Progress in ultrafast CsI-photocathode gaseous imaging photomultipliers

Abstract A large area, low-pressure gas-filled UV-imaging photomultiplier with CsI-photocathode is presented. The double-step electron multiplier with a 10 Torr CH 4 gas-filling enables stable high gain operation. The detection efficiency of photons in the wavelength range λ ∼ 170 nm (Xe scintillation light) is about 10% for 200 to 2000 nm thick photocathodes. We investigated the influence of various substrate materials, the thickness of the CsI-layer, the gas pressure and gas composition on the performance of the photocathode. Furthermore we studied the stability of the photocathode under different operating conditions and its sensitivity to air. Measurements of the timing characteristics of the device yielded an ultimate time resolution of 350 ps (FWHM).