Density of Hydroxyl Radicals Generated in an Aqueous Solution by Irradiating Carbon-Ion Beam

The density of hydroxyl radicals (•OH) produced in aqueous samples by exposure to X-ray or carbon-ion beams was investigated. The generation of •OH was detected by the electron paramagnetic resonance (EPR) spin-trapping technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin-trapping agent. When the concentration of DMPO is in excess of the generated •OH, the production of DMPO-OH (spin-trapped •OH) should be saturated. Reaction mixtures containing several concentrations (0.5–1685 mM) of DMPO were then irradiated by a 32 Gy 290 MeV carbon-ion beam (C290-beam) or X-ray. C290-beam irradiation was performed at the Heavy-Ion Medical Accelerator in Chiba (HIMAC, National Institute of Radiological Sciences, Chiba, Japan), applying different linear energy transfers (LET) (20 to 169 keV/μm). The amount of DMPO-OH in the irradiated samples was detected by EPR spectroscopy. The generation of DMPO-OH increased with the concentration of initial DMPO, displayed a shoulder around 3.3 mM DMPO, and reached a plateau. This plateau suggests that the generated •OH were completely trapped and able to be detected. A steep linear increase in DMPO-OH measured in solutions with higher DMPO concentrations suggested very dense •OH generation (> 1.7 M). Generation of •OH is expected to be localized on the track of the radiation beam, because the maximum concentration of measured DMPO-OH concentration was 40 μM. These results suggested that both sparse (≈ 3.3 mM) and dense (> 1.7 M) •OH generation occurred in the irradiated samples. The percentage of dense •OH generation increased with increasing LET. Different types of dense •OH generation may be expected for X-ray and C290-beams