A study on ion initiated photomultiplier afterpulses

Ions originating from the ionization of the gas impurities by the electrons inside the photomultiplier tube are generally responsible for the delayed pulses that arrive several hundred nanoseconds after the main pulse. These afterpulses are studied in detail for a Hamamatsu PMT (R7525). Their timing, amplitude, rate and high-voltage dependence are measured and monitored in a helium-rich environment for an extended period. The afterpulse rate due to helium is determined to be 5–10×10-410×10-4 per ppm per photoelectron. The characteristic afterpulse features are that their rate linearly depends on the main pulse charge but the afterpulse charge itself is essentially independent of it. We discuss the timing distribution and the origin of these afterpulses.