Improvement of Geant4 Neutron-HP package: Doppler broadening of the neutron elastic scattering kernel and cross sections

Whether it is for shielding applications or for safety criticality studies, numerically solving the neutron transport equation with a good accuracy requires to precisely estimate the Doppler broadened elastic scattering kernel in the thermal and epithermal energy range of neutrons travelling in a free gas. In Geant4, low energy neutrons are transported using evaluated data libraries handled by the Neutron High-Precision (Neutron-HP) package. Version 11.00.p03 of the code features in particular the Doppler broadened elastic scattering kernel, provided by the so-called 'Sampling of the Velocity of the Target' (SVT) method. However this latter fails for resonant heavy nuclei such as 238U and can severely impact the solving of the Boltzmann equation in fissile media. To overcome this shortcoming, the Doppler Broadened Rejection Correction (DBRC) method has been implemented in Geant4 and successfully validated with the reference Monte Carlo neutron transport code Tripoli4 (version 11). This development will be taken into account in the next release of the code. The cross section Doppler broadening process, which is performed on-the-fly, is also carefully investigated and ways to improve it on a simulation-by-simulation basis are presented. All the validations have been performed with an automated benchmark tool which has been designed to support the quality assurance of the Geant4 Neutron-HP package. This tool is currently available on an ad hoc Gitlab repository and will be included in Geant4.