Generation of fragment angular momentum in fission

TL;DR

This paper demonstrates that nucleon-exchange mechanisms in fission models can produce uncorrelated fragment spins, challenging previous conclusions, and explains the sawtooth pattern of average fragment spins through shell and deformation effects.

Contribution

The study shows that fragment spins are largely uncorrelated due to nucleon exchange, and explains the sawtooth pattern of average spins by including shell and deformation effects.

Findings

Nucleon exchange leads to largely uncorrelated fragment spins.

Shell and deformation effects explain the sawtooth pattern of average spins.

The model invalidates previous assumptions about spin correlation post-scission.

Abstract

A recent analysis of experimental data [J. Wilson $et. al$, Nature $\mathbf 590$, 566 (2021)] found that the angular momenta of nuclear fission fragments are uncorrelated. Based on this finding, the authors concluded that the spins are therefore determined only $after$ scission has occurred. We show here that the nucleon-exchange mechanism, as implemented in the well-established event-by-event fission model $\mathtt{FREYA}$, while agitating collective rotational modes in which the two spins are highly correlated, nevertheless leads to fragment spins that are largely uncorrelated. This fact invalidates the reasoning of those authors. Furthermore, it was reported [J. Wilson $et. al$, Nature $\mathbf 590$, 566 (2021)] that the mass dependence of the average fragment spin has a sawtooth structure. We demonstrate that such a behavior naturally emerges when shell and deformation effects are included in the moments of inertia of the fragments at scission.