  Monte Carlo simulation is used to investigate the fluctuations in the Xe
proportional electroluminescence (EL) yield H (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 H 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 H 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 H 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.
