GCR and SPE organ doses in deep space with different shielding: Monte Carlo simulations based on the FLUKA code coupled to anthropomorphic phantoms

Astronauts’ exposure to space radiation is of high concern for long-term missions, especially for those in deep space such as possible
travels to Mars. In these cases shielding optimization is a crucial issue, and simulations based on radiation transport codes and anthropomorphic
model phantoms can be of great help. In this work the FLUKA Monte Carlo code was coupled with two anthropomorphic
phantoms (a mathematical model and a ‘‘voxel’’ model) to calculate organ-averaged dose, dose equivalent and ‘‘biological dose’’ in the
various tissues and organs following exposure to the August 1972 Solar Particle Event and to Galactic Cosmic Rays under different
shielding conditions. The ‘‘biological dose’’ was characterized by the average number of induced ‘‘Complex Lesions’’ (CLs) per cell
in a given organ or tissue, where CLs are clustered DNA breaks which can play an important role in chromosome aberration induction.
Separate calculation of the contributions from secondary hadrons – in particular neutrons – with respect to primary particles allowed us
to quantify the role played by nuclear interactions occurring in the shield and in the human body. Specifically for GCR, the contributions
from the different components of the incident primary spectra were calculated separately as well. As expected, the SPE doses showed a
dramatic decrease with increasing Al shielding. Furthermore, for SPEs internal organs received much lower doses with respect to skin,
and nuclear interactions were found to be of minor importance. A 10 g/cm2 Al storm shelter turned out to be sufficient to respect the
NCRP limits for 30-days LEO missions in case of a SPE similar to the August 1972 event. In contrast with SPEs, GCR absorbed doses
remained roughly constant with increasing Al shielding. The organ-averaged dose equivalent and biological dose showed a (slight)
decrease starting from a shield thickness of 2 g/cm2, probably due the lower LET of projectile fragments.