Comparisons for the national standards of air kerma from therapy level and radiation protection gamma-ray beams were conducted by the NIST and the Bureau International des Poids et Mesures (BIPM).
In support of the Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA), the Dosimetry Group of the Radiation Physics Division, PML has recently completed two key comparisons with the BIPM for therapy and radiation protection gamma-ray beams for the radiation quantity air kerma (radiation dose in air). The SI unit for air kerma is the gray (joule per kilogram). The therapy level comparison measurements were conducted at the NIST and BIPM 60Co gamma-ray beam facilities. The NIST facility has a reference radiation beam that is used to provide traceability for measurements performed at hospitals nationwide that are critical for ensuring the safety of patients undergoing cancer treatment. The radiation protection level comparison measurements were performed in the corresponding NIST and BIPM 137Cs and 60Co gamma-ray beam facilities. The two NIST protection level facilities used in this work provide reference beams that are used to disseminate the air kerma standard nationwide for instruments used in a large number of applications including the military, protection of radiation workers, safety of power plants, homeland security, and manufacturing of radiation instruments. A total of five ionization chambers were used for the two key comparisons and were initially calibrated in terms of air kerma. The instruments were later shipped to the BIPM where measurements were performed by BIPM scientists with participation by one scientist from the NIST Dosimetry Group. The comparison results are expressed as a ratio of the NIST and the BIPM air kerma standards. The ratio of the therapy level comparison was 1.0039 with a combined standard uncertainty of 3.4 x 10–3. The ratios for the two radiation protection level comparisons were 1.0023 with a combined standard uncertainty of 3.1 x 10–3 for the 60Co reference beams, and 0.9990 with a combined standard uncertainty of 3.3 x 10–3 for the 137Cs reference beams. The details of both key comparisons have been published in Metrologia 50 (2013) Tech. Suppl. 06002 and Metrologia 51 (2014) Tech. Suppl. 06013, and the full reports are available on the BIPM key comparison data base web site.