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

Abstract

Purpose: To reanalyze experiments which determine , the product of (W / e)air, the average energy deposited per coulomb of charge released in dry air, and , the Spencer‐Attix mass collision stopping‐power ratio for graphite to air, and to calculate an average value for this product for the BIPM air kerma standard: . This value could be adopted for use with air kerma primary standards, along with corrections to account for variations due to cavity size. Methods and Materials: The experiments measured 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 is multiplied by , 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 . Results: The correction factors generally decrease the value of for each experiment, often outside the range of one standard deviation quoted with each experimental result. The ratio varies by less than 0.1% for different choices of density correction and I‐value and hence the product is also relatively insensitive to these choices.Conclusion: The preliminary analysis suggests that the accepted value of , 33.97 J/C ±0.15%, is 0.6% too high. This would have implications for primary air kerma standards worldwide and for the value of (W / e)air which is used in low energy x‐ray standards.