Assessment of lunar surface radiation risks and uncertainties using a full-chain Monte Carlo framework: From lunar radiation environment to dose

The return of humanity to the Moon necessitates an accurate assessment of the radiation risks posed by energetic particles in space, including omnipresent Galactic Cosmic Rays (GCRs) and sporadic Solar Energetic Particles (SEPs). The former has been recognized as the main source of radiation during solar quiet periods to deep space explorers and is a critical factor for mission risk assessment. Monte Carlo simulation is a useful method for investigating space radiation risks under specific conditions. This method generally requires considerable computational resources. Meanwhile uncertainties in the GCR dose estimates may arise from various factors that are not yet agreed within the community and have precluded a consensus on mission risk evaluation. This study presents a systematic deconstruction of effective dose uncertainties for astronauts on the lunar surface using a full-chain Geant4-based Monte Carlo simulation framework. This framework integrates the state-of-the-art description of GCR sources, the REDMoon (Radiation Environment and Dose at the Moon) lunar environment model, and energy deposition within ICRP (International Commission on Radiological Protection) Adult Male (AM) and Adult Female (AF) reference computational phantoms. Using the response function method, we avoid the repeated execution of the Monte Carlo program and are able to calculate GCR dose on the lunar surface under specific conditions solely through simple matrix computations. We also quantify the deviations arising from physics lists, anatomical differences in gender, and two radiobiology models: ICRP-60 and NSCR-2012 (NASA Space Cancer Risk). The obtained effective dose under the broad GCR spectrum is strongly anti-correlated with the solar activity, while our results also show a small but visible solar-cycle dependent difference between dose quantities in AM and AF phantoms. Our study further reveals that the choice of physics lists and Linear Energy Transfer (LET) calculation methods can introduce a variation range of approximately 8%-15%, while the selection of the radiobiology model introduces a variation range of approximately 2%-10%. This study assists in quick assessment of lunar mission risks and emphasizes the priority for developing more validated and unified standards to reach a consensus on mission risk evaluation across different agencies.