Significant discrepancies in air kerma rates measured with free-air and cavity ionization chambers

Air kerma rates were measured in the same narrow X-ray beams in the range 300–400 kV with a free-air ionization chamber and a graphite cavity ionization chamber. The graphite-to-air stopping-power ratios that are necessary to determine the air kerma rates according to the cavity theory were calculated by means of Monte Carlo methods based on measured energy distributions of the photon fluence and the ICRU 37 stopping-power values. As a result, it was found that the air kerma rates obtained with the cavity chamber were significantly higher, by up to about 2%. The discrepancies disappeared when different stopping-power values for graphite were used in the calculation of the graphite-to-air stopping-power ratios. The ICRU 37 values were calculated on the basis of a mean excitation energy in graphite of I=78 eV, in contrast to I=86 eV used for the calculation of those values solving the discrepancies obtained in the first approach. The results are of fundamental interest for primary standard dosimetry laboratories because all of them employ graphite cavity chambers to realize the unit of air kerma for 137Cs- and 60Co-γ-radiation.