Gamma-Ray Spectroscopy Using an Unfolding Method with Response Functions Including the Energy Dependencies of Scintillation Efficiency for the NaI(T1) Scintillator

Background: Measuring the energy spectrum of gamma rays to assess the exposure dose and identifying the leaked radioisotopes at the accident site and the surrounding environment is necessary in the case of an accident involving the leakage of radioactive material at a nuclear power plant or a radiation-related facility. High-purity germanium semiconductor detectors are utilized for gamma-ray spectrometry due to their high energy resolution, but they exhibit a high initial and operational cost due to the need for a cooling mechanism. Materials and Methods: We improved the unfolding method for scintillators with a refined response function that involves the energy dependence of scintillation efficiency for secondary electrons produced by incident gamma-ray interaction. Monte Carlo simulation code EGS5 with a mesh width of 5 keV in the energy range of 0-3 MeV was used to calculate response functions, assuming gamma rays irradiated parallel to the detector sides. The unfolding algorithm involves an iterative approximation method, which is independent of initial guesses. Results and Discussion: The measurement accuracy of the gamma-ray fluence rate was almost constant at <10% at >= 0.3 MeV caused by repeated measurements using Cs-137,Ba-133, Y-88, and Co-60 radiation sources. Additionally, we confirmed linearity concerning the gamma-ray intensity by changing the distance from the source to the detector. Conclusion: We verified that the unfolding method separated peaks for each gamma-ray energy, although the difference in gamma-ray energies was several tens of keV. Moreover, the accuracy of the unfolding method was almost constant and had linearity concerning gamma-ray intensity.