Low energy X-rays (LEX)

Ionisation chambers are calibrated directly against the primary standard of air kerma for low-energy X-rays, the Low Energy Free Air Chamber (LEFAC). During a measurement, the LEFAC is placed in the beam and the air kerma rate determined. The LEFAC is then replaced with the chamber to be calibrated, and the same X-ray beams are used to expose the chamber. A monitor chamber corrects for variations in the X-ray tube output.

Currently, low energy X-rays are provided by an ex-clinical RT100 generator. The X-ray tube has a stationary tungsten target. The beams used for radiotherapy calibrations are listed in the table below. Beam RT1 to RT7 are included in a calibration for a single fee.

Figure 2: Parallel plate chamber mounted in a horizontal X-ray beam for calibration against the Low Energy Free Ait Chamber (LEFAC) - the grey box to the left

Figure 1: Example of the type of thin-window parallel plate chambers usually calibrated for low energy X-rays

Calibration in a phantom

Calibrations are performed free in air. However, plane-parallel chambers of the type shown in Figures 1 and 2 are convenient to use when mounted in a solid phantom. ARPANSA can calibrate the chambers in such phantoms. However, it is important for users to recognise that such a calibration is for air kerma (free in air) and not for absorbed dose to water, despite the presence of the phantom. Users should contact ARPANSA for clarification if they wish to proceed down this path.