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 W-glass entrance window. Focusing electrodes demagnify the image on a Si detector of 50 mm diameter comprising 2048 pads with a surface of 1 mm ² 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 transmitting bialkali photocathodes (K₂ CsSb) has been built and successfully operated. The evaporation process is optimized for maximum quantum efficiency and lifetime 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