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

We report on the development of large area hybrid photo diodes (HPD) which are one of the proposed photodetectors for the RICH counters of the LHCb ex periment. The HPD's consist of a cylindrical vacuum envelope of 127 mm diam eter capped with a spherical borosilicate UV-glass entrance window. Focusin g electrodes demagnify the image on a silicon detector of 50 mm diameter co mprising 2048 pads with a surface of 1 mm(2) each. The analogue readout ele ctronics is integrated in the vaccum tube. As an intermediate step a HPD wi th a UV sensitive CsI photocathode has been produced which allowed to verif y the electron optics of the HPD. A large UHV evaporation plant for the pro duction 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 t he photocathode and the silicon sensor has proven to provide excellent vacu um tightness.