Hybrid Photon Detectors for the LHCb RICH Counters

Hybrid Photon Detectors (HPD) have been chosen for the Ring Imaging Cherenkov (RICH) detectors of the LHCb experiment. Photons impinging on a multi-alkali S20 photo-cathode deposited on a quartz window produce photo-electrons that are accelerated by a 20 kV potential onto a silicon pixel sensor anode. The sensor is segmented into 8192 pixels of size 0.0625 mm ° 0.5 mm with an electronic OR forming 1024 super-pixels of 0.5 mm ° 0.5 mm. The cross-focusing electron optics introduces a demagnification factor of five, resulting in an effective pixel size of 2.5 mm ° 2.5 mm at the photo-cathode. The silicon sensor is bump-bonded to a pixel chip fabricated using 0.25 µm deep submicron radiation-tolerant technology, which amplifies and digitizes the anode signals at the LHC speed of 40 MHz. The sensor/chip assembly is mounted inside the HPD vacuum envelope. Mass production of 484 HPDs for the LHCb experiment has commenced in close collaboration with industry. Measurements of HPD properties in the laboratory are presented. These include supply currents, threshold scans, dark count, ion feed-back rates, quantum efficiency and image distortions due to magnetic fields. Finally, results from studies of the HPD performance for the detection of Cherenkov light produced in particle test beams are reported