Defined Solid Angle (DSA) alpha spectrometry is a primary method for absolute activity determination of Radon-222, in which radon is cryogenically depositing onto a polished cold disk under vacuum and counted in a fixed source–detector geometry. In this approach, the detection efficiency is determined exclusively by the normalized solid angle, expressed as the Geometry Factor G. The accuracy of activity determination therefore depends directly on the precise evaluation of G and its associated uncertainty. This work presents a comprehensive numerical investigation of the sensitivity and uncertainty of G with respect to the governing geometrical parameters: source–diaphragm distance z, diaphragm radius a, source radius b, and eccentricity e. Calculations were performed using both the Knoll (Radiation detection and measurement, 4th ed. Wiley, New York, S120, 2010) and Curtis (Nucleus 13:38, 1955. https://doi.org/10.1016/S0168-9002(96)80029-5) analytical extensions, enabling systematic parameter variation and direct model comparison. The results show excellent agreement between the two models for concentric configurations (e = 0 mm). The Geometry Factor is found to be most sensitive to variations in z and a; however, uncertainty propagation analysis demonstrates that the dominant contribution to the combined uncertainty arises from z, accounting for approximately 88% – 97% of the total variance under typical National Metrology Institute (NMI) conditions. In contrast, the influence of b is significantly smaller, and eccentricities up to 1 mm produce only minor variations in G. These findings quantitatively identify the source–diaphragm distance as the critical parameter limiting uncertainty reduction in DSA-based primary standardization of Radon-222. The study provides a clear metrological framework for optimizing geometry control and supports the harmonization of high-accuracy activity measurements across NMIs, enabling more stable and reliable transfer standards for secondary laboratories and end-users.
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CC BY 4.0 ; Creative Commons Attribution 4.0 License
Themenbereich der Metrologie
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Khanbabaee, B., Honig, A., & Röttger, S. (2026). Evaluating geometrical parameters in defined solid-angle alpha spectrometry for absolute Radon-222 activity measurement. European Physical Journal: Special Topics, 1–13. https://doi.org/10.1140/epjs/s11734-026-02278-y
