Comparison of the PTB and OMH air kerma standards for 60Co and 137Cs gamma radiation

A direct comparison between the primary air kerma standards of the PTB and OMH was carried out for 60Co and 137Cs γ rays in June 2000 within a time period of 1 month. The OMH standard is a graphite cavity chamber of the cylindrical type. The PTB standard is comprised of three graphite cavity chambers, different in shape and size. Two of them are of the cylindrical type and one of them is a pancake type chamber. The main technical parameters of the standards and the applied beams are described. The correction factors of the chambers are given together with their uncertainties. In particular, it is pointed out that for the first time calculated instead of measured wall correction factors were applied to all chambers. The ratios of air kerma rates measured with the PTB standard to those measured with the OMH standard were 1,0009 and 1,0011 for 60Co and 137Cs radiation, respectively. They are well within the stated relative standard uncertainties of the ratios of 0,22% for 60Co and 0,24% for 137Cs. The consistency of the results, which was obtained with ionisation chambers characterised by wall correction factors between 0,9980 and 1,0280 for 137Cs and between 1,0004 and 1,0212 for 60Co, is of special importance because it strongly confirms the validity of the calculation procedure to determine wall correction factors. (orig.)