Electrometer Amp: Darlington NPN

I soldered up the simplest possible “electrometer amplifier” at Squidwrench, based on Charles Wenzel’s writeup:

Electrometer Amp - MPSA14 NPN Darlington
Electrometer Amp – MPSA14 NPN Darlington

It’s an MPSA14 NPN Darlington transistor, with the base soldered directly to the Victoreen 710-104 ionization chamber collector pin. The flying leads connect to an ordinary digital voltmeter set to read voltage, rather than current, so that you see the voltage created by the transistor’s collector current through the meter’s input resistance.

The MPSA14 data sheet specifies DC current gain hFE > 10 k for low collector currents, with a graph suggesting it might be somewhat larger. Alas, all those are for “ordinary” currents, not the countably finite number of electrons coming from an ionization chamber, but let’s assume 10 k is close.

I used a Radio Shack 22-805 DMM, set to auto-ranging DC volts. The specs say the input “impedance” is 10 MΩ for all voltage ranges, so let’s run with that, too.

With 24 V (actually 24.6 V) applied to both the chamber (through the red wire) and the DMM (through the yellow wire), it read 250 mV: a mere 25 nA through the 10 MΩ meter resistance.

Assuming a transistor gain of 10 k, that’s a chamber current of 2.5 pA.

The ionization chamber specs say it produces 5 pA at 0.5 röentgen/hour → 100 mR/h produces 1 pA.

No, I do not believe the Squidwrench Operating Table is bathed in gamma radiation at 250 mR/h.

I should wipe down the transistor to see if that reduces the external leakage, then try a few others, but obviously the signal will remain lost in the noise.

We replaced the DMM with an oscilloscope and 10 MΩ probe, which conclusively demonstrated that unshielded high-impedance circuits make excellent 60 Hz receivers.