Stochastic aspects and uncertainties its the prechemical and chemical stages of electron tracks in liquid water: a quantitative analysis based on Monte Carlo simulations

A new physical module for the biophysical simulation code PARTRAC has recen tly been developed, based on newly derived electron inelastic-scattering cr oss-sections in liquid water. In the present work, two modules of PARTRAC d escribing the production, diffusion and interaction of chemical species wer e developed with the specific purpose of quantifying the role of the uncert ainties in the parameters controlling the early stages of liquid water radi olysis. A set of values for such parameters was identified, and time-depend ent yields and frequency distributions of chemical species produced by elec trons of different energies were calculated. The calculated yields were in good agreement with available data and simulations, thus confirming the rel iability of the code. As the primary-electron energy decreases down to 1 ke V, the . OH decay kinetics were found to get faster, reflecting variations in the spatial distribution of the initial energy depositions. In agreement with analogous works, an opposite trend was found for energies of a few hu ndred eV, due to the very small number of species involved. The spreading e ffects shown at long times by . OH frequency distributions following 1 keV irradiation were found to be essentially due to stochastic aspects of the c hemical stage, whereas for 1 MeV tracks the physical and pre-chemical stage s also were found to play a significant role. Relevant differences in the c alculated e(aq)-yields were found by coupling the physics of PARTRAC with d escriptions of the pre-chemical and chemical stages adopted in different mo dels. This indicates a strict interrelation of the various stages, and thus a strong dependence of the parameter values on the assumptions made for th e preceding and subsequent stages of the pro- cess. Although equally accept able results can be obtained starting from different assumptions, it is nec essary to keep control of such uncertainties, since they can significantly influence the modeling of radical attack on DNA and, more generally, radiob iological damage estimation. This study confirms the need for new, independ ently derived data on specific steps of water radiolysis, to be included in comprehensive biophysical simulation codes.