Free-air ionization chambers for the measurement of air kerma in low-energy x-rays—optimum air path length and the limitations of averaging monoenergetic correction factors

Abstract The German national metrology institute Physikalisch-Technische Bundesanstalt is developing a new primary standard for the realization of the quantity air kerma free-in-air in the x-ray energy range up to 50 keV. National metrology institutes use free-air ionization chambers (FACs) for this purpose. FACs with various geometrical dimensions are used for low-energy x-rays. This study aims to (1) determine the optimum air path length of a FAC, which is the most critical geometric parameter for the low-energy x-ray range, and (2) to evaluate the limitations of averaging monoenergetic correction factors, a method that many institutes use to enable time-saving calculation of spectral correction factors for a larger number of radiation qualities. Monte Carlo simulations were performed to compute monoenergetic and spectral correction factors for air attenuation, photon scattering, inadequate charged particle equilibrium, and diaphragm effects. In addition, spectral correction factors were calculated by averaging the monoenergetic correction factors with different x-ray spectra. The results show that for low-energy FACs the predominant correction is the correction for air attenuation. Thus, it is preferable to have a minimized air path length. The minimization of the air path length is limited by the impact of electrons created by diaphragm interactions and the loss of charged particle equilibrium. If the air attenuation between the aperture and the collecting volume is considered for a 10 kV x-ray spectrum, the averaged and simulated spectral correction factors agree within less than 0.1%.