A new technique for studying the Fano factor and the mean energy per ion pair in counting gases

A new method is presented for deriving the Fano factor and the mean energy per ion pair in the ultrasoft x-ray energy range. It is based on counting electrons deposited by a photon in a low- pressure gas, and is applicable for all counting gases. The energy dependence of these parameters for several hydrocarbons and gas mixtures is presented. A fundamental topic in radiation physics and dosimetry deals with the number of ion pairs generated in matter and its fluctuation, following the passage of an ionizing particle or the absorption of an energy quantum. In the latter, the dissipation of energy released by a soft x-ray photon involves the photoelectric eect and its related processes: the Auger/Coster-Kronig transitions, fluorescence , electron shake-up and electron shake-o, as well as further gas ionization by the photo/Auger electrons. The x-ray energy absorption can be described by two energy-dependent parameters: the mean energy per ion pair (Wi) and the Fano factor (F). The mean energy per ion pair is dened by Wi=E/n where E is the deposited energy and n is the mean number of induced electrons, while F characterizes the ionization fluctuation given by : n = p F n.