Contents I. Introduction
The accurate measurement of small direct currents has wide applications both in fundamental metrology and physical sciences. For example, a picoamperemeter (picoammeter) can measure light from a photomultiplier tube whose anode current ranges from picoamps to microamps and ion beam current typically less than [1]. In radionuclide metrology, ionization chambers have been extensively used for activity and half-life determinations because of their intrinsically good linearity and long-term stability. One factor limiting the uncertainty of activity determination is the precision of the ionization current measurement [2].
