Study of mechanism of radiation effects for low energy protons by the irradiation of neutrons

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The study of relative biological effectiveness (RBE) of low-energy protons and neutrons attracts considerable interest for many scientific fields. The reason of expected increasing the RBE of protons in the energy range of the Bragg peak is still the matter of scientific discussion. In this institution we have been studied using the neutron generator and the Cf-252 fission neutron irradiator. The effects of neutrons were experienced in the cases of Hiroshima and Nagasaki atomic bomb and Tokai mura (JCO) accident. However effects of neutrons for people are not known well. We studied using the 2MV tandem accelerator at Tsukuba University. By using a parallel plate ion chamber and a silicon surface barrier detector we measured proton beam and studied the dosimetry in the small size like cell level to use for the study of DNA strand breaks. The other purpose was to present theoretical results on quantitative modeling of irradiation of V79 cells using track structure of slowing down protons and simulation of initial DNA breaks for investigation a variation of RBE values as a function of initial proton energy. We simulated yields of DNA breaks: SSB and DSB classified on complexity as a function of energy of proton affected straightforwardly on a DNA segment. The results are the clear illustration of the following statements: (i) sensitive sites for cell lethality (transformation) are located in the cell nucleus and (ii) the complex DSB are the most relevant initial lesions leading to cell killing or transformation. The most efficient for delivering absorbed energy in whole V79 cell nucleus are the protons with incident energy of 550 keV. At that the protons with 200 keV were found to be the most efficient in order to induce a complex double strand breaks of DNA affecting DNA straightforwardly. Those calculated numbers are in the range of experimental RBEMAX values.