Low-level electrical measurement system

Key measurements in the medical dosimetry laboratory are performed using ionization chambers connected to highly sensitive electrometer systems, enabling accurate measurement of charge and current under both cobalt-60 and megavoltage photon beam conditions. We provide calibration services for electrometer charge and DC current measurements, supporting traceable medical dosimetry.

Three electrometer systems are used for calibration and reference measurements. The master system is based on a Keithley 6517B electrometer equipped with an external 1 nF low-leakage air-filled feedback capacitor, providing excellent linearity and long-term stability with minimal dielectric absorption effects. The system is calibrated in situ against DC voltage and capacitance standards, including all relevant stray capacitances.

Charge measurements up to 200 nC are supported, and DC current is determined from time-resolved charge accumulation, enabling measurement of currents from the femtoampere range up to several nanoamperes. A dedicated high-voltage supply allows flexible floating or grounded configurations.

An independent second system, identical to the master electrometer setup, is used for verification and consistency checks. A third electrometer system, based on a Keithley 6514, is used for measurements with the external monitor ionization chamber during linear accelerator work and supports charge measurements up to 20 µC.

All electrometer systems are operated and synchronized using DTU-developed MELab software and a common external frequency standard, ensuring consistent timing, traceable measurements and reproducible data acquisition across all calibration and dosimetry activities.