Meesungnoen, J., Benrahmoune, M., Filali-Mouhim, A., Mankhetkorn, S. and Jay-Gerin, J-P. 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/μm: Application to 137Cs 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 variations of the primary radical and molecular yields (at 10–6 s) as a function of linear energy transfer (LET) in the range ∼0.3 to 6.5 keV/μm. The early energy deposition is approximated by considering short (∼20–100 μm) high-energy (∼300–6.6 MeV) proton track segments, over which the LET remains essentially constant. The subsequent nonhomogeneous 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 under various conditions. The results obtained are in good general agreement with available experimental data over the whole LET range studied. After normalization of our computed yields relative to the standard radical and molecular yields for 60Co γ radiation (average LET ∼0.3 keV/μm), we obtain empirical relationships of the primary radiolytic yields as a function of LET over the LET range studied. Such relationships are of practical interest since they allow us to predict a priori values of the radical and molecular yields for any radiation from the knowledge of the average LET of this radiation only. As an application, we determine the corresponding yields for the case of 137Cs γ radiation. For this purpose, we use the value of ∼0.91 keV/μm for the average LET of 137Cs γ rays, chosen 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 eV for this radiation recommended by the International Commission on Radiation Units and Measurements. The uncertainty range on those primary radical and molecular yields are also determined knowing the experimental error (∼2%) for the measured G(Fe3 ) value. The following values (expressed in molecules/100 eV) are obtained: (1) for neutral water: Ge–aq = 2.50 ± 0.16, GH· = 0.621 ± 0.019, GH2 = 0.474 ± 0.025, G·OH = 2.67 ± 0.14, GH2O2 = 0.713 ± 0.031, and G–H2O = 4.08 ± 0.22; and (2) for 0.4 M H2SO4 aqueous solutions: GH· = 3.61 ± 0.09, GH2 = 0.420 ± 0.019, G·OH = 2.78 ± 0.12, GH2O2 = 0.839 ± 0.037, and G–H2O = 4.46 ± 0.16. These computed values are found to differ from the standard yields for 60Co γ rays by up to ∼6%.
