Hybrid photon detectors for the LHCb RICH system

The LHCb experiment, currently being commissioned at CERN, uses two Ring Imaging Cherenkov (RICH) detectors as its principal particle identification system. These detectors are essential in allowing LHCb to perform the precision measurements of CP-violation in B-meson mixing and decays for which the experiment was designed. The photodetectors used in the LHCb RICH system are Hybrid Photon Detectors (HPDs), which employ a vacuum photocathode and a solid-state pixel sensor. The operating principle and production process of the HPDs are described. The performance criteria for the HPDs is outlined, along with the qualification procedures carried out at dedicated test facilities. The compiled results of tests performed on more than 500 HPDs are presented. The performance of the HPDs in a 25 ns structured test-beam and during the RICH commissioning process with the HPDs installed is discussed. Finally, the time alignment of the HPDs via a pulsed laser system is described.