Air kerma rate calibration of small sources of60Co,137Cs,226Ra and192Ir

The traceability routes for the calibration of brachytherapy sources supplied by Amersham International plc were recently summarized (Rossiter 1990) following continued interest in this topic by users. Such calibrations have depended on an ionometric comparison of the source output with one of a set of 60 Co reference sources. These reference sources have been calibrated for the measurement of brachytherapy sources by ionometric comparison, first with each other, then with an Amersham International 226 Ra reference source which itself had been calibrated at NPL by comparison with the British National Radium Standard. The air kerma rate (formerly exposure rate) at 1 m distance from each 60 Co reference source was then calculated using the ionometric ratios, the widely accepted exposure rate constant for 226 Ra and other minor corrections. The measurements have been extended to the important brachytherapy radionuclide 192 Ir. The results indicate a level of agreement between NPL and Amersham source output measurements which is within the NPL measurement uncertainties alone.