Considerations for practical dose equivalent assessment of space radiation and exposure risk reduction in deep space

Shielding from space radiation, especially galactic cosmic rays (GCRs), is a significant safety challenge for future human activities in deep space. In this study, the shielding performances of potential materials [aluminum (Al), polyethylene (PE), and carbon fiber reinforced plastic (CFRP)] were investigated using Geant4 Monte Carlo simulation considering two types of biological scale parameters, the International Commission on Radiological Protection (ICRP) quality factor (QF_ICRP) and the plausible biological effectiveness (RBE_γacute), for GCRs. The effective dose equivalent was reduced by 50% for QF_ICRP and 38% for RBE_γacute when shielding using 20 g/cm² of CFRP. A spacecraft made from CFRP will have a better radiation shielding performance than conventional Al-based spacecraft. The contribution of heavy ions for QF_ICRP based effective dose equivalent was larger by a factor of ~ 3 compared to that for RBE_γacute based effective dose equivalent. The shielding materials efficiently reduced the effective dose equivalent due to ions with QF_ICRP > 3.36 and RBE_γacute > 2.26. QF_ICRP and RBE_γacute have advantages and disadvantages in quantifying the dose equivalent of space radiation, and the establishment of a standard parameter specified for a mixed radiation environment occupied by protons and heavy ions is necessary for practical dose assessment in deep space.