The fast photon detection system of COMPASS RICH-1

A fast photon detection system has been built for the upgrade of COMPASS RICH-1, the large size gaseous RICH detector in use at the COMPASS Experiment at the CERN SPS since 2001. The photon detectors of the central region have been replaced by a new system based on multi-anode photomultipliers coupled to individual fused silica lens telescopes and a fast readout electronics system, while in the outer region the existing MWPCs with CsI photocathodes have been equipped with a new readout system, based on the APV chip. RICH-1 has been successfully operated in its upgraded version during the 2006 run. We report on the upgrade design and construction, and on the preliminary characterization of the upgraded RICH-1 performances: at saturation about 60 photons per ring have routinely been obtained and the background level has drastically been reduced thanks to the good time resolution of the new system, leading to a 2 σ π –K separation above 55 GeV/c.