New LNHB primary standard for 60Co air kerma

As a single working chamber for a national air kerma standard was not considered safe and robust enough, a new set of six ionization chambers was devised to establish the French 60Co air kerma standard. Although every new ionization chamber was treated as much as possible in the same way (manufacturing, measurements of volumes, wall effect calculations, current corrections), a maximum discrepancy of 0.2% was observed between the final measurement results from each chamber. The final value of the air kerma rate in reference conditions was determined as the mean value of the measurement results from all six chambers. Among the different factors whose determination is necessary to calculate the air kerma rate, some are considered independent of or common to all the graphite-walled ionization chambers (example: mean energy expended by an electron to produce an ion pair in dry air), while others vary for each chamber (example: air cavity ionic collection volume). Considering that the uncertainties of the individual ionization chamber measurement results seem slightly underestimated, the uncertainty on the mean of the products of the six chamber-dependent factors was taken equal to the standard deviation of the sample composed of the products of the six chamber-dependent factors (0.078%). Compared with the previous standard, the air kerma rate of the 60Co photon beam would then increase by 0.09% and the air kerma rate uncertainty would drop from 0.38% to 0.31%. (authors)