Individual differences of NACP plane-parallel chambers

When an ionisation chamber, calibrated in a 60Co γ-ray beam, is used for absorbed dose measurements in a water phantom in another beam quality, some conversion factors have to be known. If the calibration is performed in terms of air kerma a conversion factor to absorbed dose to water is needed, and if the calibration is performed in terms of absorbed dose to water, the difference in perturbation properties in the different radiation qualities has to be known. With cylindrical ionisation chambers there is a long experience of using these conversion factors. These chambers where originally designed for photon beams and the factors are tabulated for each type of chamber in a number of dosimetry protocols and elsewhere in the literature. For plane-parallel chambers, originally designed for electron beams, much less is known about the conversion factors and their application. In the present work a large number of NACP chambers from two manufacturers are studied in order to find out if one unique factor can be applied to all chambers of this type, or if there are significant chamber-to-chamber, batch-to-batch or manufacturer-to-manufacturer differences. Beside the above conversion factors, the consistency in the correction of ion recombination and polarity effect is discussed. The use of both these factors rely on the assumption that the same method for obtaining it can be used regardless of which chamber is used. It is, however, shown that different properties of various chambers may result in discrepancies when measurements with high precision is to be obtained. The influence of these discrepancies on the over all accuracy is discussed