Improvements of preequilibrium and evaporation models in Geant4

Secondary neutron spectra in proton-nucleus reactions constitute a sound test for validation of prequilibrium and equilibrium models at incident energies below 100 MeV, where they play a major role. Previous versions of these models in deexcitation (Weisskopf) and preequilibrium (exciton) packages of Geant4 showed a big discrepancy with experimental data in the secondary neutron spectrum produced by bombarding ²⁷Al with protons of 22 MeV kinetic energy, although reasonable agreement was obtained in other reactions, provided some “ad-hoc” assumptions were made. Several actions have been taken in order to improve the situation. At the equilibrium stage, much more realistic inverse reaction cross sections have been included. At preequilibrium, the same set of reaction cross sections is used and the transition from preequilibrium to equilibrium regime is now physically consistent. In general, good agreement with experimental data of (p,n) reactions is achieved.