Directional astronaut radiation dose for hemispherical galactic cosmic ray irradiation

While galactic cosmic rays (GCRs) are inherently isotropic, in several important cases they are constrained to certain solid angle region. This directional irradiation leads to non-trivial deviations in radiation exposure from simple solid angle proportionality, since human organs/tissue sensitive to radiation also exhibit distinct spatial orientations within body. In this paper we investigate GCR incidence patterns through two characteristic geometries: the upper and anterior (front) hemispherical incidence relative to the ICRP110 human voxel phantom, and make comparison with the isotropic incidence. The fluence-to-dose-equivalent conversion coefficients are calculated by the particle physics Monte Carlo toolkit GEANT4, for all the $Z=1-28$ ions and $27-36$ energy points for each ions. Our analysis encompasses both the unshielded configuration and a shielded configuration with a uniform 5 g/cm${}^2$ aluminum shell, approximating spacecraft habitat shielding. We found that upper hemispherical incidence demonstrates $\lesssim 10\text{\%}$ dose coefficient variation compared to the isotropic baselines, while anterior incidence exhibits pronounced $\sim 80\text{\%}$ higher dose coefficients within the 10 MeV/n to 1000 MeV/n energy range for the unshielded configuration. Dose equivalent rates with hemispherically filtered GCR fluxes show corresponding differences. Interestingly the deviation from solid angle proportionality can be utilized, that strategic astronaut orientation (Earth-facing with eastern alignment) may reduce cumulative radiation exposure by $\sim 15\text{\%}$.