Calibration of low activity caesium tubes and needles traceable to the therapy level standard

A technique for deriving the reference air kerma rate for low activity brachytherapy sources that is traceable to the therapy level standard at the National Physical Laboratory is presented. Correction factors have been generated to account for the finite source and detector size. The air kerma rate calibration of the secondary standard for caesium quality has been derived by polynomial curve fitting. The reference air kerma rates for several caesium tubes and needles have been determined and the results compared with the manufacturers' source test reports. For all the source types used the agreement between methods was within 2%.