Development of low-energy proton irradiation system for radiobiology study of neutrons

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Biological effects of neutrons are relatively large comparing with gamma rays and X rays. Their biological effects are not related directly to neutrons themselves but to the secondary charged particles, which were induced by neutrons. The factors are usually explained by "the relative biological effectiveness (RBE)" and are 7-10 times sometimes even 20 times larger than those of gamma rays and X rays. The reasons are that the 70 to 80% of the neutron doses are given by recoil protons. However, the experimental data of the RBE values of protons are 1.0 to 1.2, therefore our purpose of this study is to find the reason of the high RBE values of neutrons. We considered the possibility is in the protons when they loose energy and stop. At this moment the RBE values may become very high. To verify this assumption we used 3 MeV accelerator (HIRRAC) at Research Institute for Radiation Biology and Medicine, Hiroshima University, which was installed in 1995. We made proton beam extraction window from vacuum to air using a 5 micron meter thick Mylar foil or 2.2 micron meter Harvar alloy foil. To measure proton beam to estimate doses, we used plastic scintillation counter. The whole system was successful. We extracted proton beam and measured proton beam energy and fluence. The final purpose of the study was to make biological experiment using cells, however, the exposure system is now under construction and planning to perform the experiment. We are making effort to complete this study.