CHARACTERIZATION OF CSI PHOTOCATHODES FOR USE IN A FAST RICH DETECTOR

We have completed a series of measurements that provide a basic unders tanding of the properties of CsI photocathodes for use in ring imaging Cherenkov (RICH) detectors. The quantum efficiency of CsI has been me asured relative to an NIST-calibrated photodiode and is in excellent a greement with a similar measurement by Breskin et al. A representative value of the quantum efficiency is 20% at 180 nm. The quantum efficie ncy 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 wor k function. This rise occurs both due to photoelectron transport with an effective lifetime of 0.1 mu C/mm2 and due to positive-ion bombardm ent with an effective lifetime of 15 mu C/mm(2). When the CsI photocat hode is used in a chamber with gas gain greater than 150 the latter li fetime is the relevant one. This lifetime should be sufficient for use of a RICH detector at an e(+)e(-) B factory. The reduction of quantum efficiency of a CsI photocathode in a gas-filled chamber has been stu died for several gases over a wide range of reduced electric field. Th is effect can be minimized by use of atmospheric-pressure methane in a chamber with anode wires rather than a mesh. We have also demonstrate d that excellent spatial resolution for the location of the photoelect rons can be obtained using a coarse cathode-pad readout if the anode-c athode spacing is similar to the pad width.