When the high-energy electromagnetic radiation of the medical linear accelerator is irradiated in high capacity, there is a nuclide transformation due to the radio-activation. Especially for the magnet core of the bending
magnet, the magnetic field strength is changed due to the nuclide transformation, and this change in the magnetic field strength is shown as the change in flux of the electromagnetic radiation. Therefore, the effect of
radio-activation on the change in flux of the electromagnetic radiation according to the electromagnetic radiation dose is to be evaluated. Subjected to 7 medical linear accelerators planned for disposal, the total dosage,
nuclide analysis using the spectrometer of the magnet core, flatness using the water phantom & ion chamber, and the change amount of symmetry were evaluated. As a result, the relationship of change rate in flux according to the dosage is shown in 4 % error per 1,000 Gy. As a result of applying the 1 % threshold limit by the American Association of Physicists in Medicine, TG-142 Report, the tolerated dose is 250Gy. Therefore, on the use of medical linear accelerator in the future, it is recommended to perform the flux evaluation according to the magnet core radio-activation for each 250Gy along with the existing periodical quality control.