Comparison of dosimetric standards of Canada and France for photons at 60Co and higherenergies

Abstract

We report the results of a comparison of the dosimetric standards of Canadaand France for photon beams at 60Co and a few higher energies. Thepresent primary standard of absorbed dose to water for NRC, Canada is based onmeasurements made with a sealed water calorimeter. The corresponding standardof the LNHB, France is based on measurements made with a graphite calorimeterat 60Co energy and transferred to absorbed dose to water for 60Coand higher-energy photon beams using both ion chambers and Fricke dosemetersas transfer instruments. To make this comparison, we used three graphite-walledNE2571 Farmer chambers. The absorbed dose to water determined by the LNHB wasgreater than that determined by NRC by 0.20% at 60Co energy. Thisdifference is not significant given the uncertainties on the standards. Inorder to do the comparison for higher-energy photons, we interpolated the NRCdata set at the beam qualities used at the LNHB. When %dd(10)x is used as themethod of specifying beam quality, the determination of absorbed dose to waterby the LNHB is about 0.2% greater than that determined by NRC and consistent withthe results at 60Co. However, when using TPR20,10 as the beam qualityspecifier, the LNHB determination is greater than the NRC's determination by 0.8%and 1.2% at 12 and 20 MV respectively. This discrepancy, which systematicallyincreases with increasing energy, eventually exceeds the uncertainties in theratio of the standards, estimated to be 0.7%. This underscores the importanceof selecting the method of specifying beam quality, either %dd(10)x orTPR20,10, at least for the `soft' beams used by NRC in this comparison. In thecase of the air kerma standards, which were also compared at 60Co energy,the LNHB determination was greater than NRC's by 0.14%, which is notsignificant given the uncertainties on the standards.