An event-by-event computer simulation of interactions of energetic chargedparticles and all their secondary electrons in water

A computer code simulating the interaction-by-interaction discrete slowing down process of heavy charged particles (i.e. protons, alpha-particles, and bare heavier ions of z < 10) of speeds greater than about 0.3-1 MeV amu(-1 ) in water medium is presented. Along with the primary particle, the transp ort of all secondary (and subsequent generations) electrons is explicitly f ollowed down to the minimum electronic excitation potential of a water mole cule. Monte Carlo sampling techniques were applied to accurate probability distribution functions characterizing the particle-molecule interaction. Th e entire secondary electron energy spectrum has been modelled by a semi-emp irical approximation combining the leading term of Bethe's asymptotic cross section with an appropriate binary collision formula. The elastic scatteri ng cross section was modelled by a modified Rutherford formula that account s for screening, while impact excitation was modelled by an empirical formu la which exhibits the correct (Born-Bethe) asymptotic behaviour. Binding ef fects for the most important molecular transitions are explicitly accounted for by partial inelastic cross sections. By following the histories of all particles, the code scores the spatial coordinates of all elastic and inel astic collision events, as well as the partitioning of energy loss to the v arious inelastic channels.