Development of a New Primary Standard for the Dose Measurement Quantity Air Kerma in X-ray Diagnostics

Abstract

Mammography is the main screening modality for the early detection of breast cancer. However, the use of ionizing x-ray radiation may also increase the risk of cancer. This highlights the importance of quality assurance of mammography devices. In acceptance and constancy tests diagnostic dosimeters are used to measure air kerma, a quantity describing the kinetic energy transferred from indirect ion-izing radiation to charged particles in air. To ensure reliable and traceable measurements, these do-simeters must be calibrated regularly. The Physikalisch-Technische Bundesanstalt (PTB), the national metrology institute of Germany, realizes the unit gray for the quantity air kerma using free-air ioniza-tion chambers (FACs) as primary standards. Due to the increasing demand for calibrations with a pri-mary standard in the x-ray energy range below 50 keV, as used in mammography, a new air kerma primary standard, called MAKS (Mammography �ir ]Serma �tandard), was developed within the scope of this work. The objective of this work was the development a FAC for low-energy x-rays. Monte Carlo and finite element method (FEM) simulations were systematically employed to optimize the FAC geometry with the aim of minimizing required correction factors and reducing associated uncertainties. Based on these simulations, engineering tolerances for individual components were derived. MAKS was subse-quently manufactured at PTB using high-precision, state-of-the-art techniques. In the following MAKS was characterized. Monte Carlo simulations were used to determine correction factors for diaphragm effects kctia, photon scattering ksc, radial electron losses keloss, and inadequate charged particle equi-librium kcpe· FEM simulations were applied to determine the collecting length and collecting volume, as weil as the correction factor for electric field distortion kct, The default polarizing voltage and cor-rection factors for ion recombination k5, polarity effects kpo!, and photon attenuation in air kau were determined experimentally. An uncertainty budget for air kerma measurements with MAKS was es-tablished. Finally, MAKS was compared with the existing PTB primary standard PK100, developed in the 1950s. The correction factors required for MAKS are closer to unity than those of the PKl00. In particular, shortening the air path length between the reference plane at the diaphragm and the center of the collecting volume reduced the air attenuation correction, which is especially significant at low x-ray energies, by approximately 40 %. This design choice, however, introduces challenges due to electric field distortion and an increased contribution to the air kerma from particles produced by interactions with the diaphragm. Both effects were successfully reduced by optimizing the FAC geometry, resulting in correction factors kct and kctia close to unity. High precision manufacturing has reduced the com-bined standard uncertainty of air kerma measurements from 0.52 % for the PK100 to 0.38 % for MAKS. Despite successful characterization, a comparison between MAKS and PK100 revealed a systematic and significant deviation of approximately 1.4 % in the determined air kerma. Extensive investigations did not identify any systematic error in the measurements with MAKS. Consequently, further work is required prior to establishing MAKS as an air kerma primary standard. This includes long-term stability studies, reproduction of the most critical component - the collecting electrode, international compar-isons with other established primary standards, and a re-evaluation of the collecting length and cor-rection factors of the PK100. The latter is expected to be limited by incomplete historical documenta-tion on the PKl00. Nevertheless, the results of this work contribute valuable insights into the realiza-tion of air kerma for low-energy x-rays using FACs and support the further advancement of primary dosimetry in diagnostic radiology.