Monte Carlo analysis of depth dose distributions of 4He and 12C ion beams from 0.1 to 10 GeV in a human head phantom.

Long-duration human spaceflight and crewed missions to the Moon and Mars - recently renewed priorities for several space agencies - pose major technical and biomedical challenges, including reliable supplies of oxygen, water, and food. Among the most critical is protecting astronaut health from intense fluxes of energetic charged particles in space. Measurements and numerical simulations are both used to study the interactions of these energetic particles with human tissues. In this work, the Monte Carlo codes FLUKA (v4-5.0) and Geant4 (v11.3.2) were used to simulate and analyze depth-dose distributions for monoenergetic 4He and 12C ions over a broad energy range (0.1, 0.25, 0.5, 1, 5, and 10 GeV) in the Snyder head phantom. Within Geant4, results obtained with three hadronic physics lists, FTFP_BERT, FTFP_INCLXX, and FTF_BIC, were compared with each other and with FLUKA predictions; differences were quantified as percentage deviations. In addition, the results for 4He and 12C ions were analytically compared with those for protons, the most abundant component of cosmic rays, at the same energies. These analyses are essential for assessing the sensitivity of dose estimates to the underlying physical models, improving confidence in computed dose values, and ultimately supporting the investigation of the radiobiological effects of cosmic radiation. The average of the absolute value of the relative difference observed in this study between the total dose values obtained for 4He ions using FLUKA and the physics models implemented in Geant4 ranged from 0.85% to 3.55%, while for 12C ions, it ranged from 0.002% to 3.14%. This parameter was also calculated for the primary and secondary dose components, and the observed differences were discussed. It is recommended that similar studies should be conducted for all particle species present in cosmic rays to properly assess the reliability and consistency of Monte Carlo codes in this area of research.