Photon-scattering, electron-loss and shadow-effect correction factors calculation for cylindrical free-air chamber

Abstract

A cylindrical free-air ionization chamber is used as the medium X-ray air kerma primary standard at the Institute of Nuclear Energy Research (INER, Taiwan). Photon-scattering, electron-loss and shadow-effect correction factors are taken into account for the measurement of air kerma by the cylindrical free-air ionization chamber. The photon-scattering correction factor is used to deduct the ionizations caused by scattered photons. The electron-loss correction factor compensates for the loss of electrons striking the electrode shell without fully depositing their energies to charges in the air. The shadow-effect correction factor compensates for the loss of electrons striking the collecting rod inside the chamber. The photon scattering and electron loss correction factors previously used at INER were based on a least-squares fit with experimental data published in the NBS Handbook 64. The shadow-effect correction factor was not considered.In this study, photon-scattering, electron-loss and shadow-effect correction factors for each mono-energetic photon were calculated by a Monte Carlo code, EGS5. Then, the mono-energetic correction factors were substituted into the ISO 4037 radiation qualities spectrum, and the energy weighted correction factors were calculated. Comparing the calculated correction factors with the previous correction factors, the maximum differences were 0.51% and 1.22% for IN-250 and IN-300 radiation qualities, respectively.In a report detailing an international comparison of air kerma standards for the ISO 4037 narrow spectrum series (EUROMET.RI(I)-S3), which was conducted from 2004 to 2005, the ratio of differences and expanded uncertainties (Di/Ui) for INER's IN-250 and IN-300 radiation qualities were 0.9 and 1.8, respectively. If the correction factors obtained in this study are substituted, the differences can be reduced, and the (Di/Ui) ratios become 0.36 and 0.6 for IN-250 and IN-300, respectively.