Simulations of MATROSHKA experiment outside the ISS using PHITS

The radiation environment at the altitude of the International Space Station (ISS) is substantially different than anything typically encountered on Earth in both the character of the radiation field and the significantly higher dose rates. Concerns about the biological effects on humans of this highly complex natural radiation field are increasing due to higher amount of astronauts performing long-duration missions onboard the ISS and especially if looking into planned future manned missions to Mars. In order to begin the process of predicting the dose levels seen by the organs of an astronaut, being the prerequisite for radiation risk calculations, it is necessary to understand the character of the radiation environment both in- and outside of the ISS as well as the relevant contributions from the radiation field to the organ doses. In this paper the three-dimensional Monte Carlo Particle and Heavy Ion Transport code System (PHITS) and a voxel-based numerical human model NUNDO (Numerical RANDO) were used to estimate the radiation load of human organs during a long term activity outside the ISS. The baseline measured data was generated with the MATROSHKA-1 (MTR-1) experiment taking place from February 2004 up to October 2005 outside the Russian Zvezda module of the ISS, thereby simulating a long term extravehicular activity (EVA) of an astronaut. The organ absorbed dose values calculated by PHITS for the inner organs are in a good agreement with the experimental data. However, a rather large disagreement was observed for the most outer organs. This disagreement appears to be due to the strong dependence that the thickness of the applied carbon fiber container, acting as the EVA suit of the astronaut, has on the effects caused by the trapped electron (TE) component. The organ dose equivalent values for the deeper organs are a factor of two lower than the experimental data. The detailed reason behind this is still under investigation