AbStracL An indirect comparison of the absorbed dose to water standards of lhe FTB and the NRC was carried out for 18/20 MV x-rays using five ionimtion chambers as m s f e r insmmenls. The absorbed dose standard of the Fl% is based on the total absorption of 5.6 MeV electrons in Fncke solution. The NRC standard uses Wcke solution whose calibration is based on measuremenfs made with a water calorimeter Bnd a calculation of the heat defect For high-energy x-rays. the difference found between the standards of 0.4% is within the expected uncertainties. Tbe comparison was linked to existing %o absorbed dose to warer and air kema standards by comparing measurements of those quantities at the PTB, NRC and BIPM. Agreement was better than 0.78 in all cases. For the i o n i d o n chamberr used in Lhi study, the absorbed dose calibration factors for 18/20 MV x-rays are a b u t 2% lower than those for "CO gamma-rays. . IntroductionThe Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig and the National Research Council of Canada (NRC) in Ottawa have developed primary standards of absorbed dose to water for high-energy radiation. Although both groups are carrying out further developments, this comparison indicates the state of the art at the two laboratories for highenergy photon dosimeby. Section I (x-and gamma-radiation; electrons) of the Comit6 Consultatif pour les & I o n s de Mesure des Rayonnements Ionisants (CCEMRI) (BIPM 1988) recommended that primary standards for absorbed dose be compared in collaboration with the Bureau Intemational des Poids et Mesures (BIPM) so that these comparisons could be linked to the existing BIPM 6oCo reference system. Thus measurements of air kerma and absorbed dose to water for %o were made at the BIPM as well as the NRC and the PTB. The results of these measurements are presented here.As can be seen from section 2, the primary standards of absorbed dose to water for high-energy x-rays are more complex than those of air kerma since auxiliary equipment and procedures are involved. Therefore, primary standards are not transportable and comparison of these standards has to be made using transfer instruments. Since ionization chambers are easily transportable, have good precision and long-term stability, and are connected to directly readable measuring assemblies, they were chosen for use as transfer instruments.

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