High-dose-rate effects in the radiolysis of water at elevated temperatures.

Monte Carlo track chemistry simulations were used to study the effects of high dose rates on the radical (e-aq, H•, and •OH) and molecular (H2 and H2O2) yields in the low linear energy transfer (LET) radiolysis of liquid water at elevated temperatures between 25–350 °C. Our simulation model consisted of randomly irradiating water by single pulses of N incident protons of 300 MeV (LET ~ 0.3 keV/μm), which penetrate at the same time perpendicular to this water within the surface of a circle. The effect of dose rate was studied by varying N. Our simulations showed that, at any given temperature, the radical products decrease with increasing dose rate and, at the same time, the molecular products increase, resulting from an increase in the inter-track, radical-radical reactions. Using the kinetics of the decay of hydrated electrons at 25 and 350 °C, we determined a critical time (τc) for each value of N, which corresponds to the “onset” of dose-rate effects. For our irradiation model, τc was inversely proportional to N for the two temperatures considered, with τc at 350 °C being shifted by an order of magnitude to shorter times compared to its values at 25 °C. Finally, the data obtained from the simulations for N = 2,000 generally agreed with the observation that during the track stage of the radiolysis, free radical yields increase, while molecular products decrease with increasing temperature from 25 to 350 °C. The exceptions of e-aq and H2 to this general pattern are briefly discussed.