Abstract. In 1998, five national metrology institutes (NMI), an international laboratory and the Bureau International des Poids et Mesures (BIPM) took part in a comparison of activity measurements of ²³⁷Np as part of the EUROMET project No 416 comparison identifier EUROMET.RI(II)-K2.Np-237. The nominal activity per mass of the ²³⁷Np solution distributed was 150 kBq g^-1. The three ampoules received by the BIPM for this comparison were also measured in the International Reference System (SIR) for activity comparison at the BIPM (identifier BIPM.RI(II)-K1.Np-237). Two other participants to the EUROMET comparison also submitted ampoules of the same ²³⁷Np solution to the SIR. These SIR measurements enable the EUROMET comparison to be linked to the BIPM.RI(II)-K1.Np-237. The degrees of equivalence between each laboratory and the key comparison reference value (KCRV) have been calculated and the results are given in the form of a graph and matrix for the three SIR participants and four other NMIs from the EUROMET comparison.

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