CaF2 single crystals as entrance windows of a fast photosensitive MWPC for a RICH detector

Abstract CaF 2 single crystals (162 × 170 × 3.5 mm 3 ) are used as entrance windows for a photosensitive multiwire proportional chamber (MWPC) being the main component of a Fast RICH detector. Thin silver traces (100 μm wide with a pitch of 1.5 mm) have been vacuum evaporated onto the surface of the crystals. Thus the windows can be used as the cathode of the MWPC which is operating with a TEA/methane gas mixture at room temperature. VUV measurements of the optical transmissivity before and after the evaporation reveal a reduction of 7% which is consistent with the geometry of the evaporation mask.