The effect of linear energy transfer on the early, transient radiolytic oxygen depletion in the radiolysis of water by high-dose-rate irradiating protons

Monte Carlo multi-track chemistry simulations were carried out to study, from a radiation chemistry perspective, the effect of “linear energy transfer” (LET) on the transient yields and concentrations of radiolytic oxygen consumption in the high-dose-rate (∼107 Gy/s) radiolysis of both pure air-saturated (0.25 mmol/L O2) and oxygenated (30 µmol/L O2) cell water, in the interval ∼1 ps–10 µs. Our simulation model consisted of randomly irradiating water with single pulses of 5 MeV (LET ∼ 8 keV/µm), 1.5 MeV (LET ∼ 19.5 keV/µm), and 0.7 MeV (LET ∼ 33 keV/µm) protons at 25 °C. Similar to what is observed with low-LET irradiation (∼300 MeV protons, LET ∼ 0.3 keV/µm), our calculations showed that, in pure, aerated water, the concentration of depleted oxygen, [−O2], exhibits a pronounced maximum around ∼0.1–0.2 µs for all three high-LET irradiating protons studied. This maximum increased markedly with increasing LET. As expected, the effect of adding competing scavengers of both hydrated electrons and •OH radicals on the radiolytic O2 depletion in oxygenated cell water (a more bio-mimetic model of cells) irradiated by 5 MeV protons delivered at the same dose rate led to a marked decrease in the maximum of [−O2] around 1 µs. However, contrary to what is observed for low-LET irradiation, we found that the transient O2 consumption is quite substantial under high-LET irradiation conditions. This is explained by the fact that, even though their underlying mechanism of action differs, high-LET particles affect radiolysis yields in a similar way to high dose rates.