Tissue-Specific Dose Equivalents of Secondary Mesons and Leptons during Galactic Cosmic Ray Exposures for Mars Exploration

Abstract

During a human mission to Mars, astronauts would be continuously exposed to galactic cosmic rays (GCRs) consisting of high energy protons and heavier ions coming from outside our solar system. Due to their high energy, GCR ions can penetrate spacecraft and space habitat structures, directly reaching human organs. Additionally, they generate secondary particles when interacting with shielding materials and human tissues. Baryon secondaries have been the focus of many previous studies, while meson and lepton secondaries have been considered to a much lesser extent. In this work, we focus on assessing the tissue-specific dose equivalents and the effective dose of secondary mesons and leptons for the interplanetary cruise phase and the surface phase on Mars. We also provide the energy distribution of the secondary pions in each human organ since they are dominant compared to other mesons and leptons. For this calculation, the PHITS3.27 Monte Carlo simulation toolkit is used to compute the energy spectra of particles in organs in a realistic human phantom. Based on the simulation data, the dose equivalent has been estimated with radiation quality factors in ICRP Publication 60 and in the latest NASA Space Cancer Risk model (NSCR-2022). The effective dose is then assessed with the tissue weighting factors in ICRP Publication 103 and in the NSCR model, separately. The results indicate that the contribution of secondary mesons and leptons to the total effective dose is 6.173%, 9.239%, and 11.553% with the NSCR model in interplanetary space behind 5, 20, and 50 g/cm² aluminum shielding, respectively with similar values using the ICRP model. The outcomes of this work lead to an improved understanding of the potential health risks induced by secondary particles for exploration missions to Mars and other destinations.