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

低能质子和中子的相对生物效应（RBE）研究引起了许多科学领域的极大兴趣。在布拉格峰的能量范围内，质子的RBE预期增加的原因仍然是科学讨论的问题。在这个机构里，我们用中子发生器和Cf-252裂变中子辐照器进行了研究。在广岛、长崎原子弹爆炸和东海村（JCO）事故中都经历了中子的影响。然而，中子对人类的影响还不是很清楚。我们使用筑波大学的2MV串联加速器进行研究。利用平行平板离子室和硅表面势垒探测器对质子束进行了测量，并在小细胞水平上研究了用于DNA链断裂研究的剂量学。另一个目的是为研究RBE值随初始质子能量的变化而变化，提供利用减速质子轨道结构对V79细胞辐照进行定量建模和模拟初始DNA断裂的理论结果。我们模拟了DNA断裂的产量：SSB和DSB的复杂性分类为直接影响DNA片段的质子能量的函数。结果清楚地说明了以下陈述：(i)细胞致死（转化）的敏感部位位于细胞核中；（ii）复杂的DSB是导致细胞杀伤或转化的最相关的初始病变。在整个V79细胞核中最有效地传递吸收能量的是入射能量为550 keV的质子。在这种情况下，200 keV的质子被发现是最有效的，以诱导DNA的复杂双链断裂，直接影响DNA。计算结果均在RBEMAX实验值范围内。

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.