Implementation of the ISO 4037:2019 standard using voltage dividers and X-ray spectrometry

The ISO 4037:2019 standard is the reference standard for dosimetry laboratories who wish to realize dosimetric operational quantities for radiation protection calibrations. In implementing the ISO 4037:2019 standard, the X-ray radiation qualities need to be defined according to strict requirements on the material and thickness of the additional filtration, and according to metrologically traceable high voltage bias applied to the X-ray tube. This enables usage of standardized conversion coefficient from air kerma to operational quantities. However, the tube potential may vary as a function of tube current if a protective resistor is built into the protective tube housing, which, if not corrected, alters the energy distributions of the reference field and thus the value of the appropriate conversion coefficients. Particularly at low energies, the energy dependence of the conversion coefficients can be sharp and, depending on the specific realization of a radiation quality, the conversion coefficient from air kerma to dose equivalent can vary substantially. We have investigated the correspondence between the tube voltage measurements of a X-ray system based on calibrated voltage dividers and that of an X-ray spectrometer system based on a Cadmium Telluride (CdTe) detector in order to obtain an estimation of the resistance of the protective resistor that is usually unknown. A method based on X-ray spectrometry for calibration of the tube potential even in the presence of tube with protective resistor is presented. Finally, conversion coefficients were calculated using simulated spectra to study the influence of the protective resistor on the determination of these coefficients. The simulated spectra obtained from X-ray tube with and without a protective resistor resulted in differences in conversion coefficients mostly ˂2% but 5.6% for the radiation quality considered at the lowest energy.