On the influence of afterpulsing in PMTs on the trigger threshold of multichannel light detectors in self-trigger mode

In some experiments photomultiplier based multichannel detectors are used to measure short light flashes in self-trigger mode in the presence of a significant background illumination. This background can induce a sizeable afterpulse rate. Contrary to published data the afterpulses can have rather large amplitudes and might be interpreted as genuine signals. This is the case with current imaging air Cherenkov telescopes in cosmic ray experiments. We found that due to the simple trigger logic of contemporary telescopes it is not the light of the night sky (LONS), but the afterpulsing which sets the trigger threshold. Afterpulsing features for 5 types of photomultiplier tubes are presented.