Mathematical simulation of proton tracks in water vapor and their microdosimetric analysis.

The spatial pattern of primary physical events was calculated for protons in water vapor by means of a Monte Carlo program. Two different cross section data sets were used to cover the proton energy range from 0.2 to 15 MeV. From the spatial pattern of primary energy deposition, proximity functions were derived and from these the dose mean lineal energy yD was calculated. The contributions of different track components to yD for spherical target volumes of 1-100 nm were analysed. The results are compared with the LET approximation and with analytical calculations of yD based on expectation values of the radial energy deposition around the proton path (radial dose profiles). Finally the associated volume of proton tracks was calculated using the so called 'linear approximation', and energy deposition distributions were derived. These were compared with distributions calculated by means of restricted-LET.