The high voltage distribution system for the hybrid photodetector arrays of RICH1 and RICH2 at LHCb

Two ring imaging Cherenkov detectors, RICH1 and RICH2, will provide the particle identification of the LHCb experiment. The Cherenkov light cones produced in three different refractive media are projected onto four arrays of hybrid photon detectors (HPDs), with 484 HPD units in total. In order to accelerate and focus the photoelectrons resulting from the conversion of photons in the HPD photocathode, the electron optics of an HPD requires three different high voltages (HVs): -20 kV, -19.7 kV and -16.4 kV. We describe in this paper the HV distribution system that has been designed for this purpose. The HV is supplied by printed circuit boards specially developed to prevent electrostatic discharges and/or corona effects across the volume of the HPD arrays. The circuits allow for the splitting, distribution, protection and monitoring of the high voltages. Each board is covered with silicone rubber, which prevents electrostatic breakdown, and a study has been made on its radiation tolerance. A prototype HV system has demonstrated compliance with all experimental requirements. Finally, a production test system has been implemented to provide for a precise characterization of each HV board, in order to guarantee optimal yield.