Simulation of subsurface particle radiation and leakage flux on the moon from solar energetic particles and galactic cosmic rays

Abstract The moon is continuously bombarded by cosmic rays, primarily Galactic Cosmic Rays (GCR) and Solar Cosmic Rays, which generate complex secondary radiation in the lunar subsurface. Secondary radiation is a critical factor that must be considered in the design of radiation protection for future manned lunar missions and the construction of lunar research bases. Based on previous studies, this study employs the Geant4 to simulate the secondary radiation produced by nuclei from hydrogen to nickel in GCR, with a particular focus on the neutron number density and flux at a depth of 10 ms below the lunar surface. The results indicate that when the abundance of heavy elements from lithium to nickel in GCR is less than 1%, their contribution to the neutron number density exceeds 6%. Additionally, we analyzed the leakage fluxes of neutrons, gamma rays, protons, and electrons during the months of strongest and weakest solar modulation from 1951 to 2016. The total neutron leakage fluxes were 1.59 n/ $$\hbox {cm}^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>cm</mml:mtext> <mml:mn>2</mml:mn> </mml:msup> </mml:math> -s and 3.85 n/ $$\hbox {cm}^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>cm</mml:mtext> <mml:mn>2</mml:mn> </mml:msup> </mml:math> -s, respectively, with the leakage flux proportions of the three neutron types all below 2%. Furthermore, we simulated the total leakage of neutrons and gamma rays in the energy range of 10 −9 to 10 4 MeV during the second extreme Solar Energetic Particle (SEP) event in 2003, which lasted for 33 h. The neutron leakage during this event was 4.6 times and 1.9 times higher than during the months of strongest and weakest solar modulation, respectively; while the gamma ray leakage was 14.5 times and 6.6 times higher, respectively.