EPOTRAN: a full-differential Monte Carlo code for electron and positron transport in liquid and gaseous water

We describe here a novel full-differential Monte Carlo (MC) event-by-event simulation, for modelling electron and positron histories in liquid and gaseous water, with impact energies ranging from the water excitation threshold (7.4 eV) to 10 keV. This new track-structure code is named EPOTRAN, an acronym for Electron and POsitron TRANsport in water.

All the processes induced by both electrons and positrons are studied in detail via theoretical differential and total cross sections, calculated within the quantum mechanical framework by using the partial-wave method. Elastic and inelastic interactions are then successively reviewed, including in particular an original description of the positron-induced capture process leading to Positronium formation.

Total and differential cross sections are reported and compared with a large set of existing measurements. Rather good agreement is generally observed over the considered energy range.

This work reports the theoretical cross sections used in a special purpose Monte Carlo simulation suitable for electron and positron transport in gaseous and liquid water. This MC code should represent an accurate tool for dose calculation at the nanometric scale, by providing a detailed spatial distribution of energy deposits. Furthermore, positron trajectory studies made possible by this approach should prove useful for evaluating the real contribution of the positron range on the overall spatial resolution of PET (Positron Emission Tomography) imaging.