Development of the new PTB air kerma primary standard—optimization of the geometry for minimal electric field distortions and assessment of the associated uncertainties using the finite element method

Abstract The German national metrology institute, Physikalisch-Technische Bundesanstalt (PTB), currently develops a new free-air ionization chamber (FAC) for use as a primary standard for realizing the unit of the air kerma free-in-air. The aim of this study was to optimize the FAC geometry to minimize electric field distortions and to analyze how engineering tolerances affect the uncertainty of the electric field correction factor. We simulated the electric field in the FAC using finite element method simulations and systematically varied the geometry of the FAC. The results of this study invalidated the common conclusion in the literature, that applying half the high-voltage potential to the housing of the FAC minimizes electric field distortions. Instead, we present two novel approaches for reducing electric field distortions by modifying the geometry of the FAC that yielded to an optimal geometry for the new PTB primary standard. The uncertainty of the resulting correction factors for electric field distortion was quantified as 0.04% by considering the engineering tolerances of the FAC.