Astronaut dose coefficients calculated using GEANT4 and comparison with ICRP123.

Fluence-to-dose conversion coefficients are fundamental ingredients to calculate astronaut radiation dose in space. For this purpose, the conversion coefficients for isotropic radiation provided by the International Commission on Radiological Protection in Publication 123 (ICRP123) are widely used. Understanding the uncertainties in these coefficients is important for a precise calculation of radiation dose. In this work, we present a systematic study of unshielded dose coefficients calculated by means of the GEANT4 Monte Carlo simulation toolkit and the human voxel phantoms defined in ICRP Publication 110. Four GEANT4 physics lists, featured with two variations of electromagnetic and two variations of hadronic interaction models, were used in the study. Absorbed dose and dose equivalent coefficients with both the ICRP60 and NASA quality factors were calculated, for individual cosmic nuclei with charge from Z $=$ 1 to Z $=$ 28 and a kinetic energy range from 1 MeV/n to 100 GeV/n. The effective dose equivalent rates in free space at 1 AU were then calculated for each set of dose coefficients. The four effective dose equivalent rates calculated with each physics list agreed within $\pm 3\%$ , and on average they were larger than the ICRP123 results by $\sim 7\%$ and $\sim 1\%$ using the ICRP60 and the NASA quality factor, respectively. These results shed light on the systematic uncertainty of astronaut radiation exposure calculation, particularly from the physics interaction models.