Giblin S P, Lorusso G
National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom.
Rev Sci Instrum. 2019 Jan;90(1):014705. doi: 10.1063/1.5052717.
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.
我们比较了一款商用电流表和一台自制积分静电计在读取小于100 pA电离室电流时的性能。积分静电计用未知电流对一个电容器充电，并测量由此产生的电压变化率，而电流表则使用一个高值电阻作为放大器的反馈元件，该放大器将电流转换为电压。对于小于1000 s的平均时间，两个系统的噪声性能非常相似。两个系统均使用精度为百万分之一（ppm）的参考电流源进行校准，结果显示静电计指示电流存在460 ppm的误差，误差来源不明。该误差完全在放射性核素校准的不确定度预算范围内，但远大于电容、电压和时间的可溯源校准中的个体不确定度。电离室电流中的噪声远大于两台仪器的本底噪声，测试有力地表明，过量噪声源自用于给电离室供电的高压源。
