Workers in accelerator facilities, aircraft crews, and astronauts are subjected to exposure of high-energy radiations. Among them, astronauts are irradiated by not only neutrons and protons but also heavy ions - nuclei of atoms heavier than hydrogen, to which we have no chance to be exposed on the earth's surface (Fig. 3-8). The biological effects of radiation depend on the distribution of ionized energies in a human body. It is well known that heavy ions can cause severe effects because of their greater ionizing power compared with protons, but the severity of this effect is unknown.
Why has the effect not been estimated precisely until now? The answer is that it was very difficult to analyze the motions of heavy ions in the human body because of their induced complex nuclear reactions, and consequently, it was impossible to evaluate the radiation dose - an indispensable quantity to estimate the biological effect of radiation - due to the irradiation of heavy ions. This difficulty has been overridden by developing a sophisticated simulation code that is capable of analyzing the motion of heavy ions in three-dimensional geometry. Using this code, we evaluated the radiation dose in a human body irradiated by heavy ions with unit fluence - dose conversion coefficient - by analyzing the energy deposition process in the body. Based on the results, we proposed a simple fitting formula that can predict the dose conversion coefficient for all heavy ions in space.As examples, the dose conversion coefficients for protons and helium (He), carbon (C), and iron (Fe) ions are depicted in Fig. 3-9.
The radiation doses for astronauts can be easily evaluated by employing the dose conversion coefficients (Fig. 3-10). This study can be regarded as the first step to elucidate biological effects of exposure to space radiations on astronauts, which should be solved before attempting long-term space missions such as manned Mars exploration.
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