W value measurements for 241Am alpha particles in various gases

Measurements of W, the mean energy expended per ion pair formed, have been performed for 241Am alpha particles stopping in a number of the gases commonly used in ionisation chambers. Both absolute and relative measurements were made with nitrogen used as the reference gas for the relative measurements. The absolute method entailed a simultaneous measurement of a quantity of charge and the number of alpha particles which produced that charge, whilst the relative approach simply required a determination of the ratio of ion currents. Results were obtained for methane-based tissue-equivalent gas, for its constituents, i.e. methane, carbon dioxide and nitrogen, for argon, and also for a gas mixture with a composition approximately equivalent to A-150 plastic.