STAFF OF THE DOSIMETRY AND MEDICAL RADIATION PHYSICS (DMRP) SECTION

An indirect comparison of the standards for absorbed dose to water and air kerma of the National Research Council of Canada (NRC) and of the International Atomic Energy Agency (IAEA) was carried out in the Co radiation beam of the NRC in August 2016. Both comparison results were calculated based on the calibration coefficients for two transfer standards and expressed as the ratios of the IAEA and the NRC standards. The comparison results for absorbed dose to water and air kerma were 0.9997 (uc=0.66%) and 0.9963 (uc=0.47%) respectively. Introduction 1. The Dosimetry Laboratory of the International Atomic Energy Agency (IAEA), Seibersdorf, Austria, calibrates reference standards in Co gamma beams for IAEA/WHO SSDL Network members (more than 80 laboratories worldwide) free of charge. As a signatory of the Mutual Recognition Arrangement (CIPM MRA), a Quality Management System (QMS) complying with ISO 17025 is maintained in the IAEA laboratory and dosimetry calibration and measurement capabilities (CMCs) are published in Appendix C of the CIPM MRA key comparison database (KCDB). To maintain the validity of CMCs, updated “supporting evidence” for the measuring capabilities is required periodically in addition to the traceability of the measured quantities. An indirect (i.e. using transfer ionization chambers) comparison of the standards for absorbed dose to water and air kerma of the National Research Council of Canada (NRC) and of the IAEA was performed in the Co radiation beam of the NRC. The measurements were carried out in June and September 2016 at the IAEA and in August 2016 at the NRC. Materials and Methods 2. 2.1 Determination of reference values Determination of the absorbed dose to water at the NRC The absorbed dose to water, determined at the NRC, is based upon measurements made directly in water using the primary standard sealed water calorimeter. This is a Domen type water calorimeter [1] with modifications as described by Seuntjens et al [2] and was the basis for the NRC key comparisons with the Bureau International des Poids et Mesures (BIPM) in Co-60 [3]. The measured absorbed dose to water rate was around 0.75 Gy/min. Determination of the air kerma rate at the NRC The air kerma standard of the NRC for Co is a graphitewalled cylindrical cavity ionization chamber (designated 3C) with a volume of approximately 3 cm, constructed at the NRC in 1958 as described by Shortt and Ross [4]. It was used in the most recent comparison of the NRC and BIPM standards in 2009 [5]. The measured air kerma rate was around 0.84 Gy/min. Determination of the absorbed dose to water at the IAEA At the IAEA a secondary standard ionization chamber NE2611 calibrated at the BIPM is used to determine the absorbed dose rate to water using the relation i i BIPM w D k N I D P ⋅ = , ,  where I is the current measured with the IAEA standard, ND,w,BIPM is the calibration coefficient of the standard determined at the BIPM and ki is the product of the correction factors to be applied to the measured current (pressure and temperature correction). The measured Comparison of the Standards of Air Kerma and Absorbed Dose to Water of the NRC and the IAEA for Therapy Level Co Gamma Radiation Ladislav Czap, Paula Toroi, István Csete, John Paul Archambault, Brad Downton, Malcolm R. McEwen IAEA International Atomic Energy Agency, Vienna, Austria Measurement Science and Standards National Research Council Canada / Government of Canada