A comparison of absorbed dose standards for high-energy X-rays

An indirect comparison of the absorbed dose to water standards of the PTB and the NRC was carried out for 18/20 MV X-rays using five ionization chambers as transfer instruments. The absorbed dose standard of the PTB is based on the total absorption of 5.6 MeV electrons in Fricke solution. The NRC standard uses Fricke solution whose calibration is based on measurements made with a water calorimeter and 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. The comparison was linked to existing 60 Co absorbed dose to water and air kerma standards by comparing measurements of those quantities at the PTB, NRC and BIPM. Agreement was better than 0.7% in all cases. For the ionization chambers used in this study, the absorbed dose calibration factors for 18/20 MV X-rays are about 2% lower than those for 60 Co gamma-rays.