Air kerma rate calibration of small sources of 60Co, 137Cs, 226Ra and 192Ir

The results presented indicate a level of agreement between NPL and Amersham source output measurements which is within the NPL measurement uncertainties alone. The 3% discrepancy reported earlier was not reproduced in this work which may reflect an improved performance of the secondary standard ionization chamber in its present form. These results provide assurance on source air kerma rate figures provided by the manufacturer and which are based on a traceability route to standards of activity. Alternatively the results could be used as a basis to change the traceability route to air kerma standards. (author)