WE‐E‐BRD‐03: Re‐Evaluation of the Product of W/e and the Graphite to Air Stopping Power Ratio for Co‐60 Air Kerma Standards
Purpose: To reanalyze experiments which determine [formula omitted], the product of (W / e)air, the average energy deposited per coulomb of charge released in dry air, and [formula omitted], the Spencer‐Attix mass collision stopping‐power ratio for graphite to air, and to calculate an average value for this product for the BIPM [formula omitted] air kerma standard: [formula omitted]. This value could be adopted for use with [formula omitted] air kerma primary standards, along with corrections to account for variations due to cavity size. Methods and Materials: The experiments measured [formula omitted] by various methods, often involving calorimeters and ionization chambers. Correction factors, e.g., to account for gaps about a calorimeter core or perturbations due to a cavity's presence, are calculated as needed for each experiment using the EGSnrc user‐codes CAVRZnrc, DOSRZnrc, and CAVITY. Stopping power ratios are evaluated using SPRRZnrc for different choices of graphite density (bulk 1.70 g/cm3 or grain 2.265 g/cm3) for the density effect correction and average excitation energy for graphite (I=78 or 87 eV). For each experiment, the corrected value of [formula omitted] is multiplied by [formula omitted], the quotient of the stopping power ratios for the BIPM chamber and the experiment in question. A least squares technique is used to compute an average value of [formula omitted]. Results: The correction factors generally decrease the value of [formula omitted] for each experiment, often outside the range of one standard deviation quoted with each experimental result. The ratio [formula omitted] varies by less than 0.1% for different choices of density correction and I‐value and hence the product [formula omitted] is also relatively insensitive to these choices. Conclusion: The preliminary analysis suggests that the accepted value of [formula omitted], 33.97 J/C ±0.15%, is 0.6% too high. This would have implications for primary [formula omitted] air kerma standards worldwide and for the value of (W / e)air which is used in low energy x‐ray standards.
Thomson, R, & Rogers, D.W.O. (2009). WE‐E‐BRD‐03: Re‐Evaluation of the Product of W/e and the Graphite to Air Stopping Power Ratio for Co‐60 Air Kerma Standards. In Medical Physics (Vol. 36). doi:10.1118/1.3182555