We compared the performance of a commercial ammeter and a home-made integrating electrometer in reading ionisation chamber currents less than 100 pA. The integrating electrometer charges a capacitor with the unknown current and measures the resulting rate of change of voltage, whereas the ammeter uses a high-value resistor as the feedback element to an amplifier which converts current to voltage. The noise performance of both systems was very similar for averaging times less than 1000 s. Both systems were calibrated using a reference current source with 1 part per million (ppm) accuracy, revealing an error of 460 ppm in the electrometer indicated current, of unknown origin. This error is well within the uncertainty budget for radionuclide calibrations but much larger than the individual uncertainties in the traceable calibrations of capacitance, voltage, and time. The noise in the ionisation chamber current was much larger than the noise floor of both instruments, with tests providing strong indication that the excess noise originated in the high voltage source used for energising the chamber.
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We could have used the 100 pA range, achieving slightly lower instrument noise at the expense of slower response time. However, the noise in IA is roughly two orders of magnitude higher than the instrument noise floor so no benefit is obtained by using a lower range.
The term “noise,” frequently used in this paper, means random fluctuations in a measured signal, irrespective of the origin of those fluctuations.
Instability in the analogue-to-digital conversion of the preamp output voltage can be ameliorated by using the ammeter’s auto zero function. However, each reading then takes at least twice as long.
The averaging time at which the Allan deviation changes from white-noise behaviour to frequency-dependent behaviour depends on the instrument range, the stability of the environmental conditions and the particular unit of instrument (of the same model number).
