spherical "magic" nuclei are very resistant to octupole deformation, which hinders their production as fission fragments. These findings may explain surprising observations of asymmetric fission of lighter than lead nuclei.