Effect of multiple ionization on the yield of H2O2 produced in the radiolysis of aqueous 0.4 M H2SO4 solutions by high-LET 12C6+ and 20Ne9+ ions

Monte Carlo track structure simulations were performed to investigate the effect of multiple ionization of water on the primary (or "escape") (at approximately 10(-6) s) yield of hydrogen peroxide (G(H2O2)) produced in the radiolysis of deaerated 0.4 M H2SO4 solutions by 12C6+ and 20Ne9+ ions at high linear energy transfer (LET) up to approximately 900 keV/microm. It was found that, upon incorporating the mechanisms of double, triple and quadruple ionizations of water in the calculations, a quantitative agreement between theory and experiment can be obtained. The curve for G(H2O2) as a function of LET reaches a well-defined maximum of approximately 1.4 molecules/100 eV at approximately 180-200 keV/microm, in very good accord with the available experimental data. Our results also show that, for the highest LET values considered in this study, the H2O2 escape yields obtained in 0.4 M sulfuric acid solutions are about 45% greater in magnitude than those found in neutral water. Contrary to a recent assumption suggesting that the limiting value of G(H2O2) at infinite LET should be approximately 1 molecule/100 eV, somewhat similar for neutral and acidic water, our simulations show a clear decrease in the primary H2O2 yields with increasing LET at high LET, indicating that the question of the limiting value of G(H2O2) at very high LET for both neutral and acidic liquid water is still open.