Photoelectron transmission efficiency in Ar-CH4 and Xe-CH4 mixtures: Experimental results

The transmission efficiency of photoelectrons emitted from CsI photocathodes operated in Ar-CH4 and Xe-CH4 mixtures is experimentally investigated and compared to Monte Carlo simulation results; the ratio between the number of photoelectrons collected in the gas media and in vacuum is determined as a function of the pressure-reduced electric field E/p at the photocathode surface, for irradiation with the VUV photons from an Hg(Ar) lamp. The addition of a small percentage of CH4 to Ar and Xe results in a significant increase of the photoelectron transmission, in particular for electric fields below E/p∼1 V cm−1 Torr−1. Below E/p∼0.5 V cm−1 Torr−1 the transmission efficiency f in Ar-CH4 mixtures reaches values higher than in pure CH4, going through a maximum in the range of 10-30% CH4. The addition of 5% and 20% CH4 to Xe increases f by a factor of about 2 and 3, respectively, when compared to pure Xe.