APMP/TCRI key comparison report of measurement of air kerma for medium-energy x-rays (APMP.RI(I)-K3.2013)

The APMP/TCRI Dosimetry Working Group initiated the APMP.RI(I)-K3.2013 key comparison of measurement of air kerma for medium-energy x-rays (100 kV to 250 kV); measurements took place between 2015 and 2017. In total, 11 institutes took part in the comparison, among which 8 were APMP member laboratories, two from AFRIMETS and one from SIM. Three commercial cavity ionization chambers were used as transfer standards circulated among the participants. All the participants established 100, 135, 180 and 250 kV x-ray beam qualities equivalent to those of the BIPM. The ARPANSA and the NMIJ were linking laboratories and two participants subsequently made comparisons with the BIPM. The 7 other participants' degrees of equivalence fell within 11 parts in 103 of unity, well within the relevant expanded uncertainty (k = 2) in all cases. Excluding the BARC, the results were within 5.7 parts in 103, well within the relevant combined standard uncertainty. These results indicated that the calibration capabilities of most laboratories were in very good agreement and there were no serious discrepancies between them. The effect of implementing the recommendations of ICRU Report 90, which occurred on 1 January 2018 (after this comparison), is discussed and we deduce that the effect on the degrees of equivalence is negligible. The increase in the uncertainty in Wair has been added to all laboratories so that the degrees of equivalence and the uncertainties published here are applicable to post-ICRU 90 measurement results.