A bilateral comparison between LNHB and PTB to determine the activity concentration of the same 125 I solution.

A bilateral comparison to determine the activity concentration of the same ¹²⁵ I solution was organized. As electron-capture radionuclide with a rather high atomic number, ¹²⁵ I must be regarded as difficult to measure. The situation is partly exacerbated by the fact that some established standardization methods, like photon-photon coincidence counting, can no longer be applied due to the unavailability of appropriate equipment and expertise. One aim of this work is to compare modern liquid scintillation counting methods for the standardization of ¹²⁵ I. Both participating metrology institutes have used their custom-built triple-to-double-coincidence ratio (TDCR) counters and the determined activity concentrations are in excellent agreement even though the ways to analyze the data and to compute counting efficiencies were widely independent. The results also agree with the outcome of 4π-γ counting that was carried out at LNHB. In both laboratories, the measurements were complemented by measurements with several secondary standardization methods which even allow to establish a link to the CCRI(II)-K2.I-125(2) comparison started in 2004. A good agreement between the TDCR results and the key comparison reference value of the 2004/2005 comparison was obtained.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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