Introduction

The abrasion model is a simplified macroscopic model for nuclear-nuclear interactions based largely on geometric arguments rather than detailed consideration of nucleon-nucleon collisions. As such the speed of the simulation is found to be faster than models such as G4BinaryCascade, but at the cost of accuracy. The version of the model implemented is interpreted from the so-called abrasion-ablation model described by Wilson et al [1],[2] together with an algorithm from Cucinotta to approximate the secondary nucleon energy spectrum [3]. By default, instead of performing an ablation process to simulate the de-excitation of the nuclear pre-fragments, the Geant4 implementation of the abrasion model makes use of existing and more detailed nuclear de-excitation models within Geant4 (G4Evaporation, G4FermiBreakup, G4StatMF) to perform this function (see section 26.5). However, in some cases cross sections for the production of fragments with large $\Delta$A from the pre-abrasion nucleus are more accurately determined using a Geant4 implementation of the ablation model (see section 26.6).

The abrasion interaction is the initial fast process in which the overlap region between the projectile and target nuclei is sheered-off (see figure 26.1) The spectator nucleons in the projectile are assumed to undergo little change in momentum, and likewise for the spectators in the target nucleus. Some of the nucleons in the overlap region do suffer a change in momentum, and are assumed to be part of the original nucleus which then undergoes de-excitation.

Less central impacts give rise to an overlap region in which the nucleons can suffer significant momentum change, and zones in the projectile and target outside of the overlap where the nucleons are considered as spectators to the initial energetic interaction.

The initial description of the interaction must, however, take into consideration changes in the direction of the projectile and target nuclei due to Coulomb effects, which can then modify the distance of closest approach compared with the initial impact parameter. Such effects can be important for low-energy collisions.