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) and the plausible biological effectiveness (RBE), for GCRs. The effective dose equivalent was reduced by 50% for QF and 38% for RBE when shielding using 20 g/cm 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 based effective dose equivalent was larger by a factor of ~ 3 compared to that for RBE based effective dose equivalent. The shielding materials efficiently reduced the effective dose equivalent due to ions with QF > 3.36 and RBE > 2.26. QF and RBE 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.