Photomultiplier pulses only at first few channels? I have an old Hamamatsu PMT, and I have a scintillator that can detect neutrons, gammas, alphas. But no matter what source I use, I'm only seeing counts at the first few channels (up to about the 15th channel) and then no counts for any of the other channels. I've been using this PMT since a while ago, and previously it would show counts (for the same source and same conditions) upto channel 700 even. What could be an issue to investigate? I only have one PMT.
 A: When you expose a PMT with full light, you can seriously affect its gain. Years ago I was involved in a project where a PMT had its gain stabilized by monitoring the output due to a (dim) LED that flashed at 1 kHz. We noticed that the voltage needed to maintain constant output increased over time; when we reduced the intensity and frequency of the LED pulse, the "gain loss curve" slope decreased significantly.
The mechanism for this was believed to be electron bombardment of the dynodes - they cause changes to the surface of the dynodes, reducing the electron multiplying qualities.
The other thing that might have happened - you could have created a microcrack when you "knocked it". In that case, air enters the enclosure and this would show up as a discoloration of the PMT surface (for bialkali cathodes, it would go from light brown to clear). 
The third thing that can sometimes cause gain loss (but this is usually a slower effect) is leakage current across the bulb (yes, I had to measure that to believe it). With the cathode at - 1500 V, mounted to a scintillator in a grounded enclosure, coupled with grease, we were getting fA of current through the envelope. This apparently caused ion migration (Na+ from the glass) that "poisoned" the cathode, resulting in a reduced gain. Adding a guard ring (at HV) made the problem go away.
And of course, it's possible that knocking the PMT+scintillator assembly would have knocked the scintillator off the PMT. If the light is no longer coupled efficiently, the peak will essentially disappear.
Three final ideas - HV supply is fried, or the preamp is. Or the preamp power is off... unlikely, but I thought I had to mention it.
It's hard to know which of these is your problem, but I hope it helps with your troubleshooting.
See also this earlier answer about PMT troubleshooting.
