Hybrid photon detectors for the LHCb RICH

The LHCb Ring Imaging Cherenkov (RICH) counters use the pixel Hybrid Photon Detector (HPD) as a photo-sensitive device. Photo-electrons are produced in semi-transparent multi-alkali photo-cathode (S20) and are accelerated by a voltage of 20 kV onto a pixelated silicon anode. The anode is bump-bonded to the LHCBPIX1 pixel readout chip which amplifies and digitises the anode signals at the LHC speed of 40 MHz. Using a demagnification of five, the effective pixel size at the HPD window is 2.5 x 2.5 mm$^2$. Over the course of 18 months, 550 HPSs will undergo a quality-assurance programme to verify the specifications and to characterise the tubes. The tested parameters include the threshold and noise behaviour of the chip, the response to light emitting diode (LED) light, the demagnification of the electron optics, the leakage current and the depletion of the silicon sensor, the quality of the vacuum, the signal efficiency and the dark count rate. Results of tests of the first nine HPDs of the final design are presented and compared to the specifications.