Changes to the NPL air kerma standard for 137Cs and 60Co.

The NPL maintains a primary standard of air kerma defined as the mean corrected response of three cavity ionisation chambers, which were designed to operate in 2 MV X-rays at therapy-level air kerma rates. The therapy level exposure calibration service for 2 MV X-rays was first instituted at the NPL in 1956; a chamber correction factor F was applied to the mean response of the standard cavity chambers. This factor was revised in 1969 and again in 1992 for 1 MV and 2 MV X-rays and for 137Cs and 60Co gamma-rays. This report documents the changes to the correction factors and presents new work re-evaluating the chamber correction factors for 137Cs and 60Co gamma-rays.