Using Photomultipliers in Two-Wire Probe Configurations

by Charlie Thompson

Buda Seismic Observatory

The following diagram depicts how a PMT operates in the two-wire probe configuration.

The probe wire must supply dynode resistor string voltage while simultaneously providing signal pickoff for counting. While we show the dynodes as "grids" they are more like "cathodes" in a triode. They *will* source current pulses into the dynode resistor string. For this reason the upper dynodes must be bypassed to ground so that they are not pulled upwards with each photon current pulse. There is no bypassing at the anode because this is where the signal is tapped off. Failure to bypass properly will result in little or no signal output from the PMT. Because the anode "sinks" current the output at the signal pickoff is a negative-going pulse. For this scheme to work properly the anode load resistor must be large enough to provide a decent pulse amplitude but small enough to avoid excessive voltage drop while supplying dynode resistor string current. Typical values for the anode load are in the 1 Meg ohm range. Typical values for the dynode resistor string are in the 5 Meg to 20 Meg (each) range. Dynode resistors need to be substantially larger than the anode resistor. Upper dynodes should be bypassed to GND. Placing bypass caps across dynode resistors is a way to lower capacitor voltage rating but must still have at least one bypass capacitor to GND.

If you take a look at the R1307 hookup diagram here you'll see that I've modified the R1307 subassembly for the exact same two wire configuration described above. Another example of conversion of an R1538 subassembly for two-wire operation can be seen here. Study these two examples and you'll see the similarities.

The diagram above is also a good tutorial on the electrical behavior of a PMT. They all work this way. Only the number of dynodes and the max voltage changes. They are friendly animals....give them a try.

Remember - NEVER apply full voltage to a PMT in any ambient lighting whatsoever. You can apply drastically reduced voltages safely...like maybe 100-200 volts for a 900V tube. This is because the gain of a PMT varies exponentially with applied voltage.



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