A Photon Counter of High Sensitivity for the Vacuum Ultra-violet Region

Abstract

PHOTO-EMISSIVE receivers for the region of the spectrum below 2000 A. have generally used for the sensitive surface pure metals (platinum, tungsten, copper) or alloys (beryllium–copper¹, chrome–iron²) the photo-thresholds of which lie near 2000 A., thus rendering them insensitive to stray light in the near ultra-violet. Such materials, while lending themselves to reproducibility of surface, have not shown high quantum efficiencies except at the very short wavelengths³—below 1000 A. In the measurement of low light-levels, for which purpose counting of photo-electrons, whether by electron multiplier or gas amplification, is best suited, quantum efficiency may best be defined as the fraction of incident quanta detected, that is, which cause emission of one or more electrons detectable as a group. Values of the efficiency defined in terms of the average number of electrons per incident quantum, which may be rather higher, have generally been found to range from about 10⁻³ (for beryllium–copper) to only 10⁻⁷ (for chrome–iron) at 2000 A.