An SI traceable electrometer system for radionuclide metrology

A measurement system for ionisation chamber currents below 1 nA is presented. The system is based on a standard capacitor charging technique, and uses a commercially available electrometer and air-gap capacitor. We find that the uncertainties in the current range 100 fA-1 nA are up to an order of magnitude better than those available from a state-of-the-art commercial electrometer alone.