Abstract. A key comparison has been made between the air-kerma standards of the NIM and the BIPM in the medium-energy x-ray range. The results show the standards to be in general agreement at the level of twice the stated relative standard uncertainty of 3.1 × 10^-3, although there is evidence of a trend in the results at different radiation qualities. The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database.

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