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Self-reaction of the hydrated electron Rate constant Yield of H 2 Linear energy transfer (LET) Monte Carlo track chemistry calculationsIt has been a longstanding issue in the radiation chemistry of water that, even though H 2 is a molecular product, its "escape" yield g(H 2 ) increases with increasing temperature. A main source of H 2 is the bimolecular reaction of two hydrated electrons (e − aq ). The temperature dependence of the rate constant of this reaction (k 1 ), measured under alkaline conditions, reveals that the rate constant drops abruptly above ~150°C. Recently, it has been suggested that this temperature dependence should be regarded as being independent of pH and used in hightemperature modeling of near-neutral water radiolysis. However, when this drop in the e − aq self-reaction rate constant is included in low (isolated spurs) and high (cylindrical tracks) linear energy transfer (LET) modeling calculations, g(H 2 ) shows a marked downward discontinuity at ~150°C which is not observed experimentally. The consequences of the presence of this discontinuity in g(H 2 ) for both low and high LET radiation are briefly discussed in this communication. It is concluded that the applicability of the sudden drop in k 1 observed at ~150°C in alkaline water to near-neutral water is questionable and that further measurements of the rate constant in pure water are highly desirable.

The stochastic modeling of the (60)Co γ/fast-electron radiolysis of the ceric-cerous chemical dosimeter has been performed as a function of temperature from 25-350°C. The system used is a dilute solution of ceric sulfate and cerous sulfate in aqueous 0.4 M sulfuric acid. In this system, H(•) (or HO2(•) in the presence of dissolved oxygen) and H2O2 produced by the radiolytic decomposition of water both reduce Ce(4+) ions to Ce(3+) ions, while (•)OH radicals oxidize the Ce(3+) present in the solution back to Ce(4+). The net Ce(3+) yield is given by G(Ce(3+)) = g(H(•)) + 2 g(H2O2) - g((•)OH), where the primary (or "escape") yields of H(•), H2O2 and (•)OH are represented by lower case g's. At room temperature, G(Ce(3+)) has been established to be 2.44 ± 0.8 molecules/100 eV. In this work, we investigated the effect of temperature on the yield of Ce(3+) and on the underlying chemical reaction kinetics using Monte Carlo track chemistry simulations. The simulations showed that G(Ce(3+)) is time dependent, a result of the differences in the lifetimes of the reactions that make up the radiolysis mechanism. Calculated G(Ce(3+)) values were found to decrease almost linearly with increasing temperature up to about 250°C, and are in excellent agreement with available experimental data. In particular, our calculations confirmed previous estimated values by Katsumura et al. (Radiat Phys Chem 1988; 32:259-63) showing that G(Ce(3+)) at ∼250°C is about one third of its value at room temperature. Above ∼250°C, our model predicted that G(Ce(3+)) would drop markedly with temperature until, instead of Ce(4+) reduction, Ce(3+) oxidation is observed. This drop is shown to occur as a result of the reaction of hydrogen atoms with water in the homogeneous chemical stage.