Recent progress of photocathodes for PMTs

Abstract

The quantum efficiency (QE), the conversion efficiency of a photon to an electron, is one of the most important performance of photon sensors. In general, the QE of opto-semiconductor devices is superior to that of photocathodes for PMTs. As a result of our continuous efforts to improve them, an ultra bialkali photocathode (UBA) released a few years ago shows QE higher than 40% at 380nm, GaAsP does 50% in 550nm, and GaAs 30% in 750nm. In the case of UBA, however, linearity is a problem, when it is cooled down to liquid xenon temperature (-108 degree centigrade) for dark matter experiments. For these experiments, a high linearity photocathode at low temperature has been developed. This photocathode shows a little less QE than UBA, but much lower dark current than UBA even at room temperature, thus it's a good compromise in S/N ratio. Including these topics, recent progresses of photocathode is described.