Fluence-to-dose conversion coefficients for space dosimetry calculated using PHITS and the ICRP/ICRU adult reference computational phantoms.

Astronauts are subjected to be exposed to a variety of cosmic-rays such as neutrons produced inside spacecraft as well as protons and heavy ions over wide energy ranges.  Evaluation of dose for astronauts due to the cosmic-ray exposure is thus of great importance in the planning of long-term space missions. According to the NCRP report 153, the organ dose equivalent, which can be calculated from the dose distribution in terms of LET coupled with the Q(L) relationship defined in ICRP60, may be used as a surrogate for the equivalent dose for space dosimetry. We therefore calculated the conversion coefficients from fluence to organ dose equivalents for neutrons, protons and heavy ions up to Z=28 (Ni) and E=100 MeV/nucleon for the isotropic irradiation geometry, using the PHITS code [1] and the ICRP/ICRU adult reference computational phantoms [2], under the supervision of ICRP Task Group on Dose Calculations (DOCAL). Additionally, the conversion coefficients for dose equivalents obtained using the Q(y) relationship defined in ICRU40 instead of Q(L) were also calculated, where y is the lineal energy for the site diameter 1 mm. The Q(y)-based dose equivalent has a potential to be a key quantity in constructing a more sophisticated radiation protection system in space, since the cosmic-rays generally produce a large number of high-energy d-rays around their trajectories, and their radiation qualities cannot be uniquely determined from their LET. Note that the conversion coefficients for the Q(y)-based dose equivalents had never been calculated before due to the computational difficulty in estimating the probability density of absorbed dose in lineal energy y, d(y), in macroscopic matter. We solved this difficulty by providing PHITS with the capability of calculating d(y) by incorporating the results of microdosimetric track-structure simulation into the code [3]. The calculated conversion coefficients for neutrons and protons were given elsewhere [4]. Hence, we will show the results for heavy ions at the meeting, together with their mean quality factors calculated using the Q(L) and Q(y) relationships