Performance Optimizations and Evaluations for the Small Direct Currents Measurement System

Abstract

Ionization chambers are essential for activity determinations in radionuclide metrology. We have developed a high-precision integrating-differentiating (int-diff) system for measuring small currents. It is anticipated to enhance the ionization current measurement capability of the 4{\pi}{\gamma} ionization chamber radioactivity standard at the National Institute of Metrology (NIM), China. Besides, it has broad application prospects in physical experiments and fundamental metrology. The design of the measurement system is optimized through circuit analysis and simulation. The structure of the integrating capacitor array is redesigned to reduce the error of the amplification gain, and a relay is used as the reset switch to achieve improved noise and leakage performance. The digital readout and control module is also enhanced in terms of flexibility and functionality. High-precision test platforms utilizing the standard small current source at NIM China and an ionization chamber were developed to evaluate the performance of the system. The results demonstrate an ultra-low noise floor (<1 fA/\sqrt{Hz}) and a low current bias of fA-level, as well as a low temperature coefficient of the amplification gain of 2.1 ppm/{\textdegree}C. The short-term stability and linearity of the gain are also tested and exhibit comparable indicators to those of the Keithley 6430. Reasonable results are obtained in the long-term reproducibility test. Therefore, the system enables high-precision measurements for small direct currents and shows promise for applications in ionization chambers.