The LHCb experiment at CERN employs two Ring Imaging Cherenkov (RICH) detectors to provide particle identification over the range 1 to 100 GeV/$c$. The RICH detectors use custom-built Hybrid Photon Detectors (HPDs) to detect the Cherenkov photons over the wavelength range 200-600 nm. Three colums of 16 HPDs with final readout electronics and data acquisition were installed in a prototype Cherenkov gas enclosure. Two gas radiators were studied, nitrogen and $C_{4}F_{10}$. Saturated Cherenkov rings ($\beta \approx$ 1) were focused onto the plane of HPDs by exposing the radiators to a beam of 80 GeV/$c$ charged pions from the CERN SPS, operated with a 25 ns bunch structure. The HPDs and the on- and off-detector electronics performed to specification, and the DAQ system was verified under the sustained 40 MHz readout mode required for LHC running. The analysis verified the local alignment across the HPDs and their relative timing. The photon yields from both radiators are in good agreement with expectations.
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