Stochastic aspects and uncertainties in the prechemical and chemical stages of electron tracks in liquid water: a quantitative analysis based on M.C. simulations.

A new physical module for the biophysical
simulation code PARTRAC has recently been developed,
based on newly derived electron inelastic-scattering
cross-sections in liquid water. In the present work, two
modules of PARTRAC describing the production, diffusion
and interaction of chemical species were developed
with the specific purpose of quantifying the role of the
uncertainties in the parameters controlling the early stages
of liquid water radiolysis. A set of values for such parameters
was identified, and time-dependent yields and
frequency distributions of chemical species produced by
electrons of different energies were calculated. The calculated
yields were in good agreement with available data
and simulations, thus confirming the reliability of the
code. As the primary-electron energy decreases down to
1 keV, 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
hundred eV, due to the very small number of species involved.
The spreading effects shown at long times by
•OH frequency distributions following 1 keV irradiation
were found to be essentially due to stochastic aspects of
the chemical stage, whereas for 1 MeV tracks the physical
and pre-chemical stages also were found to play a
significant role. Relevant differences in the calculated
eaq
– yields were found by coupling the physics of PARTRAC
with descriptions of the pre-chemical and chemical
stages adopted in different models. This indicates a strict
interrelation of the various stages, and thus a strong dependence
of the parameter values on the assumptions
made for the preceding and subsequent stages of the process.Although equally acceptable results can be obtained
starting from different assumptions, it is necessary
to keep control of such uncertainties, since they can significantly
influence the modeling of radical attack on
DNA and, more generally, radiobiological damage estimation.
This study confirms the need for new, independently
derived data on specific steps of water radiolysis,
to be included in comprehensive biophysical simulation
codes