Development of radioactivity analyzer using NaI(Tl) scintillator with unfolding technique

We have developed a radioactivity analyzer for foodstuffs using a NaI(Tl) scintillator with an unfolding technique to measure the radioactive concentration of foodstuffs in a short time to assure the safety of foodstuffs. It is difficult to determine each radioactive concentration when the gamma ray energies are close to each other such as ¹³⁴Cs (0.605 MeV) and ¹³⁷Cs (0.662 MeV) through conventional methods due to the poor energy resolution of the NaI(Tl) scintillator. In order to improve energy discrimination performance we have established an iterative unfolding method using the response functions including pulse height broadening to the same energy deposition due to the statistical fluctuations of scintillation photon collection, photoelectron conversion, photoelectron collection and electrical noise. By applying this method to an NaI(Tl) scintillator, we have confirmed the discrimination of ¹³⁷Cs (0.662 MeV) and ¹³⁴Cs (0.605 MeV). The results of a performance evaluation with certified reference brown rice demonstrated that the accuracy of sensitivity is less than ±10%. In addition, the detection limit in 60 seconds of measurement was less than 25 Bq/kg in the case of 2 L of foodstuff. (author)