Check of the scaling procedure of track structures of ionizing radiation in nanometric volumes

In nanodosimetry, the track structure of ionizing radiation is characterized by the probability distribution of the number of ionizations produced by an ionizing particle in a target volume. By Monte-Carlo simulations of the track structure, this so called ionization cluster size distribution can be determined for nanometric cylindrical water targets used as substitute for the radiation sensitive biological target, namely a DNA-segment of about 20 base pairs length. Measurements of ionization cluster size distributions are carried out in an ion-counting nanodosimeter filled with an appropriate working gas at low pressure. A scaling procedure was proposed by Grosswendt (2006) to derive an operating pressure depending on the working gas for which the measured ionization cluster size distribution becomes equivalent to the ionization cluster size distribution within a nanometric target of liquid water. This scaling procedure was previously tested by means of Monte-Carlo simulations and is now tested experimentally.Ionization cluster size distributions produced by mono-energetic proton and alpha particle beams in the energy range between 0.1 MeV and 20 MeV were measured at the accelerator facilities of the PTB. The working gases used in this experiment were C₃H₈ and N₂. According to the NIST databases for stopping power and mean ionization energy (NIST, 2009), equivalent cluster size distributions for protons and alpha particles should be obtained for pressures of 0.25 mbar C₃H₈ and 1.2 mbar N₂. Measurements reveal the best agreement with pressures of 0.425 mbar C₃H₈ and 1.2 mbar N₂ for protons and 0.46 mbar C₃H₈ and 1.2 mbar N₂ for alpha particles.