Proposed specification for a primary standard of air kerma for 60Co, 137Cs and 192Ir -ray sources.

The three cavity chambers, the mean response of which constitutes the primary standard of sir kerma for 60Co and 137Cs -rays in the United Kingdom, have been in continuous, almost daily, use at the National Physical Laboratory (NPL) since 1956. These chambers were initially designed for use with 2 MV X-rays at therapy level air kerma rates. However since 1978 that have also been used for protection level air kerma rates, initially, with X-rays generated at 1 MV and 2 MV and more recently with 60Co and 137Cs -rays. They have been used for therapy level air kerma calibrations with 60Co -rays since 1997 following the demise of the NPL 2 MV Van de Graaff generator. This report describes the proposals for a new primary standard and methods that will be used to give better performance than the present standard when used with air kerma rates from as high as 1 Gy min-1 down to 10 mGy hr-1. The design will also seek to ensure that the standard will be capable of providing traceably calibration for 192Ir high doserate brachytherapy sources.