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 ionizat ion chambers as transfer instruments. The absorbed dose standard of th e PTB is based on the total absorption of 5.6 MeV electrons in Fricke solution. The NRC standard uses Fricke solution whose calibration is b ased on measurements made with a water calorimeter and a calculation o f the heat defect. For high-energy x-rays, the difference found betwee n the standards of 0.4% is within the expected uncertainties. The comp arison was linked to existing Co-60 absorbed dose to water and air ker ma standards by comparing measurements of those quantities at the PTB, NRC and BIPM. Agreement was better than 0.7% in all cases. For the io nization chambers used in this study, the absorbed dose calibration fa ctors for 18/20 MV x-rays are about 2% lower than those for Co-60 gamm a-rays.