Results of an international comparison of 57Co

As part of a Cooperative Research Project (CRP) aimed at improving the state of radioactivity measurement in nuclear medicine, the International Atomic Energy Agency (IAEA) organized a comparison of 57Co solutions among the participants of the project. The comparison solutions were prepared from a single master stock solution and distributed to the participating laboratories, who measured the activity concentration of the solution using either the laboratory's radionuclide activity calibrator or primary standardization methods. A total of 9 sets of results were received, with 5 laboratories reporting results of primary measurements, one reporting results of secondary measurements calibrated against primary standards, and three laboratories reporting values based on measurements in commercial re-entrant ionization chambers using manufacturer-recommended calibration figures. Most of the laboratories reporting primary standardizations also provided results from secondary standardizations. The Comparison Reference Value was calculated from the mean of the five primary standardizations and was found to be 35.54 MBq g−1, with a standard deviation of the mean of 0.17 MBq g−1. Degrees of equivalence were calculated for each reporting laboratory and demonstrated that equivalence to within about 4% could be achieved, even in the case of those laboratories that used instruments calibrated by third parties.