Numerical simulation of multiple ionization and high LET effects in liquid water radiolysis

We present a complete simulation of liquid water radiolysis by swift ions that takes into account explicitly the multiple ionization of water molecules. For high linear energy transfer (LET), we show that this process is indeed not negligible with respect to single ionization, which controls water radiolysis in the low-LET range. As suggested qualitatively by several authors, the rearrangement of multiply ionized water molecule is consistent with the production of atomic oxygen. We present a predictive and quantitative analysis of the possible role of this atomic oxygen. All radicals generated during the physico-chemical phase are followed during the chemical phase by means of the independent reaction time (IRT) method. The comparison of the simulation with and without multiple ionizations shows that the latter are responsible for creation of a large amount of HO2 radicals and O-2 molecules. The simulated HO2 yield G(HO2) is in excellent agreement with direct optical measurements for Ar ions at 65 MeV/u. The variations of the yield GO(2) + G(HO2) versus LET are in satisfactory agreement with indirect measurements reported in the literature over a wide range of LET. The yield of H2O2 is also sensitive to multiple ionization for high LET projectiles, above 100 keV/mu m. On the contrary, for other species formed by radiolysis of water like OH, H-2 and e(aq)(-), the yield is almost unsensitive to our considerations regarding multiple ionization. For all species, we observe that two projectiles with the same LET but different velocities produce different yields because of differences in the track structure. (c) 2005 Elsevier Ltd. All rights reserved.