Abstract:

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, 127mm 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 50mm diameter silicon sensor, containing 2048 pads of size 1 * 1mm². 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.