The backscattering of low energy protons in gases of importance for ionisation measurements

For gases frequently employed in ionisation chambers and proportional counters, in the energy range between 1 keV and 100 keV, the backscattering of protons has been studied experimentally by using a plane parallel ionisation chamber and theoretically by performing Monte Carlo proton transport simulations. Three types of backscatter factors were investigated: (i) the backscatter factor with respect to then ionisation yield, (ii) the energy absorption, and (iii) the number of primary particle endpoints. Whereas the first of these backscatter factors could be determined experimentally, the latter two were calculated assuming first that the energy loss due to inelastic proton interactions can be handled in the framework of the continuous slowing down approximation, and secondly that the spatial distribution of all possible interaction events is exclusively determined by elastic scattering. A comparison of the calculated and experimental backscatter factors for nitrogen, carbon dioxide, methane, ethane, propane, butane, and two tissue-equivalent gases based on methane or propane confirms the validity of the concept, at least within the uncertainties of the experimental and theoretical data. (author)