Activity concentration of a solution of 125I: results of an international comparison

Nineteen laboratories have taken part in an international comparison of activity measurement of a solution of 125I organized by the Bureau International des Poids et Mesures (BIPM). Eight methods have been used by the participants. The total range of the results is 6.4% (2.5% if an outlier is excluded). The unweighted mean value of the 19 results, evaluated from the experimental data communicated by the laboratories, is (1 427.6 ± 4.4) Bq mg−1 at reference date. The uncertainty of the unweighted mean is 0.3% (1σ), and 1.3% for a single laboratory. Two of the values reported are rather distant from the other results, but no convincing reason has been found to exclude them. Six laboratories, two of them during the trial comparison, determined the half life of 125I and reported values between 59.3 d and 59.9 d.