Microdosimetric event size distributions in small sites in water vapour irradiated by protons

The author describes a calculation of the statistical distribution of energy deposition events in small spherical sites in water (with diameters of 1 mu m or smaller in unit density material) irradiated by protons. The calculation is based on the use of a semi-empirical differential ionisation cross section for water vapour given by Wilson and Miller, and takes into account proton energy loss straggling and the transport of energy by secondary electrons. Results are given for irradiations by monoenergetic protons with energies from 1 to 20 MeV, and for irradiations with protons set in motion by interactions of 14 and 20 MeV neutrons with hydrogen.