Hybrid Photon Detectors for the LHCb RICH: Performance and operational experience

Pion/kaon discrimination in the LHCb experiment will be provided by two Ring Imaging Cherenkov (RICH) counters. These use arrays of 484 Hybrid Photon Detectors (HPDs) to detect the Cherenkov photons emitted by charged particles traversing the RICH. The results from comprehensive quality assurance tests on the 550 HPDs manufactured for LHCb are described. Leakage currents, dead channel probabilities, dark count rates and ion feedback rates are reported. Furthermore, results from two specialised measurements carried out on a sample of HPDs to determine the efficiencies of the HPD pixel chip and photocathode, respectively, are described. Finally, an overview is given of the in situ performance of the HPDs which are now installed in the RICH.