Ohm's Law Low-current Calibration System for Ionization Chambers

We describe our progress toward a system to calibrate electrometers that measure currents from ionization chambers. The calibration system uses a 1 GΩ standard resistor in series with a stable voltage source to generate calibration currents from 1 pA to 20 nA, traceable to quantum voltage and resistance standards through an 8-1/2 digit digital multimeter and the standard resistor. Expanded uncertainties ( k=2) of 100×10 ^-6 or better for the calibration of the electrometer are needed for the ionization chamber measurements. We show our preliminary results including our initial calibration correction.