An R&D project is presented, aimed to develop a high performance gaseous detector of single photons, for the upgrade of the Ring Imaging Cherenkov Counter RICH-1 of the COMPASS Experiment at CERN SPS. The detector has to stably operate at high gain and high rate, to provide good time resolution and insensitivity to magnetic field, and to offer the possibility to cover very large areas at affordable cost. The proposed solution is based on the use of a novel and robust electron multiplier, the Thick GEM (THGEM), arranged in a multilayer architecture, where the first layer is coated with a photosensitive CsI film. A systematic study of the response of THGEMs with various geometrical and production parameters and in different conditions was performed, leading to the choice of a set of optimal parameters. Prototypes of THGEM-based photon detectors able to efficiently detect Cherenkov photons have been built, tested in laboratory and operated in test beam exercises with typical gain of 105 and time resolution better than 10 ns. The engineering aspects of building large area (600×600 mm2) THGEM-based photon detectors are presently being investigated.
