THE BACKSCATTERING OF LOW-ENERGY PROTONS IN GASES OF IMPORTANCE FOR IONIZATION MEASUREMENTS

For gases frequently employed in ionisation chambers and proportional counters, in the energy range between 1 keV and 100 keV, the backscatt ering of protons has been studied experimentally by using a plane para llel ionisation chamber and theoretically by performing Monte Carlo pr oton transport simulations. Three types of backscatter factors were in vestigated: (i) the backscatter factor with respect to the ionisation yield, (ii) the energy absorption, and (iii) the number of primary par ticle 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 interacti on events is exclusively determined by elastic scattering. A compariso n of the calculated and experimental backscatter factors for nitrogen, carbon dioxide, methane, ethane, propane, butane, and two tissue-equi valent gases based on methane or propane confirms the validity of the concept, at least within the uncertainties of the experimental and the oretical data.