Speaker
Description
Due to a constant demand for standards of the radionuclide 125I, LNHB and PTB decided to verify their calibration capabilities by means of a bilateral comparison to determine the activity concentration of the same 125I solution. As electron-capture radionuclide with a rather high atomic number, 125I must be regarded as difficult to measure. The situation is partly exacerbated by the fact that some established standardisation methods, like photon-photon coincidence counting, can no longer be applied due to the unavailability of appropriate equipment and expertise.
On the other hand, the liquid scintillation counting (LSC) methods have been further developed in recent years and today even allow the standardization of 125I. Both participating metrology institutes have used their custom-built triple-to-double-coincidence ratio (TDCR) counters to measure aliquots of the same 125I solution, that was provided by LNHB. The activity concentrations are in excellent agreement even though the ways to analyse the data and to compute counting efficiencies were completely 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 ionization chambers serving as important tools for secondary standardization. Eventually, PTB could also apply an LSC-based secondary standardization method. The results of all secondary standardization methods are slightly higher than those from the TDCR measurement, but still in very good agreement. The secondary standardization techniques allow us to establish a link to the comparison CCRI(II)-K2.I-125 organized in 2004. Even if this link is fraught with some uncertainty, it allows us to draw some important conclusions. We find a good agreement between the TDCR results and the key comparison reference value of the 2004 comparison. Moreover, the results make it possible to evaluate calibration factors for the ionization chambers which are usually the working horses for routine activity determinations in metrology institutes.
