A study of the electroluminescence yield and fluctuations in Xe doped with CH4 or CF4: The role of electron cooling and attachment

Monte Carlo simulation is used to investigate the Xe secondary scintillation (electroluminescence) yield and the corresponding fluctuations in gaseous Xe doped with CH₄ and CF₄, comparing with the results in pure Xe. Electron drift velocities and characteristic energies ekT and ekL are also analyzed. The addition of CH₄ or CF₄ to Xe reduces electron diffusion, a desirable effect when electrons may have to drift across a long absorption/drift region of a detector. However, for detectors where amplification is achieved in a secondary scintillation region, it is found that the presence of CF₄ will not only reduce but bring undesirable high fluctuations to the Xe scintillation yield; this is due to electron attachment by the CF₄ molecules, a process which becomes important for the range of scintillation fields. On the other hand, the calculations show that Xe scintillation is affected by CH₄ to a much lower extent, indicating that CH₄ concentrations < 1% may represent an adequate choice.