Monte Carlo calculation of the primary radical and molecular yields of liquid water radiolysis in the linear energy transfer range 0.3-6.5 keV/mu m: Application to Cs-137 gamma rays

Monte Carlo simulations of the radiolysis of neutral liquid water and 0.4 M H2SO4 aqueous solutions at ambient temperature are used to calculate the v ariations of the primary radical and molecular yields (at 10(-6) s) as a fu nction of linear energy transfer (LET) in the range similar to0.3 to 6.5 ke V/mum. The early energy deposition is approximated by considering short (si milar to 20-100 mum) high-energy (similar to 300-6.6 MeV) proton track segm ents, over which the LET remains essentially constant. The subsequent nonho mogeneous chemical evolution of the reactive species formed in these tracks is simulated by using the independent reaction times approximation, which has previously been used successfully to model the radiolysis of water unde r various conditions. The results obtained are in good general agreement wi th available experimental data over the whole LET range studied. After norm alization of our computed yields relative to the standard radical and molec ular yields for Co-60 gamma radiation (average LET similar to0.3 keV/mum), we obtain empirical relationships of the primary radiolytic yields as a fun ction of LET over the LET range studied. Such relationships are of practica l interest since they allow us to predict a priori values of the radical an d molecular yields for any radiation from the knowledge of the average LET of this radiation only. As an application, we determine the corresponding y ields for the case of Cs-137 gamma radiation. For this purpose, we use the value of similar to0.91 keV/mum for the average LET of Cs-137 gamma rays, c hosen so that our calculated yield G(Fe3+) for ferrous-ion oxidation in air -saturated 0.4 M sulfuric acid reproduces the value of 15.3 molecules/100 e V for this radiation recommended by the International Commission on Radiati on Units and Measurements. The uncertainty range on those primary radical a nd molecular yields are also determined knowing the experimental error (sim ilar to2%) for the measured G(Fe3+) value. The following values (expressed in molecules/100 eV) are obtained: (1) for neutral water: G(e-aq) = 2.50 +/ - 0.16, G(H .) = 0.621 +/- 0.019, G(H2) = 0.474 +/- 0.025, G(. OH) = 2.67 /- 0.14, G(H2O2) = 0.713 +/- 0.031, and G(-H2O) = 4.08 +/- 0.22; and (2) fo r 0.4 M H2SO4 aqueous solutions: G(H .) = 3.61 +/- 0.09, G(H2) = 0.420 +/- 0.019, G(. OH) = 2.78 +/- 0.12, GH(2)O(2) = 0.839 +/- 0.037, and G(-H2O) = 4.46 +/- 0.16. These computed values are found to differ from the standard yields for Co-60 gamma rays by up to similar to6%. (C) 2001 by Radiation Re search Society.