Radiolysis of liquid water at temperatures up to 300 degrees C: A Monte Carlo simulation study

Monte Carlo simulations were performed to calculate the temperature depende nce of the primary yields (g-values) of the radical and molecular products of the radiolysis of pure, deaerated liquid water by low linear-energy-tran sfer (LET) radiation. The early energy deposition was approximated by consi dering short segments (similar to 100 mum) of 300-MeV proton tracks (corres ponding to an average LET of similar to0.3 keV/mum). The subsequent nonhomo geneous chemical evolution of the reactive species formed in these tracks w as simulated by using the independent reaction times approximation, which h as previously been used successfully to model the radiolysis of liquid wate r at ambient temperature under various conditions. Our calculated g-values for the radiolytic species: e(aq)(-), OH, H, H-2, and H2O2, are presented a s a function of temperature over the range 25-300 degreesC. They show an in crease in g(e(aq)(-)), g(OH), and [g(H) + g(H-2)] and a decreasein g(H2O2) With increasing temperature, in agreement with existing experimental data. The sensitivity of the results to the values of reaction rate constants and to the temperature dependence of electron thermalization distances (r(th)) was also investigated. It was found that the best agreement with experimen t occurs when the distances of electron thermalization decrease with increa sing temperature, a result that is at variance with the predictions of prev ious modeling studies. Such a decrease in r(th) as the temperature increase s could be linked to an increase in the scattering cross sections of subexc itation electrons that would account for the corresponding decrease in the degree of structural order of water molecules. Our simulations also suggest that the variations of the g-values with temperature, and especially that of g(H-2), are better described if we account for the screening of the Coul omb forces between the two e(aq)(-) in the bimolecular self-reaction of the hydrated electron. Finally, the time-dependent yields of e(aq)(-) and OH a re presented as functions of temperature, in the range 10(-12)-10(-6) s. It was found that the temporal variation of g(e(aq)(-)) at elevated temperatu res is sensitive to the temperature dependence of r(th), suggesting that me asurements of the decay of hydrated electrons as functions of time and temp erature could, in turn, provide information on the thermalization of subexc itation electrons. The good overall accord of our calculated results with t he experimental data available from the literature demonstrates that Monte Carlo simulation methods offer a most promising avenue at present to furthe r develop our understanding of temperature effects in the radiolysis of liq uid water.