The Electronic Amplification Chain

The dynode amplification chain is mounted on a PC board, as is the case of the 2" base. A schematic diagram of the resistor chain is shown in figure . As in the case of the 2" base, it incorporates an adjustable potentiometer (0-500 Ohm) and the nominal critical matching value is 90 Ohm. It has, in addition, a 51 Ohm resistor in series with the 14th dynode, to further improve the shape of the anode pulse. A safe -HV to start with is -1,800 V; it provides the best timing resolution with adequate pulse height for electrons during in beam testing.

  

Figure: The 5" 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.