One of the most important steps in the near-future space age will be a crew mission returning to the Moon and even a manned mission to Mars. Unfortunately, such a mission will expose astronauts to unavoidable cosmic radiation in deep space and on the Martian or lunar surface. Thus, a better understanding of the radiation environment for such a mission and the consequent biological impacts on humans, in particular the human brains, is critical. The need for this better understanding is strongly suggested by investigations on animal models and on human patients who were undergoing irradiation for cancer therapy in the head. These have revealed unexpected alterations in the central nervous system behavior and sensitivity of mature neurons in the brain to charged particles. However, such experiments shall not be carried out realistically in space using humans. Therefore, to investigate the impact of cosmic radiation on human brains and the potential influence on the brain functions, we model and study the cosmic particle-induced radiation dose in a realistic head structure. Specifically speaking, 134 slices of computed tomography (CT) images of an actual human head have been used as a 3D phantom in Geant4 (GEometry ANd Tracking), which is a Monte Carlo tool for simulating energetic particles impinging into different parts of the brain and deliver radiation dose therein. As a first step, we compare the influence of different brain structures (e.g., with or without bones, with or without soft tissues) to the resulting dose therein to demonstrate the necessity of using a realistic brain structure for our investigation. Afterward, we calculate energy-dependent functions of dose distribution, for the most important (some of the most abundant and most biologically-relevant) particle types encountered during a deep space mission inside a spacecraft or habitat such as protons, helium ions, neutrons and some major heavier ions like carbon, nitrogen, and iron particles. Furthermore, two different scenarios have been modeled as a comparison: a human head without shielding protection and a human head with an aluminum shielding shell around (of varying thickness). These functions can then be used to fold with energetic cosmic-ray particle spectra of the ambient environment for obtaining the dose rate distribution at different lobes of the human brain. Our calculation of these functions can serve as a ready tool and a baseline for further evaluations of the radiation in the brain encountered during a space mission with different radiation fields, such as on the surface of the Moon or Mars.
Keywords: Brain simulation; Geant4; Radiation Risk; Space Exploration.
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