Changes of KRISS primary standards by implementing ICRU 90 recommendation

Abstract

The International Commission of Radiation Units and Measurements (ICRU) published report No. 90 (i.e., Key Data for Ionizing Radiation Dosimetry: Measurement Standards and Applications) in 2016. The present study explores the impact of ICRU 90 on the primary standards of the Korea Research Institute of Standards and Science (KRISS). To this end, first, two new correction factors (kii and kw) for the free-air ionization chambers were introduced; it influences the standards related to kV X-ray beams. The new correction factors were calculated by simulating the initial electrons’ generations using the GEANT4 code. The second recommendation considered was for changes in the mean excitation energies of graphite and water, and the use of the crystalline density of graphite in the stopping power evaluation. These recommendations should be adopted for the graphite-walled cavity chamber and graphite calorimeter, which are the primary standards of the KRISS for air kerma in gamma rays and absorbed dose to water in high-energy photons, respectively. EGSnrc code-based evaluations of the electron stopping-power ratios for gamma rays were performed for the KRISS graphite-walled cavity chamber. Regarding graphite calorimetry in high-energy photon beams, the recommendations on graphite and water have been studied and presented in a previous paper. The third consideration is the recommendation for increasing uncertainty of the mean energy of electrons expended in dry air per ion pair production, $${\left(W/e\right)}_{\mathrm{air}}$$ , from 0.15% to 0.35%. The impact of the increased uncertainty under the conditions of the use of the existing components was summarized, together with the overall changes to the KRISS primary standards related to air kerma. Consequently, the difference between the products of the calculated kii and kW for the free-air ionization chamber standard and the corresponding values given in the ICRU 90 report was less than 0.10% for mono-energetic photons in the range of 1–400 keV. Regarding the X-ray beams recommended by the Consultative Committee for Ionizing Radiation (CCRI), the calculated correction factors deviated from those of the Bureau International des Poids et Mesures (BIPM) standards by 0.03%. In the case of the graphite-walled cavity chamber, changes of 0.65–0.77% were observed for the gamma-ray air kerma standards. The observed changes were reasonable, considering that similar changes were found in those reported by other research groups. Regarding the combined uncertainties, the uncertainties for the air kerma primary standards increased up to approximately 0.40%, primarily because of $${\left(W/e\right)}_{air}$$ . The changes to the KRISS primary standards that were observed in the present study will be adopted for the measurements performed since 2021.