DEVELOPMENT OF SODIUM IODIDE SCINTILLATOR-TYPE GAMMA-RAY SPECTROMETER USING AN UNFOLDING METHOD WITH REFINED RESPONSE FUNCTIONS

Abstract

We developed a sodium iodide scintillator-type γ-ray spectrometer with improved energy resolution using an unfolding method for gamma ray (γ-ray) spectroscopy and dosimetry during emergencies such as radiation leakage. To improve the energy resolution, the response function was refined by weighting the energy dependence of the scintillation efficiency of the sodium iodine on the energy of the secondary electrons produced by the interaction with the incident γ-rays. Response functions were calculated using a Monte-Carlo simulation code EGS5 with a mesh width of 5 keV in the energy range of 0 to 3 MeV, assuming γ-rays irradiated parallel to the sides of the detector. The unfolding algorithm was an iterative approximation method, which is independent of the initial guess. As a result of repeated measurements using ¹³⁷Cs, ⁶⁰Co, ¹³³Ba, and ²²Na radiation sources, the energy resolution is about 0.01 MeV and the measurement accuracy of γ-ray fluence rate is almost constant at less than 10% in at 0.3 MeV and above.