Development of large area hybrid photodiodes for the LHCb ring imaging Cherenkov detectors

We report on the development of large area hybrid photodiodes (HPD) which are one of the proposed photodetectors for the RICH counters of the LHCb experiment. The HPD´s consist of a cylindrical vacuum envelope of 127 mm diameter capped with a spherical borosilicate UV-glass entrance window. Focusing electrodes demagnify the image on a silicon detector of 50 mm diameter comprising 2048 pads with a surface of 1 mm² each. The analogue readout electronics is integrated in the vacuum tube. As an intermediate step a HPD with a UV sensitive CsI photocathode has been produced which allowed to verify the electron optics of the HPD. A large UHV evaporation plant for the production of HPD´s with visible light transmissive bialkali photocathodes (K₂ CsSb) has been built and successfully operated. The evaporation process is optimized for maximum quantum efficiency and life time of the photocathodes. A cold indium sealing technique developed for a minimum thermal load of the photocathode and the silicon sensor has proven to provide excellent vacuum tightness