Correlated fission fragment spin dynamics

TL;DR

This paper investigates how nucleon exchange influences fission fragment spins using Langevin simulations and transport theory, revealing how spin components evolve and correlating observables with mass asymmetry.

Contribution

It introduces a detailed transport model for fragment spin dynamics during fission, combining Langevin simulations with nucleon exchange theory.

Findings

Rotational modes fall out of equilibrium before scission due to rapid temperature rise.

Neck shrinking suppresses further nucleon exchange during fission.

Distributions of fragment spin magnitude and orientation are characterized.

Abstract

This study explores the role of nucleon exchange for the generation of the fission fragment angular momenta. For a number of typical fission cases, samples of 10,000 shape evolutions are generated by Langevin simulation and, subsequently, for each such evolution, the nucleon exchange transport theory previously developed for damped nuclear reactions is used to obtain the development of the fragment spin-spin distribution within the Fokker-Planck transport framework. The characteristic evolution of both parallel and perpendicular spin components is discussed. A common feature is that the rotational modes fall out of equilibrium before scission when the temperature rises rapidly while the concurrent shrinking of the neck suppresses further exchange. A number of fission observables are extracted from the event ensembles: the distribution of the magnitude of the fragment spin and its orientation relative to the fission axis, as well as the correlation between the two spins and the distribution of their opening angle. The dependence of these observables on the mass asymmetry is also examined.