Are non-invasive tube voltage measurements sufficiently accurate for establishing reference X-ray fields for calibration of radiation protection dosimeters?

Radiation protection calibrations are performed in reference radiation fields established according to ISO 4037-1 standard. Accurate X-ray tube voltage measurements are necessary for X-ray radiation qualities, because many relevant parameters, most importantly, conversion coefficients from air kerma to operational quantities, depend on tube voltage. The standard requires invasive measurements using a calibrated voltage divider, but this requires significant additional investments and is relatively complicated. Other, non-invasive, methods exist, but they are not recommended by the standard. In this paper, the invasive and non-invasive methods are compared and their uncertainty budgets are analyzed. Tube voltage range between 40 kV and 150 kV was investigated. Comparison between different methods is excellent within the reported uncertainties, with only 2 non-conforming results, out of 91. The results show that non-invasive methods, such as linear extrapolation by spectrometry, spectrometric practical endpoint, and X-ray multimeters can be used to achieve the measurement uncertainties that meet the requirements for most of standard X-ray radiation qualities, which makes them a viable alternative to voltage dividers.