Simulation of He(+) induced afterpulses in PMTs.

We report on a Monte Carlo simulation study of afterpulses due to trace gases in EMI 9235QA photomultipliers that are widely used in many luminescence detection systems operating in single photon counting mode. The numerical simulation takes into account the nonuniform electric field distribution and processes including elastic scattering: e + He → e + He, excitation: e + He → e + He(∗), ionization: e + He → 2e + He(+), elastic scattering: He(+) + He → He(+) + He, charge transfer: He(+) + He → Hef + He(+) (Hef indicates a fast neutral) and elastic scattering: Hef + He → Hef + He(f). The simulated and the measured time of flight distributions agree well. In addition, the above simulated processes demonstrate mechanisms of the observed series of pulses brought about by a single helium atom ionization.