ELECTRIC-FIELD EFFECTS ON THE QUANTUM EFFICIENCY OF CSI PHOTOCATHODESIN GAS MEDIA

We have measured the quantum efficiency (QE) of CsI photocathodes as a function of the electric field strength in a parallel-plate geometry, in CH4, C2H6 and i-C4H10 both in charge collection and multiplication modes. It was found that in the collection plateau the QE value in ga ses is lower compared to that of vacuum and is independent of the fiel d; in gas media the QE starts to increase at the transition between co llection and multiplication modes and reaches the vacuum value at high gas gain. We explain this effect by a decrease of the electron-molecu le elastic backscattering while entering the multiplication mode. We c onclude that the electric field effects observed here, would also appl y for other photocathodes and gas mixtures. An enhancement of the QE a fter microdischarges was observed and is discussed in detail.