The w-value and the Fano factor for X-rays in xenon-neon gas mixtures: a Monte Carlo simulation study

The w-value and Fano factor F for the absorption of X-rays in Xe-Ne gas mixtures are investigated using a Monte Carlo simulation study. The w-value is the mean energy required to produce a primary electron in the gas, and the Fano factor is the relative variance of the number of primary electrons produced per incident X-ray photon. F and w are two fundamental characteristics of a detection medium, since the product Fw establishes the intrinsic energy resolution that can be achieved. Xenon is a preferred filling for gas detectors, but the energy resolution is known to deteriorate for low energy X-rays, due to electron loss to the entrance window. Since the addition of a lighter noble gas increases the absorption depth, Xe-Ne mixtures have been investigated as a potential medium for the detection of low energy X-rays. Monte Carlo simulation results for F and w are presented for X-ray energies up to 5 keV in pure xenon, pure neon and in Xe-Ne mixtures with 90%, 70%, 50%, 40%, 30%, 20%, 10%, 5% and 1% Xe, at a total pressure of 1 atmosphere. The simulation reproduces the photoionization process of both species, the vacancy cascade decay of the residual ions, and the elastic and inelastic scattering of electrons by the gas atoms. Energy and charge transfer mechanisms such as the Penning effect and transfer- and associative-ionization are taken into account