Pixel hybrid photon detector magnetic distortions characterization and compensation

The LHCb experiment requires positive kaon identification in the momentum range 2-100 GeV/c. This is provided by two ring imaging Cherenkov detectors. The stringent requirements on the photon detectors are fully satisfied by the novel pixel hybrid photon detector, HPD. The HPD is a vacuum tube with a quartz window, S20 photo-cathode, cross-focusing electron optics and a silicon anode encapsulated within the tube. The anode is a 32×256 pixels hybrid detector, with a silicon sensor bump-bonded onto a readout chip containing 8192 channels with analogue front-end and digital read-out circuitry. An external magnetic field influences the trajectory of the photoelectrons and could thereby degrade the inherent excellent space resolution of the HPD. The HPDs must be operational in the fringe magnetic field of the LHCb magnet. This paper reports on an extensive experimental characterization of the distortion effects. The characterization has allowed the development of parameterisations and of a compensation algorithm. A calibration procedure based on the imaging of pre-defined test patterns that has been developed for the RICH detectors is also proposed.