W-VALUES FOR LOW-ENERGY PROTONS IN METHANE-BASED TISSUE-EQUIVALENT GAS AND ITS CONSTITUENTS

The mean energy, W, expended per ion pair formed has been determined experimentally for protons completely stopped in methane-based tissue-equivalent gas (TE gas) and its constituents, methane, carbon dioxide and nitrogen, covering the energy range from 1 to 100 keV. The standard deviations of the W values obtained are less than 1%. Differential omega-values were derived from the energy dependence of W. While the W values for TE gas and methane decrease monotonously with increasing energy up to 100 keV, the results for carbon dioxide and nitrogen show a flat minimum around 20 keV. For practical application, analytical functions have been fitted to the experimental data. Using a simple additivity rule of the ionization yield, it is possible to determine the W values for this TE gas mixture from those of the pure gas components within the limits of experimental uncertainty. The present data are compared with other values available.