Origins of Slow Aftfrpulsfs in High Gain First Dynode PMTs

To study the origins of the afterpulses in RCA 8850 and similar types of tubes, temperature and interstage-voltage dependences of the time distribution and afterpulsing probability have been extensively examined. It has been confirmed that temperature has no influences on both of two different types of afterpulsing which exhibit a peak at around 5 μs and 40 μs, respectively, in the time distribution. The experimental results also reveal that the afterpulses occurring around 40 μs or later than the main pulse are closely related to the interstage voltages at first few stages beyond the first dynode, and that those arising predominantly around 5 μs after the main pulse are exclusively correlated with the interstage voltages at last few stages as well as with anode current. These findings should provide not only a basis for optimum setting of interstage voltages to control afterpulsing, but also a valuable hint to identify the origins of those afterpulses.