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NIST Ionization Chamber "A" Sample-Height Corrections



Ryan P. Fitzgerald


For over 30 years scientists in the NIST radioactivity group have been using their ionization chamber “A” to make measurements of radioactivity and radioactive half-lives. We now have evidence that some of those reported measurements were incorrect due to slippage of the source positioning ring over time. The temporal change in the holder caused an error in the source-height within the chamber, which was thought to be invariant. This unaccounted-for height change caused a change in the detector response and thus an error in measured activity on the order of 10^-5 to 10^-3 per year, depending on the radionuclide. The drifting detector response affected calibration factors and half-life determinations. After discovering the problem, we carried out historic research and new sensitivity tests. As a result, we have created a quantitative model of the effect and have used that model to estimate corrections to some of the past measurement results from ICA. In this paper we report the details and results of that model. Meanwhile, we have fixed the positioning ring and are recalibrating the detector using primary measurement methods and enhanced quality control measures.
Journal of Research (NIST JRES) - 117.003
Report Number


Half-life, Ionization Chamber


Fitzgerald, R. (2012), NIST Ionization Chamber "A" Sample-Height Corrections, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed June 16, 2024)


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Created February 2, 2012, Updated November 10, 2018