Comparison of dosimetric standards of Canada and France for photons at Co-60 and higher energies

We report the results of a comparison of the dosimetric standards of Canada and France for photon beams at Co-60 and a few higher energies. The presen t primary standard of absorbed dose to water for NRC, Canada is based on me asurements made with a sealed water calorimeter. The corresponding standard of the LNHB, France is based on measurements made with a graphite calorime ter at Co-60 energy and transferred to absorbed dose to water for 60Co and higher-energy photon beams using both ion chambers and Fricke dosemeters as transfer instruments. To make this comparison, we used three graphite-wall ed NE2571 Farmer chambers. The absorbed dose to water determined by the LNH B was greater than that determined by NRC by 0.20% at Co-60 energy. This di fference is not significant given the uncertainties on the standards. In or der to do the comparison for higher-energy photons, we interpolated the NRC data set at the beam qualities used at the LNHB. When %dd(10)(x) is used a s the method of specifying beam quality, the determination of absorbed dose to water by the LNHB is about 0.2% greater than that determined by NRC and consistent with the results at Co-60. However, when using TPR20,10 as the beam quality specifier, the LNHB determination is greater than the NRC's de termination by 0.8% and 1.2% at 12 and 20 MV respectively. This discrepancy , which systematically increases with increasing energy, eventually exceeds the uncertainties in the ratio of the standards, estimated to be 0.7%. Thi s underscores the importance of selecting the method of specifying beam qua lity, either %dd(10)(x) or TPR20,10, at least for the 'soft' beams used by NRC in this comparison. In the case of the air kerma standards, which were also compared at Co-60 energy, the LNHB determination was greater than NRC' s by 0.14%, which is not significant given the uncertainties on the standar ds.