A Monte Carlo study of the fluctuations in Xe electroluminescence yield: pure Xe vs Xe doped with CH₄ or CF₄ and planar vs cylindrical geometries

Monte Carlo simulation is used to investigate the fluctuations in the Xe proportional electroluminescence (EL) yield (also known as secondary scintillation) produced by sub-ionization primary electrons drifting in the gas under appropriate electric fields, comparing pure Xe gas with Xe doped with CH₄ or CF₄. The work is modeled on gas detectors of the gas proportional-scintillation type, where amplification is achieved through the production of EL under a charge-multiplication free regime. The addition of the molecular gases to Xe reduces electron diffusion, a desirable effect in large size detectors where primary electrons drift across a long absorption/drift region. However, the presence of the molecules reduces and increases its fluctuations. In the case of CF₄, the effects are very strong due to significant electron attachment in the EL field range, ruling out CF₄ as an acceptable additive. The addition of CH₄ affects and its fluctuations to a much lower extent, and CH₄ concentrations lower than ~1% may be an appropriate choice. In addition, Monte Carlo calculations in pure Xe under cylindrical geometry in a regime below charge multiplication have shown that fluctuations in the EL yield are an order of magnitude higher than for planar geometry. For both geometries, though, the fluctuations have a negligible effect on the energy resolution, and variations of the anode radius in cylindrical geometry or grid parallelism in planar geometry may be a more significant cause of concern.