Pixel hybrid photon detectors for the ring imaging Cherenkov detectors of LHCb

A Pixel Hybrid Photon Detector (pixel HPD) has been developed for the LHCb Ring Imaging Cherenkov (RICH) detectors. The pixel HPD is a vacuum tube with a multi-alkali photocathode, high-voltage cross-focused electron optics and an anode consisting of a silicon pixel detector bump-bonded to a CMOS readout chip; the readout chip is thus fully encapsulated in the device. The pixel HPD fulfils the stringent requirements for the RICH detectors of LHCb, combining single photon sensitivity, high signal-to-noise ratio and fast readout with an ∼8 cm diameter active area and an effective pixel size of 2.5 mm×2.5 mm at the photocathode. The performance and characteristics of two prototype pixel HPDs have been studied in laboratory measurements and in recent beam tests. The results of all measurements agree with expectations and fulfill the LHCb RICH requirements. In readiness for production of the ∼500 pixel HPDs for the RICH detectors, a test programme was designed and implemented to ensure component quality control at each stage of the manufacturing process.