Wall Correction Factors Estimated by Monte Carlo Calculation for Spherical Graphite-Walled Cavity Chambers in INER's ⁶⁰Co Air-Kerma Standard

The purpose of this research is to estimate the wall effect of spherical graphite-walled cavity chambers using the Monte Carlo method to establish a ⁶⁰Co air-kerma standard at the National Radiation Standard Laboratory of the Institute of Nuclear Energy Research (NRSL/INER), Taiwan. For more than a decade, the validity of the wall correction term k_wall determined by linear-extrapolation methods has been strongly challenged by the Monte Carlo method. In this paper, one goal was to evaluate in detail k_wall for spherical chambers varying with wall thickness (0.1 to 2.5 cm), cavity size (1 to 1000 cm³), and incident photon energy (0.02 to 1.33 MeV). The other goal was to obtain k_wall for self-fabricated, spherical chambers and then compare it with the historical values in 2003. A significant increase of 0.3% for air kerma in the ⁶⁰Co field was expected. The difference of bilateral comparison between NRSL/INER and the Australian Radiation Protection and Nuclear Safety Agency was reduced when the calculated k_wall, instead of the original estimated value of k_wall, was applied for the derivation of the calibration factor. The NRSL/INER primary standards for air kerma will be adjusted in the near future to reflect the changes in k_wall described in this work.