Establishment of a low dose rate 133Ba γ-ray calibration field for photon energy in the 300–400 keV range

A γ-ray calibration field using a sealed 133Ba source for photon energy in the 300-400 keV range was established at the Japan Atomic Energy Agency to evaluate radiation protection devices under low dose rate conditions. To suppress low-energy γ-ray components, a 2-mm lead filter was applied. The photon fluence spectrum was measured using a CdTe spectrometer and an unfolding method, yielding a spectrum with a mean energy of 321 keV. Reference air kerma rates were determined using ionization chambers calibrated in a 137Cs field, with energy correction factors applied. Spectrometry using a CeBr₃ scintillation spectrometer combined with the G(E) function method was employed to validate these corrections. Spectrum-averaged conversion coefficients were derived from the measured spectrum, indicating equivalence to the N-400 radiation quality. This field provided air kerma rates from 0.22 to 2.13 μGy h-1, suitable for testing devices used in environmental monitoring and nuclear emergency response.