Subfemtoampere Resolved Ionization Current Measurements Using a High-Resistance Transimpedance Amplifier

In low-dose-rate dosimetry using ionization chambers, the predominant component in the uncertainty budget is a measurement uncertainty of current at a measurement range of around 10 fA. To improve this, in this article, we report on the development of a system for measuring a small ionization current based on a transimpedance amplifier utilizing a high-resistance feedback resistor. In this amplifier, we used a feedback resistor of 1 T Omega made of a metalized glaze thick film, which has clear traceability to a resistance standard. In addition, the nonlinear current-to-voltage profile of the feedback resistor and its contribution to the measurement uncertainty were carefully evaluated. Type-A uncertainty well below 1 fA can be achieved within a practical measurement time shorter than an hour. As a result, current measurements with relative expanded uncertainty from 0.1 % to 2 % were demonstrated for the ionization currents ranging from sub-fA to sub-pA generated by a Cs-137 gamma-ray source. This transimpedance amplifier with high- resistance feedback is appropriate for low-dose- rate radiation dosimetry needed for medical and environmental monitoring applications.