Frustrated fragmentation and re-aggregation in nuclei: a non-equilibrium description in spallation

TL;DR

This paper presents a microscopic, non-equilibrium model based on the Boltzmann-Langevin equation to describe fragmentation and re-aggregation in nuclei during high-energy spallation, highlighting the role of phase-space fluctuations and mean-field dynamics.

Contribution

It introduces a novel non-equilibrium approach to distinguish between fragmentation and fission-like events in nuclear reactions, emphasizing the importance of phase-space fluctuations and mean-field effects.

Findings

Fragmentation initiated by phase-space fluctuations can be confused with fission when using yield data.

The model explains re-aggregation processes leading to fission-like events with only two fragments.

The approach can be applied to heavy-ion collisions near the fragmentation threshold.

Abstract

Heavy nuclei bombarded with protons and deuterons in the 1 GeV range have a large probability of undergoing a process of evaporation and fission; less frequently, the prompt emission of few intermediate-mass fragments can also be observed. We employ a recently developed microscopic approach, based on the Boltzmann-Langevin transport equation, to investigate the role of mean-field dynamics and phase-space fluctuations in these reactions. We find that the formation of few IMF's can be confused with asymmetric fission when relying on yield observables, but it can not be assimilated to the statistical decay of a compound nucleus when analysing the dynamics and kinematic observables: it can be described as a fragmentation process initiated by phase-space fluctuations, and successively frustrated by the mean-field resilience. As an extreme situation, which corresponds to non-negligible probability, the number of fragments in the exit channel reduces to two, so that fission-like events are obtained by re-aggregation processes. This interpretation, inspired by nuclear-spallation experiments, can be generalised to heavy-ion collisions from Fermi to relativistic energies, for situations when the system is closely approaching the fragmentation threshold.