Cross talk study to the single photon response of a flat panel PMT for the RICH upgrade at LHCb

The Ring Imaging Cherenkov (RICH) detector at LHCb is now readout by Hybrid Photon Detectors. In view of its upgrade a possible option is the adoption of the flat panel Photon Multipliers Tubes, PMT. An important issue for the good reconstruction of the Cherenkov rings is a negligible level of cross-talk. We have experimentally studied the cross talk from the 16×16 pixels of the Hamamatsu H9500 PMT. Results have shown that for the single photon signal, as expected at LHCb, the statistics tied to the small number of electrons generated at the first dynode of the PMT chain (a few units) leads to a number of cross-talk signals that are a small fraction of the fired pixel. Due to the discrete nature of the electron charge, those few cross-talk signals have amplitude that is a significant fraction of the fired pixel, which is in consequence depleted.