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 teste d several prototypes of fast-RICH detectors suitable for the construct ion of large systems. The detectors used a solid photocathode consisti ng of a 500 nm CsI layer evaporated on a conventional pad readout boar d of a MWPC operated with a methane/isobutane mixture at atmospheric p ressure. The results indicate that such a detector may be used for lon g periods of time in stable operating conditions provided the gas mixt ure is continuously flushed through the detector. Evidence is presente d for a significant loss of transmission of Cherenkov photons in the N aF radiator due to internal anisotropy of the crystal. We present the results of the performance of the photocathodes, the quantum efficienc y of the CsI we have extracted from the measured Cherenkov photons and the identification achievable in a high density environment such as t he detector ALICE planned for the heavy ion LHC. The results of a stud y of primary and secondary electron photoemission of CsI layers indica te that the surface structure and electron yield depend on the source material and on the heat treatment of the layer.