Contribution of cosmic alphas to generation of albedo neutrons and gammas in lunar regolith: A computational study based on GEANT4 simulations

Abstract

The lunar surface is constantly and directly exposed to Galactic Cosmic Rays (GCRs) due to the lack of the atmosphere and the magnetic field around it. The high-energetic alpha particles subsequently contribute to the generation of the secondary neutrons through either the spallation reaction or the low-energetic $(\alpha, n)$ process along with the cosmic protons. In this study, the contribution of the alpha particles to the production of the albedo neutrons as well as the albedo gamma rays in the lunar regolith is studied by using the Monte Carlo simulations based on the GEANT4 toolkit. The incident alpha particles are referenced from the PAMELA spectrometer over the interval between 0.14 and 52 GeV and executed as a probability-based primary particle source. The present simulations demonstrate the production of the albedo neutrons and gamma rays upon the process of cosmic alpha irradiation in the lunar regolith. The albedo neutron production is predominantly situated within the near-surface region of the lunar regolith and progressively diminishes with the increasing depth. The population of the secondary neutrons is primarily distributed over the low-energy region and reduces towards the high energies. These results imply that the cosmic alpha particles have also quantifiable contribution in the generation of the secondary radiation at the lunar surface together with the cosmic protons, and this study demonstrates a computational framework in the evaluation of the lunar albedo, the radiation transport, the surface composition, and the radiation environment for the further lunar exploration missions.