Chemistry induced during the thermalization and transport of positrons and secondary electrons in gases and liquids

The recent availability of cross sections for positron (and positronium) interactions has made it possible to calculate transport properties and rates of collisions, and study in a quantitative fashion some aspects of positron-induced processes and their effects on living tissue. This paper models the interaction of primary positrons, and their secondary electrons, with water vapour (and subsequently liquid) using complete sets of cross sections predominately based on experimental binary collision data. We use a simple procedure to represent the presence of organic molecules where we look for dissociation of methane as a prototype of organic molecule dissociation. We isolate this particular process in order to establish whether the degree of damage is directly associated with the energy deposited in the tissue or whether some specific processes may cause excessive damage even with little energy deposition. We thus report on the relative contributions of initial positrons and secondary electrons in inducing dissociation, the spatial and energy profiles of individual collisional events, and positron/secondary electron tracks. It was found that secondary electrons induce 2-3 times more dissociations than the original positrons and with a longer range.