Experiments which determine the product of (W/e)air, the average energy deposited per coulomb of charge of one sign released by an electron coming to rest in dry air, and, the Spencer-Attix mean restricted mass collision stopping-power ratio for graphite to air, in a 60Co or 137Cs beam are reanalysed. Correction factors, e.g., to account for gaps about a calorimeter core or perturbations due to a cavity's presence, are calculated using the EGSnrc Monte Carlo code system and these generally decrease the value of (W/e)air for each experiment. Stopping-power ratios are calculated for different choices of density correction and average excitation energy (I-value) for graphite. To calculate an average value (W/e)air for the BIPM air kerma standard, each experimental result is multiplied by the ratio/. While individual values of are sensitive to the I-values and density corrections assumed, this ratio varies by less than 0.1% for different choices. Hence, the product (W/e)air is relatively insensitive to these choices. The weighted mean of the updated data is (W/e)air J C -1 0.2%, suggesting that the accepted value of 33.97 J C-1 0.1% is 0.8% too high. This has implications for primary 60Co air kerma standards worldwide and potentially for the choice of graphite I-value and density correction for the calculation of the graphite stopping power, as well as the value of (W/e)air.

Physics in Medicine and Biology
Department of Physics

Thomson, R.M, & Rogers, D.W.O. (2010). Re-evaluation of the product of (W/e)air and the graphite to air stopping-power ratio for 60Co air kerma standards. Physics in Medicine and Biology, 55(13), 3577–3595. doi:10.1088/0031-9155/55/13/001