It has been observed that healthy tissues are spared at ultra-high dose rate (UHDR: >40 Gy/s), so called FLASH effect. To elucidate the mechanism of FLASH effect, we evaluate changes in radiation chemical yield (G value) of 7-hydroxy-coumarin-3-carboxylic acid (7OH-C3CA), which is formed by the reaction of hydroxyl radicals with coumarin-3-carboxylic acid (C3CA), under carbon ions (140 MeV/u) and protons (27.5 and 55 MeV) in a wide-dose-rate range up to 100 Gy/s. The relative G value, which is the G value at each dose rate normalized by that at the conventional dose (CONV: 0.1 Gy/s >), 140 MeV/u carbon-ion beam is almost equivalent to 27.5 and 55 MeV proton beams. This finding implies that UHDR irradiations using carbon-ion beams have a potential to spare healthy tissues. Furthermore, we evaluate the G value of 7OH-C3CA under the de-oxygenated condition to investigate roles of oxygen to the generation of 7OH-C3CA effect. The G value of 7OH-C3CA under the de-oxygenated condition is lower than that under the oxygenated condition. The G value of 7OH-C3CA under the de-oxygenated condition is higher than those under UHDR irradiations. By direct measurements of the oxygen concentration during 55 MeV proton irradiations, the oxygen concentration drops by 0.1%/Gy, which is independent of the dose rate. When the oxygen concentration directly affects to yields of 7OH-C3CA, the rate of decrease in the oxygen concentration may be correlated with that of decrease in the G value of 7OH-C3CA. However, the reduction rate of G value under UHDR is significantly higher than the oxygen consumption. This finding implied that the influence of the reaction between water radiolysis species formed by neighborhood tracks could be strongly related to the mechanisms of UHDR effect.

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