Comparison of the air kerma standards for 137Cs and 60Co gamma-ray beams between the IAEA and the NIST

A comparison of the air kerma standards for Cs-137 and Co-60 gamma ray beams was performed between the NIST and the IAEA. Two reference class ionization chambers owned by the IAEA were used as part of this comparison and were calibrated at each facility. The calibration coefficients, N-K, were determined for both chambers and in both gamma-ray beams. The measurements were performed at the IAEA and NIST facilities starting in the fall of 2009 and were completed in 2010. The comparison ratio of the calibration coefficients for each chamber, N-K,N-IAEA/N-K,N-NIST, between the IAEA and NIST were 0.999 and 0.997 for the Cs-137 and Co-60 gamma ray beams respectively. The relative standard uncertainty for each of these ratios is 0.5 %.