Characterisation and operations of the Multianode Photomultiplier Tubes for the LHCb RICH detectors

The upgraded LHCb RICH detectors are equipped with Multianode Photomultiplier Tubes, covering a total area of approximately four square metres. In order to achieve the same excellent hadron identification performance as during LHC Runs 1 and 2 at five times the instantaneous luminosity, the photon detectors have to be sensitive to single photons with repetition rates of up to 100MHz/cm2 and to have a very low noise count rate. The main properties of the photomultipliers are presented, together with the characterisation of an unexpected source of noise observed in the Hamamatsu R11265 Multianode Photomultiplier Tubes, extending up to several microseconds after the primary signal. The quality control results, the mitigation strategies for the aforementioned noise to perform optimal single-photon counting at 40 MHz, and the first operations of the RICH system are described.