Characterization of CsI photocathodes for use in a fast RICH detector

We have completed a series of measurements that provide a basic understanding of the properties of CsI photocathodes for use in ring imaging Cherenkov (RICH) detectors. The quantum efficiency of CsI has been measured relative to an NIST-calibrated photodiode and is in excellent agreement with a similar measurement by Breskin et al. A representative value of the quantum efficiency is 20% at 180 nm. The quantum efficiency of a fresh photocathode is unaffected by temperature, but heating the photocathode can be helpful if it has been exposed to water vapor, or has been aged by a large integrated photocurrent. Detailed studies of aging show a “fast” component that appears to be associated with a rise in the work function, and a “slow” component associated with conversion of the bulk CsI to Cs. We judge that a practical lifetime of a CsI photocathode is until it has lost 20% of its initial quantum efficiency, which process is dominated by the “fast” rise in the work function. This rise occurs both due to photoelectron transport with an effective lifetime of 0.1 kc/mm’ and due to positive-ion bombardment with an effective lifetime of 15 &/mm’. When the CsI photocathode is used in a chamber with gas gain greater than 150 the latter lifetime is the relevant one. This lifetime should be sufficient for use of a RICH detector at an e+eB factory. The reduction of quantum efficiency of a CsI photocathode in a gas-filled chamber has been studied for several gases over a wide range of reduced electric field. This effect can be minimized by use of atmospheric-pressure methane in a chamber with anode wires rather than a mesh. We have also demonstrated that excellent spatial resolution for the location of the photoelectrons can be obtained using a coarse cathode-pad readout if the anode-cathode spacing is similar to the pad width.