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

Xenon gas proportional-scintillation counters (GPSC) have many applications in the detection of soft x rays where their energy resolution, R, is compa rable to solid-state detectors when large window areas are required. Howeve r, R is known to deteriorate for energies E-xr below 2-3 keV due to electro n loss to the entrance window. Since the addition of a lighter noble gas in creases the absorption depth, we have investigated the use of Xe-Ne gas mix tures at atmospheric pressure as detector fillings. The results of a Monte Carlo simulation study of the Fano factor, F, the w value, and the intrinsi c energy resolution, R = 2.36(Fw/E-xr)(1/2), are presented for Xe-Ne mixtur es and pure Xe and Ne. The results show that the addition of Ne to Xe reduc es the intrinsic energy resolution R but this never compensates for the red uction in scintillation yield in GPSC applications, implying that the instr umental energy resolution R will only improve with the addition of Ne when electron loss to the window in pure Xe is significant. The simulation repro duces the photoionization process of the Xe and Ne atoms, the vacancy casca de 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 discuss ed in detail. It is shown that Penning and associative ionization are the c rucial indirect ionization processes which determine the behavior of F and w at low concentrations of Xe. The importance of the nonmetastable Ne state s is also assessed. (C) 2001 American Institute of Physics.