For reliable and comparable measurements of the dose quantity air kerma, dosimeter measurements must be traceable to a primary standard. Primary standard laboratories use free-air ionization chambers (FACs) for the primary realization of the unit of the air kerma free-in-air. Correction factors must be applied to convert measured charge to air kerma. One such correction factor is the correction factor for diaphragm effects (k_dia). This study investigated the impact of the geometry of the diaphragm on k_dia, as established FACs from different metrology institutes use different diaphragm geometries. The aim was to find the optimal diaphragm thickness and aperture shape to minimize the required diaphragm correction for the new PTB primary standard. Monte Carlo simulations were performed to determine k_dia for various diaphragm geometries of a low-energy x-ray FAC. The influence of the diaphragm thickness and the aperture shape were investigated. The results showed that the diaphragm needs to be sufficiently thick to prevent transmission yet as thin as possible to reduce scattering at the inner surface of the aperture. The optimal diaphragm thickness, which depends on the air path length of the FAC, ranges from 0.8 mm to 1 mm. Using a diaphragm geometry with a more complex geometry than a simple 1 mm thick diaphragm with a cylindrical aperture is not advantageous.

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