Comparison of 192Ir air kerma calibration coefficients derived at ARPANSA using the interpolation method and at the National Physical Laboratory using a direct measurement

The reference air kerma rate from Ir-192 High Dose Rate (HDR) brachytherapy sources can be measured using a suitably calibrated Fanner chamber and an appropriate in-air calibration jig. When a primary standard for Ir-192 gamma rays is available, a calibration coefficient for the chamber and jig combination can be determined directly. In Australia, due to the absence of such a standard, the chamber must be calibrated by interpolation of the response in Co-60 and in a kilovoltage x-ray beam. Corrections for the effect of the jig, scatter and beam non-uniformity must then be measured or calculated before the reference air kerma rate can be determined. We compare the air-kerma calibration coefficient of a PTW 30010 PMMA/Al Fanner chamber (referred to as Farmer chamber throughout this report) obtained from the Ir-192 primary standard at the National Physical Laboratory in the UK with the corresponding coefficient obtained by interpolating Australian calibrations using Co-60 and 250 kV x-rays and determining suitable correction factors. The resulting chamber/jig calibration coefficients differ by 0.2% which is well within the combined standard uncertainties of 1.2% and 0.6% reported by ARPANSA and NPL respectively.