A Monte Carlo track structure code for electrons(~10 eV-10 keV) and protons (~0.3-10 MeV) in water:partitioning of energy and collision events

Abstract

An event-by-event Monte Carlo simulation code for track structure studies isdescribed. In the present form the code transports protons(~0.3-10 MeV) and electrons (~10 eV-10 keV) in a watermedium in the gas phase approximation. For the type of particles and energyrange considered, ionization, electronic excitation and electron elasticscattering are the most important collision events accounted for in thetransport simulation. Efforts were made to ensure that the analyticrepresentation of the various interaction cross sections rests on wellestablished experimental data and theory. For example, the secondary-electronspectrum as well as partial and total ionization cross sections arerepresented by a semitheoretical formulation combining Bethe's asymptoticexpansion and binary-encounter theory. Binding effects for five levels ofionization and eight levels of electronic excitation of the water molecule areexplicitly considered. The validity of the model cross sections is examinedagainst available experimental data and theoretical predictions from othersimilar studies. Results pertaining to the partitioning of energy loss andinteraction events for the first-collision probability and nanometre-sizetrack segments are presented.