A growing range of medical treatments and diagnostic procedures use radioactive sources. The strength of these sources is often measured using ionisation chambers whose output is an electrical current in the range from picoamperes (10^-12 A) to microamperes (10^-6 A). Robust, traceable calibration of the electrical current readout is vital for validating the stability and linearity of the ion chamber systems.

Simplified traceability scheme from primary standards (top) to ion chamber readout (bottom) via configurable electrometer and portable current references (middle)

The NPL Medical Radiation Science group operates 10 ion chamber systems, as well as a clinical linac. They provide calibration services to the National Health Service, enabling the calibration and validation of test equipment in hospitals.

Each ionisation chamber has its own readout electrometer. In practise these are calibrated against a transportable current source, which is calibrated using an electrometer via traceable standards of voltage and resistance (for currents above 1 nA) and capacitance, voltage and time (for currents below 1 nA). One key question is, how stable is the transportable current source? If it is calibrated and then moved to another lab, how well does it retain its calibration?

To answer this requires a reference source/meter superior to the instrumentation under investigation. The Ultrastable Low-noise Current Amplifier (ULCA) is a new and powerful tool developed at PTB for addressing precisely this kind of question. NPL are presently using an ULCA unit on loan from PTB as part of the e-SI-Amp project to check the robustness of the transportable current. This has revealed instability of the portable source they normally used in their calibration chain (see figure) at the ~10 ppm scale, for instance after power-cycling the current source. One solution being considered is to substitute this for an ULCA, cutting out some intermediate calibrations and simplifying the traceability chain.

The ULCA will also make new tests of the ion chamber electrometers themselves possible. These instruments largely rely on a “capacitor ramp” principle, which can be affected by non-ideal behaviour of the capacitors giving an apparent frequency dependence. The ULCA is the first instrument with the required accuracy and practical ease of use to properly investigate these factors.

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