Radiation Environment at the Surface and Subsurface of the Moon: Model Development and Validation

• Published in: Journal of geophysical research. Planets Vol. 126; no. 11
• Main Authors: Dobynde, Mikhail I., Guo, Jingnan
• Format: Journal Article
• Language: English
• Published: 01.11.2021
• Subjects: albedo radiation; GCR; GEANT4; Moon; space radiation
• ISSN: 2169-9097; 2169-9100; 2169-9100
• DOI: 10.1029/2021JE006930

A comprehensive understanding of the lunar radiation environment is essential in preparing for future human exploration of the Moon. The radiation environment on the Moon includes primary space radiation and secondary radiation, which is induced in the lunar soil. Both primary and secondary radiation may pose severe health issues to future crews on the Moon. In this work, we build a detailed radiation environment model “Radiation Environment and Dose at the Moon (REDMoon)” for the lunar surface and subsurface. We use the GEANT4 (GEometry ANd Tracking) Monte‐Carlo code and “response function” approach to calculate type‐, energy‐, angular‐, depth‐, and time‐dependent particle spectra induced by galactic cosmic rays at the surface and subsurface of the Moon. Calculated radiation particle fluxes on and beneath the surface are in good agreement with previous experimental and numerical results while offering more details on the lunar radiation fields, such as angular and depth information. The depth profile of secondary particle spectra in the lunar soil has a maximum between 0.5 and 1 m below the surface, depending on particle type and energy. The angular distribution of secondary particles (in particular neutron, γ‐rays, electrons) with energy ≲1 MeV is mostly isotropic, while higher energy particles preferentially propagate downward. Our model provides full coverage of the spatial, directional, and energy information of the radiation field at the surface and subsurface of the Moon, which can serve for designing future human bases on the Moon. Plain Language Summary Interplanetary space is filled with energetic particles that can affect astronauts' health, causing long‐term diseases or acute radiation poisoning. Our Moon lacks a magnetosphere and an atmosphere and is directly exposed to space radiation. Besides, energetic particles can also generate secondary particles in the lunar soil, which are transported therein or scattered to the surface of the Moon. This mixed radiation field poses considerable threats to future human exploration of the Moon. Future lunar bases are likely to use surface material to mitigate space radiation; thus, it is important to find optimized shielding depths as shielding could be counter‐productive if not carefully evaluated. In this work, we build and validate a detailed model to describe the radiation environment on the lunar surface and in the lunar soil. As a first iteration of the model, we provide the particle flux and its dependence on particle type, energy, zenith angle, soil depth, and solar cycle. Our model can be used to evaluate the radiation exposure for future humans on the Moon and design lunar habitats. Key Points We model the radiation field on the surface and subsurface (down to 10 m) of the Moon We obtain energy spectra of albedo and secondary particles at different depths and angles The results are comparable with previous studies while providing more details on the lunar radiation field