Ionisation chambers are calibrated directly against the primary standard of air kerma for medium-energy X-rays, the Medium Energy Free Air Chamber (MEFAC). During a measurement, the MEFAC is placed in the beam and the air kerma rate determined. The MEFAC is then replaced with the chamber to be calibrated, and the same X‑ray beams are used to expose the chamber. A monitor chamber allows for variations in the X‑ray tube output (Figure 1).
Figure 1: Chambers are calibrated directly against the primary standard (the Medium Energy Free Air Chamber - MEFAC) by shifting each chamber into the beam consecutively.
Currently, medium energy X-rays are provided by a Gulmay 320 kVp generator. The X-ray tube has a stationary tungsten target. The beams used for radiotherapy calibrations are listed in the table below. The calibration procedure is largely automated and so all the beams are included for a single fee.
Many therapy ionisation chambers such as the Farmer chamber have build-up caps designed for megavoltage radiation. This cap is removed for kilovoltage beams. The highest energy beams are also measured with the build-up cap on for the purposes of calculating a calibration coefficient for Iridium-192.
|Beam ID||kVp||Additional filtration||Air kerma rate (mGy/s)||First HVL (mm AI)||First HVL (mm Cu)||Effective energy (approx.) (keV)|
|AI (mm)||Cu (mm)|
Note that polarity and recombination corrections are not provided for kilovoltage calibrations. The uncertainties resulting from these omissions are small for the air kerma rates used in the calibration and the chamber types calibrated. Nevertheless the user should use the chamber at the polarity for which it is calibrated.