Key comparison BIPM.RI(I)-K1 of the air-kerma standards of the NIST, USA, and the BIPM in ⁶⁰Co gamma radiation

A new key comparison of the standards for air kerma of the National Institute of Standards and Technology (NIST), USA, and the Bureau International des Poids et Mesures (BIPM) was carried out in the ⁶⁰Co radiation beam of the BIPM in October 2023. The comparison result, based on the calibration coefficients for two transfer chambers and expressed as a ratio of the NIST and the BIPM standards for air kerma, is 1.0044 with a combined standard uncertainty of 3.4 parts in 10³. The result agrees within the uncertainties with the indirect comparison carried out in 2011, when updated with the changes implemented to the standards at each laboratory. The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database.