Effects of LET on oxygen-dependent and-independent generation of hydrogen peroxide in water irradiated by carbon-ion beams

Linear energy transfer (LET) dependence of yields of O₂-dependent and O₂-independent hydrogen peroxide (H₂O₂) in water irradiated by ionizing radiation was investigated. The radiation-induced hydroxyl radical (•OH) generation in an aqueous solution was reported to occur in two different localization densities, the milli-molar (relatively sparse) and/or molar (markedly-dense) levels. In the milli-molar-level •OH generation atmosphere, •OH generated at a molecular distance of ∼7 nm are likely unable to interact. However, in the molar-level •OH generation atmosphere, several •OH were generated with a molecular distance of 1 nm or less, and two •OH can react to directly make H₂O₂. An aliquot of ultra-pure water was irradiated by 290-MeV/nucleon carbon-ion beams at the Heavy-Ion Medical Accelerator in Chiba (HIMAC, NIRS/QST, Chiba, Japan). Irradiation experiments were performed under aerobic or hypoxic (<0.5% oxygen) conditions, and several LET conditions (13, 20, 40, 60, 80, or >100 keV/μm). H₂O₂ generation in irradiated samples was estimated by three methods. The amount of H₂O₂ generated per dose was estimated and compared. O₂-independent H₂O₂ generation, i.e. H₂O₂ generation under hypoxic conditions, increased with increasing LET. On the other hand, the O₂-dependent H₂O₂ generation, i.e. subtraction of H₂O₂ generation under hypoxic conditions from H₂O₂ generation under aerobic conditions, decreased with increasing LET. This suggests that the markedly-dense •OH generation is positively correlated with LET. High-LET beams generate H₂O₂ in an oxygen-independent manner.