I have an H7195 assembly type Photo Multiplier Tube (PMT) used to be tested for single photon measurement. I would like to use the dynode output to check the spectrum by connecting to a charge sensitive pre-amplifier (Canberra 2005) and then shaping with an amplifier before transferring the signals to Multi-Channel Analyzer (MCA).
Based on the manual of the preamplifier, the output will have a $50 \mu s$ decay time while my measurement only giving around $2 \mu s$ decay time. I checked using the anode output 1 as the input, it gave me around 100 us decay time if I did not terminate the anode output 2. With 50 ohm termination in anode output 2, the decay time decrease to 4 us. I also checked using a ramp down signal from a function generator and the output provides around 100 us decay time.
As far as I know, the decay time of this preamp is only determined by the resistance-capacitance (RC) value in the integrator circuit. As long as the input is a charge, the output should give 50 us decay time. what could be the issue here and how can I investigate this problem?