| Abstract | The National Research Council of Canada’s standard for air-kerma in a ⁶⁰Co beam has remained fixed since 1990. As a result of a re-evaluation of various correction factors and an effort to make a realistic assessment of the uncertainties on Monte Carlo calculated correction factors, the NRC standard changed on Oct 1, 2003. The overall increase in the standard is 0.59% and the overall uncertainty on the standard has been reduced to 0.28%. This is a correction in the factors used, not the measurement data, and thus ⁶⁰Co air-kerma calibration factors received from NRC prior to Oct 1, 2003 can be converted to values consistent with the new standard by multiplying the old factor by 1.0059. The present change has no impact on either the absorbed dose to water standards at NRC or the air-kerma standards based on free-air chambers. On the other hand, the change directly affects the standards in ¹³⁷Cs beams which are obtained using chambers calibrated using the ⁶⁰Co standard. A result of the changes at NRC and related changes at NIST is that the ratio of the ⁶⁰Co air-kerma standards at NRC and NIST is now 1.0015, well within the uncertainties of the comparison. This compares well to the previous value of 1.0061. |
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