NPL's new absorbed dose standard for the calibration of HDR 192Ir brachytherapy sources

A novel graphite calorimeter for absorbed dose rate measurements close to high dose rate (HDR) Ir-192 brachytherapy sources has been designed and built at the UK National Physical Laboratory (NPL). The graphite calorimeter allows a more direct calibration of HDR Ir-192 sources in terms of absorbed dose rate to water at a distance of 1 cm, (D) over dot(w, 1 cm), compared with the current air kerma-based calibration method, where (D) over dot(w, 1 cm) is determined as the product of the measured air kerma strength and the dose rate constant, Lambda, resulting in overall standard uncertainties of up to 5%. With the new absorbed dose standard, (D) over dot(w, 1 cm) can be measured directly with standard uncertainties between 0.7% and 1.0%, depending on the operating mode of the calorimeter. A Nucletron microSelectron-v1 Classic HDR Ir-192 source was calibrated with the calorimeter in terms of (D) over dot(w, 1 cm). The same source was also calibrated with NPL's HDR Ir-192 air kerma primary standard in terms of reference air kerma rate. Combining both measurements yielded the experimentally determined dose rate constant of the Ir-192 source.