Development of Large-Area Fast-Rich Prototypes with Pad Readout and Solid Photocathodes

In the frame of the RD26 collaboration at CERN we have built and tested several prototypes of fast-RICH detectors suitable for the construction of large systems. The detectors used a solid photocathode consisting of a 500 nm CsI layer evaporated on a conventional pad readout board of a MWPC operated with a methane/isobutane mixture at atmospheric pressure. The results indicate that such a detector may be used for long periods of time in stable operating conditions provided the gas mixture is continuously flushed through the detector. Evidence is presented for a significant loss of transmission of Cherenkov photons in the NaF radiator due to internal anisotropy of the crystal. We present the results of the performance of the photocathodes, the quantum efficiency of the CsI we have extracted from the measured Cherenkov photons and the identification achievable in a high density environment such as the detector ALICE planned for the heavy ion LHC. The results of a study of primary and secondary electron photoemission of CsI layers indicate that the surface structure and electron yield depend on the source material and on the heat treatment of the layer.