Comparison of dosimetric standards of Canada and France for photons at 60Co and higher energies.
We report the results of a comparison of the dosimetric standards of Canada and France for photon beams at 60Co and a few higher energies. The present primary standard of absorbed dose to water for NRC, Canada is based on measurements made with a sealed water calorimeter. The corresponding standard of the LNHB, France is based on measurements made with a graphite calorimeter at 60Co 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-walled NE2571 Farmer chambers. The absorbed dose to water determined by the LNHB was greater than that determined by NRC by 0.20% at 60Co energy. This difference is not significant given the uncertainties on the standards. In order 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 as 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 60Co. However, when using TPR20,10 as the beam quality specifier, 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 systematically increases with increasing energy, eventually exceeds the uncertainties in the ratio of the standards, estimated to be 0.7%. This underscores the importance of selecting the method of specifying beam quality, 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 60Co energy, the LNHB determination was greater than NRC's by 0.14%, which is not significant given the uncertainties on the standards.
Shortt
K
K
National Research Council, Ottawa, Canada. kshortt@irs.phy.nrc.ca
Ross
C
C
Seuntjens
J
J
Delaunay
F
F
Ostrowsky
A
A
Gross
P
P
Leroy
E
E
D002151:Calorimetry; D002170:Canada; D003037:Cobalt Radioisotopes / Q000592:standards / Q000627:therapeutic use; D005602:France; D017785:Photons; D011786:Quality Control; D011829:Radiation Dosage; D015203:Reproducibility of Results; D014867:Water
D003037:Cobalt Radioisotopes; D014867:Water