First international comparison of primary absorbed dose to water standards in the medium-energy X-ray range

This report presents the results of the first international comparison of primary measurement standards of absorbed dose to water for the medium-energy X-ray range. Three of the participants (VSL, PTB, LNE-LNHB) used their existing water calorimeter based standards and one participant (ENEA) recently developed a new standard based on a water-graphite calorimeter. The participants calibrated three transfer chambers of the same type in terms of absorbed dose to water (N-Dw) and in addition in terms of air kerma (N-K) using the CCRI radiation qualities in the range 100 kV to 250 kV. The additional N-K values were intended to be used for a physical analysis of the ratios N-Dw / N-K. All participants had previously participated in the BIPM. RI(I)-K3 key comparison of air kerma standards. Ratios of pairs of NMI's N-K results of the current comparison were found to be consistent with the corresponding key comparison results within the expanded uncertainties of 0.6 % - 1 %. The N-Dw results were analysed in terms of the degrees of equivalence with the comparison reference values which were calculated for each beam quality as the weighted means of all results. The participant's results were consistent with the reference value within the expanded uncertainties. However these expanded uncertainties varied significantly and ranged between about 1-1.8 % for the water calorimeter based standards and were estimated at 3.7 % for the water-graphite calorimeter. It was shown previously that the ratios N-Dw / N-K for the type of ionization chamber used as transfer chamber in this comparison were very close (within less than 1 %) to the calculated values of ((mu) over bar (en)/rho)(w,a)(d) , the mean values of the water-to-air ratio of the mass-energy-absorption coefficients at the depth d in water. Some of the participant's results deviated significantly from the expected behavior.