The Electronic Amplification Chain

The arrangement of the resistor dynode chain is shown in figure . The cathode is connected to the mu-metal shield through a 10 MOhm resistor, in addition to the 1 MOhm resistor between the cathode and the negative HV. The dynode chain incorporates an adjustable potentiometer (0-500 Ohms) to allow a match between the PMT and the external load, in order to eliminate after-pulse ringing. This potentiometer should be adjusted at first to 250 Ohm and then make fine adjustments as needed by observing the anode pulses on the oscilloscope for critical matching. It is not advised to do the adjustments with HV on. Instead, the process should be done with HV off, remove the rear tubular housing, adjust the potentiometer, replace the rear housing, and then HV on again. Iterate until the matching is accomplished. In addition to the obvious safety concerns, one does not want to remove the light sealing rear housing from an active PMT and induce a large light leak which could destroy the PMT.

  

Figure: The 2" PMT base dynode resistor chain

Both 2" and 5" bases have been extensively tested under beam conditions. They have several safety related features but these cannot protect anyone who is bent on violating operating procedures and common sense. They allow the removal of the PMT/Base assembly, for repairs of the electronics or replacement of a PMT, without decoupling the housing and collets from the light guide. Thus, replacement of PMTs can be done in minutes without the need to remove the scintillator counters from their subframes.