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W value and the Fano factor in rare gases and rare gas mixtures

The distribution of the number of electron-ion pairs produced by interaction of many incident electrons having identical initial energy T0 can be described by its mean value, Ni, and its variance, Vi. In practice, the mean energy required to form an ion pair (W = T0/Ni) and the Fano factor (F = Vi/Ni) are used. The present status of both experimental and theoretical values of the W and F values for electrons and photons in pure rare gases (He, Ne, Ar, Kr, Xe) and their mixtures is reviewed. A good correlation between the F and the W/I ratio (I is the ionization potential) is found for rare gases except xenon. Xenon has exceptionally low F value for relatively high W/I ratio. A good correlation between the Fano factor and the ratio of the total inelastic to total ionization cross section is obtained for all rare gases. The electron W and F values are compared with the corresponding values measured for a-?particles. Binary mixtures of rare gases with the metastable and with the non-metastable Penning effects are discussed. New experimental and Monte Carlo data on neon-xenon mixtures are also presented.

W value; Fano factor; rare gas; gas mixtures

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