The key measurements in the medical dosimetry laboratory are carried out using ionization cambers connected to highly sensitive electrical measurement equipment.
We provide a calibration service for electrometer charge and current measurements..
Three electrometers systems are used for calibrations in cobalt and MV photon beams from the linear accelerator:
(1) The master electrometer is a Keithley 6517B electrometer with an external 1 nF low-leakage, air-filled, feedback capacitor. This air capacitor makes the system highly linear and stable as there is minimal influence of dielectric absorption normally found in other capacitors. Depending on the measurement task, the electrometer range can be set to 2 V, 20 V, or 200 V which corresponds to a maximum charge accumulation of 2 nC, 20 nC, or 200 nC, respectively. The system is calibrated in-situ against standards for DC voltage and capacitance, and the calibration therefore includes stray capacitance related to, for example, cables for the feedback capacitor. For DC current measurements, the charge accumulation is recorded every second triggered by an external 1 Hz frequency standard, and the linear regression slope of the charge vs. time recordings gives the current. With typical integration times from 30 s to 100 s, we can measure currents in the range from <10 fA to 6 nA. Current measurements in the femto ampere range are relevant mainly in the context of measuring cable and ionization chamber leakage.
A separate Keithley 6517 is used for the high voltage supply (up to 1000V), and the system can therefore be configured for floating or grounded measurements using connectors such as TNC, BNT, BNC/banana, or M-type.
(2) A second electrometer system identical to the master system described above is used for independent checks.
(3) A third electrometer system based on a Keithley 6514 electrometer is used for recording the output from a PTW7862 external monitor chamber used during measurement in the linear accelerator. An important feature of this electrometer is that it covers the range up to 20 µC. A separate Keithley 6517 is used for high-voltage supply.
The three electrometer systems are controlled using MELab software, a general-purpose data acquisition system developed by DTU Health Tech. For current measurements, the recording of charge vs. time takes place internally within each individual Keithley electrometer using so-called buffered data acquisition. The three electrometer systems are synchronized to the same external 1.000 Hz frequency standard.