  Motivated by the interaction of the cosmic ray protons with the lunar surface
in the absence and presence of hydrogen, a series of GEANT4 simulations are
employed in the current study to unveil the influence of hydrogen on the energy
spectrum of the albedo neutrons that escape from the lunar surface. Initially,
a discrete 69-bin proton energy spectrum between 0.4 and 115 GeV based on the
PAMELA spectrometer is implemented into GEANT4. Subsequently, a single-volume
lunar surface of 2-m thickness is constructed where it is assumed that a single
layer of either 1.6 or 1.93 g/cm$^{3}$ constitutes the lunar regolith. The
material composition in the present study includes 43.7 wt.$\%$ oxygen, 0.3
wt.$\%$ sodium, 5.6 wt.$\%$ magnesium, 9 wt.$\%$ aluminum, 21.1 wt.$\%$
silicon, 8.5 wt.$\%$ calcium, 1.5 wt.$\%$ titanium, 0.1 wt.$\%$ manganese, and
10.2 wt.$\%$ iron in accordance with another study. Then, 0.1 wt.$\%$ hydrogen
is introduced by replacing oxygen in order to assess the impact of hydrogen on
the secondary neutrons. In the wake of irradiating a lunar surface of
3$\times$2$\times$3 m$^{3}$ with a planar vertical PAMELA proton beam of
20$\times$20 cm$^{2}$, the depth profile of the generated neutrons as well as
the corresponding initial energy spectrum is primarily obtained in the absence
and presence of 0.1 wt.$\%$ hydrogen by voxelizing the entire geometry with 100
cells of 2-cm thickness. Next, a surface detector is placed at the top of the
lunar surface in order to collect the albedo neutrons in both cases, and the
energy spectra of the albedo neutrons are acquired in terms of thermal,
epithermal, and fast neutrons. From the GEANT4 simulations in this study, it is
shown that the presence of 0.1 wt.$\%$ hydrogen is observable in each energy
regime of the albedo neutrons at the lunar surface, thereby providing an
indication about the elemental variation of the lunar soil.
