Monte Carlo simulation study of the Fano factor, w value, and energy resolution for the absorption of soft x rays in xenon–neon gas mixtures

Abstract

Xenon gas proportional-scintillation counters (GPSC) have many applications in the detection of soft x rays where their energy resolution, R, is comparable to solid-state detectors when large window areas are required. However, R is known to deteriorate for energies Exr below 2–3 keV due to electron loss to the entrance window. Since the addition of a lighter noble gas increases the absorption depth, we have investigated the use of Xe–Ne gas mixtures at atmospheric pressure as detector fillings. The results of a Monte Carlo simulation study of the Fano factor, F, the w value, and the intrinsic energy resolution, R=2.36(Fw/Exr)1/2, are presented for Xe–Ne mixtures and pure Xe and Ne. The results show that the addition of Ne to Xe reduces the intrinsic energy resolution ℛ but this never compensates for the reduction in scintillation yield in GPSC applications, implying that the instrumental energy resolution R will only improve with the addition of Ne when electron loss to the window in pure Xe is significant. The simulation reproduces the photoionization process of the Xe and Ne atoms, the vacancy cascade decay of the residual ions, and the elastic and inelastic scattering of electrons by the gas atoms. The contribution of energy and charge transfer mechanisms such as Penning, associative, and transfer ionization is discussed in detail. It is shown that Penning and associative ionization are the crucial indirect ionization processes which determine the behavior of F and w at low concentrations of Xe. The importance of the nonmetastable Ne states is also assessed.