Amplitude-time distributions for XP2020Q, FEU-87, and FEU-30 PMT after
pulses ranging from 20 ns to 1.4 mu s ate studied with the autocorrela
tion method, Afterpulses caused by ion feedback are identified. Afterp
ulse intensities are measured as a function of PMT power supply voltag
e and main pulse amplitude at the PMT output.