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Archive for September 22nd, 2015

Victoreen 710-104 Ionization Chamber: LMC6081 Electrometer Amp

The simplest possible electrometer amplifier that might work:

Electrometer amp - LMC6081 schematic

Electrometer amp – LMC6081 schematic

The general idea is that the op amp will drive the (essentially) open-circuited inverting input to match the 2 V offset at the noninverting terminal, so that the output will stabilize above the LMC6081’s minimum useful output voltage (of about 1 V) and the gamma-ray pulses will go downward from there (it’s an inverting amp). The rebiasing network downstream from the output cap doesn’t appear in the hardware.

The small cap across the feedback resistor that would compensate / roll off the high-frequency response isn’t possible, unless you have a stockpile of teeny glass vacuum capacitors: the leakage resistance must be far more than 100 GΩ and my collection lacks anything like that. A Teflon-insulated gimmick capacitor wouldn’t be stable enough and would probably still leak crazy current.

Normal electrometer amps operate at essentially DC and amplify an actual current from the ionization chamber. In this case, the radiation level is zero, there’s no chamber current, I’m looking for small pulses generated by gamma ray events, and the amplifier must have reasonable AC response. It’s not clear that circuit can work, but it’s a starting point.

It turned into a hairball:

Electrometer Amp - 10 Gohm Rf

Electrometer Amp – 10 Gohm Rf

The LMC6081IN is socketed, with the inverting input pin bent outward and soldered to the flying junction of the 100 kΩ input / OMG resistor (which prevents inadvertent shorts from the 24 V chamber bias) and the glass-body feedback resistor. The socket sits end-on in a puddle of epoxy, so I can swap op amps as needed.

The 100 GΩ feedback resistor didn’t work well at all:

LMC6081 100 G - out - noninv level - matched

LMC6081 100 G – out – noninv level – matched

The lower trace shows the 10.2 VDC offset at the noninverting input, which is what it took to drive the output from near 0 V to the 11.4 V in the upper trace, with no proportional change in between. I replaced the schematic’s 1 MΩ / 220 kΩ resistors with the 20 turn trimpot shown in the photo in order to find that bias condition. Obviously, the op amp acts as an open-loop comparator, not a linear amplifier, and no amount of waiting for it to stabilize would change that outcome.

It’s possible the resistor has failed open, but, frankly, the difference between “100 GΩ” and “open circuit” probably doesn’t amount to much. Note, however, that there’s absolutely no 60 / 120 Hz interference or noise, which continues to surprise me; removing the shield cap and teasing the twiddlepot slams the output with a 60 Hz trapezoid:

 

LMC6081 100 G - out - noninv level

LMC6081 100 G – out – noninv level

Swapping in a 10 GΩ resistor produced a smoothly changing output for biases between about 1 V and 8 V, so it’s behaving like an op amp should. Setting the non-inverting terminal to 8 V puts the output voltage at 6.3 V, which means the 10 GΩ resistor drops about 1.7 V to pull 170 pA from the inverting input (which is presumably at 8 V give-or-take a bit) and the chamber electrode. The LMC6081 spec says a maximum of 4 pA (for the I version, which is what I have), so:

  • My cleanliness isn’t up to par
  • The chamber delivers quite a bit more zero-radiation current than I expected
  • The op amp’s input got toasted despite my efforts toward a static-free installation

Hard to choose among those options, it is, indeed.

With the bias set to produce a 6 VDC output, the AC coupled signal doesn’t seem promising:

LMC6081 10 G Rf - out and Vplus in - AC 2 mV div

LMC6081 10 G Rf – out and Vplus in – AC 2 mV div

The lower trace is the bias voltage applied to the noninverting input, which looks reasonably clean. The sweep triggers from the power line; there’s still no 60 Hz interference.

All those flying components are, as you’d expect, microphonic beyond belief: jumping on the concrete basement floor produces a corresponding bounce in the trace that may be due to air currents or noise, for all I can tell. Even with the chamber sitting on a loose cloth pad, tapping the workbench produces 10 s of slowly decaying oscillation, admittedly at a much lower frequency than the noise in that trace.

A single gamma ray event producing an unreasonably high 10 fA chamber current will cause a downward (it’s an inverting amplifier) pulse that amounts to a mere 100 µV, a pulse that obviously isn’t visible against all that racket. You might convince yourself that the event at the center of the top trace comes from a gamma ray, but you’d probably be wrong.

In a normal electrometer amp, a stiff low-pass filter discards all the noise and isolates the DC signal corresponding to the steady-state chamber current from the ionizing radiation. Given that the pulses are on the order of 5 ms wide, there’s no obvious way to discard most of the noise without also tossing the signal.

Pending more thought, I’d say this was definitely fun while it lasted…

 

 

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