Portable Gamma Irradiation System with 57Co, 133Ba, 137Cs, and 60Co for On-site Calibration of Environmental Radiation Monitoring Devices

Background: A portable gamma irradiation system was developed for on-site calibration of environmental radiation monitoring devices. This system uses a portable collimator and gamma- ray sources such as 57 Co, 133 Ba, 137 Cs, and 60 Co, each with a nominal radioactivity of 10 MBq or less. Materials and Methods: The air kerma rate produced by the system was measured using a spherical ionization chamber with a 10 L sensitive volume. This chamber was calibrated against the air kerma rate measured with primary standard ionization chambers. The system's performance was evaluated by comparing the calibration results of the monitoring device with those obtained using a conventional method involving a standard ionization chamber for outdoor use. Results and Discussion: The calibration coefficient of the spherical ionization chamber was within +/- 2% of the value of 3.029x103 Gy/C for photon energies ranging from 50 keV to 1,250 keV. The air kerma rate of the irradiation field produced by the portable system ranged from 0.17 mu Gy/hr to 4.7 mu Gy/hr, with an uncertainty between 2.4% and 10%. The calibration coefficients obtained using the portable system were consistent with those from the conventional method. Conclusion: The portable gamma irradiation system offers significant advantages, including preventing damage to standard ionization chambers used outdoors and reducing measurement time compared to conventional methods. Due to its ease of construction and implementation, the system is expected to be useful not only for on-site calibration of monitoring devices but also in various radiation safety management scenarios.