Highly segmented large-area hybrid photodiodes with bialkali photocathodes and enclosed VLSI readout electronics

We report on the principles, design, fabrication, and operation of a highly segmented, large-area hybrid photodiode, which is being developed in the framework of the LHCb RICH project. The device consists of a cylindrical, 127 mm diameter vacuum envelope capped with a spherical borosilicate UV-glass entrance window, with an active-to-total-area fraction of 81%. A fountain-focusing electron optics is used to demagnify the image onto a 50 mm diameter silicon sensor, containing 2048 pads of size 1×1 mm2. The sensor pads are read out by 16 analogue multiplexed readout chips enclosed in the vacuum envelope. A large ultra-high vacuum plant for the evaporation of visible-light transmittive bialkali (K2CsSb) photocathodes and subsequent tube encapsulation has been built and successfully operated. The characteristics and performance of the first HPDs have been measured to be in full agreement with expectations.