Monte carlo simulation of fast electron and proton tracks in liquid water-II. Nonhomogeneous chemistry

Monte Carlo simulation techniques based on the independent reaction times (IRT) approximation are used to model the nonhomogeneous chemistry that takes place between 10 super(-12) and approximately 10 super(-6) s in the tracks that are generated by ionizing radiations in liquid water. Our simulation code TRACIRT, whose execution follows that of the codes TRACPRO and TRACELE described in the preceding paper, allows one to account adequately for the stochastic nature of the occurrence of the reactions. In addition to its accuracy and rapidity, the IRT approach also allows one to incorporate the most fundamental elements that control the reaction kinetics, that is, the Brownian diffusion of the reactive species, their mutual Coulombic interactions, the effects of radical spin correlations, and the activated processes that make the reactions only partially diffusion-controlled. The code TRACIRT is able to simulate the nonhomogeneous chemistry that pertains to proton and electron tracks of various linear energy transfers (LET) ranging from approximately 0.3 to 20 keV mu m super(-1). The time dependence that we obtain for the radiolytic yields compares well with the observed values. The simulations also provide valuable information on the time range over which the different reactions take place and on their relative importance in the resulting global chemical transformation. The use of short segments of proton tracks allows one to vary systematically the LET (through appropriate choices of the incident proton energy) and to characterize quantitatively its influence on the time-dependent yields.