Crosstalk Study of 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 read out by hybrid photon detectors. In view of its upgrade, a possible option is the adoption of the flat-panel photon multiplier tube (PMT). An important issue for good reconstruction of the Cherenkov rings is a negligible level of crosstalk. We have experimentally studied the crosstalk from the 16 × 16-pixels Hamamatsu H9500 PMT. Results have shown that, for the single-photon response, 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 crosstalk signals that are a small fraction of the fired pixel. With the H9500, in a Cherenkov ring, only one or two pixels are expected to generate crosstalk. As a consequence, crosstalk cannot be considered a limit in the use of the H9500 for the upgrade of the RICH at LHCb.