Abstract. Since 1978, six national metrology institutes (NMI) have submitted eleven samples of known activity of ¹⁶⁹Yb to the International Reference System (SIR) for activity comparison at the Bureau International des Poids et Mesures (BIPM), with comparison identifier BIPM.RI(II)-K1.Yb-169. The activities ranged from about 200 kBq to 46 MBq. The degrees of equivalence between each equivalent activity measured in the SIR and the key comparison reference value (KCRV) have been calculated and the results are given in the form of a matrix for four NMIs. A graphical presentation is also given. The results of a EUROMET regional comparison, comparison identifier EUROMET.RI(II)-K2.Yb-169, completed in 1997 for this radionuclide have been linked to the SIR results through those of the BNM-LNHB and the IRA. This has enabled five other NMIs and an international laboratory to have degrees of equivalence in the KCDB and two other NMIs to update their earlier results in the SIR.

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The final report has been peer-reviewed and approved for publication by Section II of the Consultative Committee for Ionizing Radiation (CCRI(II)), according to the provisions of the Mutual Recognition Arrangement (MRA).