LHCb: Characterisation and magnetic field properties of Multianode Photomultiplier tubes for the use in LHCb Upgrade RICH detectors

A key feature of the LHCb upgrade, scheduled for 2019, is to remove the first level trigger and its data reduction from 40MHz to 1MHz, which is implemented in the on-detector readout electronics. The consequence for the LHCb Ring Imaging Cherenkov (RICH) detectors is that the Hybrid Photon Detectors need to be replaced as the readout chip is embedded in the tubes. Baseline for replacement are Multianode Photomultiplier tubes (MaPMT) and new readout electronics. Here we report about characterisation studies of the model Hamamatsu R11265 in the effort to qualify them for use in the LHCb RICH upgrade. Comparisons to the known model R7600 are also made. Two types of readout electronics are used. Most measurements to characterise the properties of the MaPMTs are taken with a VME based reference readout, using a x100 linear amplification and the CAEN V792 12-bit charge integrating digitiser. This allows to derive the signal properties from fits to the single photon spectra. In addition a prototype readout using the MAROC3 chip was used to read out the MaPMT directly, capable to provide fast binary signals at a speed of 40MHz. Using pulsed light (470nm, 15ns pulse width) the MaPMT were characterised for pixel-to-pixel and tube-to-tube gain variations, for the signal loss below threshold, for cross-talk between pixels, for the gain dependency on high voltage and for their effective pixel size. A subset of these measurements was repeated with the MaPMT been subjected to a controlled magnetic field, applied sequentially to all three space axes. Using signle photon spectra additional signal loss due to photoelectrons missing the entry window and charge getting lost in the multiplication process can be disentangled. Results from these studies will be presented and discussed.