The X-Ray beam in the kilovoltage energy range is characterised in terms of its ability to penetrate material. It is customary to describe the quality of kilovoltage-energy X-Ray beams in terms of the half-value layer (HVL), together with the peak voltage.
The HVL is defined as the amount of material placed in the path of the beam required to attenuate the intensity of a narrow unidirectional X-Ray beam at a point distant from the absorber to half of its original value. For orthovoltage X-Rays, both aluminium and copper are used as the absorbing material for the determination of the HVL; aluminium (Al) is used in the low energy range (lower than or equal to 100 kV) and copper (Cu) is used in the medium energy range (higher than 100 kV—AAPM TG-61).
The ideal setup for measurement is to place the attenuating material between the applicator and a small aperture—just large enough for the narrow beam to cover the measuring radiation detector, which is usually the ionisation chamber (IC). The absorbing materials (Al or Cu) are placed at least half way between the X-Ray target and the IC. The chamber is positioned so that scatter from the attenuating material and backscatter from material behind the chamber (floors, walls, etc.) are minimised.
Detailed descriptions of measurement technique are well documented in textbooks and dose measurement protocols such as the IPEM and AAPM TG-61.
Half value layers
The IC should ideally have limited beam quality dependence for accurate HVL measurements. As long as the calibration factor of the chamber varies by less than 2% over the quality range being measured, this will not significantly affect the results. Most dosimetry services only offer a calibration in terms of half value layer, although for a given HVL, the dose to water calibration factor varies by a little over 2% depending on the accelerating potential and the filtration.
In the low energy kilovoltage range parallel plate chambers specifically designed for low kV dosimetry should be used. The thin entrance window of these chambers is sufficient to remove electron contamination and provide full build-up conditions up to 1 mm HVL (approx. 50 kV). If the chambers are used with a beam with a greater HVL plastic foils must be placed in the way of the beam, to ensure full build-up conditions . These foils must be included with the ionisation chamber when it is calibrated. In the medium energy range a cylindrical ion chamber with a volume less than 1.0 cm3 must be used, whether the calibration is performed in-air or in-phantom. A typical chamber would be a ‘farmer’ type chamber such as an NE2571 or NE2561.