Measurement of the Photoelectron Detection Efficiency of the HPD Anode

This paper reports on measurements carried out on the Hybrid Photon Detectors (HPDs) of the LHCb RICH detectors. The purpose of these tests is to determine the photoelectron detection efficiency $\eta$ of the HPD anode. Knowledge of $\eta$ is required for an accurate simulation of the RICH detectors. It is found that this efficiency is $(93.3\pm0.7)\%$ for a 50 ns digital readout window, and $(87.9\pm1.4)\%$ for a 25 ns digital readout window. The 25 ns result exceeds the LHCb-RICH requirement of 85\%, and is in agreement both with direct $\eta$ measurements using preseries HPDs, and with indirect measurements from testbeams using preseries and production HPDs.