High quantum efficiency S-20 photocathodes in photon counting detectors
  Based on conventional S-20 processes, a new series of high quantum efficiency (QE) photocathodes has been developed that can be specifically tuned for use in the ultraviolet, blue or green regions of the spectrum. The QE values exceed 30% at maximum response, and the dark count rate is found to be as low as 30 Hz/cm$^2$ at room temperature. This combination of properties along with a fast temporal response makes these photocathodes ideal for application in photon counting detectors, which is illustrated with an MCP photomultiplier tube for single and multi-photoelectron detection. 
Dmitry A. Orlov, Jeff DeFazio, Serge Duarte Pinto, Rene Glazenborg and   Emilie Kernen
physics.ins-det