Unveiling neutron and gamma spectrum at Martian surface in presence and absence of hydrogen: A computational study based on GEANT4 simulations

Abstract

Motivated by the interactions of the GCR protons with the Martian atmosphere and regolith in the absence and presence of hydrogen, a series of GEANT4 simulations are employed in this study to reveal the influence of hydrogen on the energy spectra of secondary and albedo neutrons together with the associated gamma rays at the Martian surface. Following irradiation of the Martian atmosphere with a planar vertical PAMELA proton beam, the attenuation of the primary proton spectrum through the atmosphere and the range and energy deposition of protons within the regolith are first obtained. The depth profile of the generated neutrons and gamma rays, along with the kinetic energy spectra of the atmospherically produced neutrons and gammas reaching the surface, is then acquired in the absence and presence of 0.11 wt.% hydrogen by voxelizing the regolith into 100 cells of 2-cm thickness. Finally, a surface detector is placed atop the regolith, and the atmospherically produced neutron and gamma spectra are compared with the total neutron and gamma spectra recorded at the surface detector to assess the relative contribution of the atmosphere and the regolith to the overall radiation field reaching the Martian surface. The energy spectra of the albedo neutrons are also obtained in terms of thermal, epithermal, and fast neutrons. The GEANT4 simulations show that the Martian regolith, rather than the atmosphere alone, constitutes the dominant source of the neutron and gamma population arriving at the surface, and that the presence of 0.11 wt.% hydrogen is observable in each energy regime of the albedo neutrons at the Martian surface, while proton transport and the overall neutron and gamma production depth profile remain comparatively insensitive to hydrogen, thereby indicating the elemental variation of the Martian regolith.