A free-air ionization chamber is used as the primary standard instrument for absolute measurement of air kerma for of X-ray beams. The evaluation of correction factors is a key importance for establishing the standard. Photon-scattering, and diaphragm transmission and scattering were taken into the accounts to reduce some photon phenomena. On the other hand, electron-loss factor which scatter from an electrode were used for electron compensates. In this research, Correction factors were calculated for each mono-energetic photon from 2 to 60 keV for low energy x-rays and 10 to 320 keV for medium energy x-rays by EGS5 code. The calculated corrections from a mono-energetic photon of W/Mo mammography x-rays were compared between effective energy and spectrum method. There is no significant difference between two methods.

Free-air ionization chambers (FAC) are accepted as the primary standard of x-ray dosimetry. The low-energy x-rays are important for basic beam quantities of diagnostics at the mammography level. This research explains the FAC characteristics of the Office of Atoms for Peace (OAP) that was developed by cooperation between the OAP, Thailand, and the Korea Research Institute of Standards and Science, Republic of Korea. The applied correction factors that are necessary for determination of the air kerma rate were investigated using an experimental method and Monte Carlo simulation methods with their uncertainties. The beam characteristics, computed using the Monte Carlo method and demonstrated in this paper, are important for calculation of the correction factors. Finally, the correction factors and uncertainties of the standard beam qualities, which are recommended by the Consultative Committee for Ionizing Radiation, and W/Mo beam qualities are shown in this paper.

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