WALL CORRECTION FACTORS ESTIMATED BY MONTE CARLO CALCULATION FOR SPHERICAL GRAPHITE-WALLED CAVITY CHAMBERS IN INER'S ~(60)Co AIR-KERMA STANDARD

摘要

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 ~(60)Co 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 cm~3), and incident photon energy (0.02 to 1.33 MeV).rnThe 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 ~(60)Co 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.