P037] Implementation of a new free-air ionization chamber as a primary air kerma standard for mammography radiation qualities

Abstract

Purpose The Physikalisch-Technische Bundesanstalt (PTB) offers air kerma calibrations of secondary standards for standard dosimetry laboratories worldwide. Due to the significant increase in the demand for such calibrations in mammography X-ray beams (tube high voltages ranging from 20 kV to 50 kV), it was decided to construct and manufacture a novel free-air ionization chamber (FAC) with a design especially optimized for radiation qualities used in mammography. This work describes the final technical construction of the new FAC and its implementation as a primary air kerma standard at PTB. Methods The new FAC is of the parallel-plate type optimized for the photon energy range of 5 keV to 50 keV. Special emphasis has been given to the design of a chamber without potential wires in the beamline to minimize the scattering and attenuation of the X-rays. The design of the chamber was optimized with respect to electrical field homogeneity using the software package SIMION Version 8.1. This software was also used to calculate k d , the correction factor for the distortion of the electric field of the final chamber design. The other correction factors for photon attenuation, scattering, aperture transmission and scattering, electron loss and violating charged-particle equilibrium have been calculated using the user code ’egs_fac’ of the EGSnrc Monte Carlo transport simulation software package. Finally, air kerma rates measured with the PK50, the new free-air chamber, were compared with those measured with the other well-established primary air kerma standards of PTB. Results The PK50 was fully characterized by all the necessary correction factors. The correction for the field distortion is below 0.8%, thus the design goal has been achieved. For the ISO narrow series, the correction factors calculated with egs_fac are below 1.0%, proving that the chamber is well optimized for energies below 50 keV. Comparisons of air kerma measurements with the PK50 and established FACs yielded results which were consistent within the uncertainties of a few parts in 1000. Conclusions The new chamber designated as the PK50 is now ready for use as a primary air kerma standard for low-energy X-rays at PTB.