Extraction of full energy peak of 137Cs from in situ NaI (Tl) gamma-ray spectrum

Abstract

A Levenberg–Marquardt Gaussian fitting algorithm has been used for analyzing the overlap of three peaks (the 583-keV peak of 208Tl, the 609-keV peak of 214Bi, and the 662-keV peak of 137Cs) using an in situ NaI (Tl) scintillation spectrometer. The algorithm, in addition, was compared with a genetic algorithm used for multiple deconvolution. The three fitted peak areas (583, 609, and 662 keV) were calculated from the measured gamma-ray spectra obtained from a simulation experiment in which a 137Cs source was buried at different soil depths (from 18 to 38 cm). The application of the Levenberg–Marquardt algorithm yielded similar results compared to the genetic algorithm. A lack-of-fit test showed that the fitting is good when the instrumental noise levels were estimated from replicated analyses. The relative fitting error of the total net area and the residual standard deviation were within 5 % and 0.04, respectively, and the goodness of the fitting was better than 0.98. While the methods used in this paper give high performance, the results may lead to incorrect estimation when the signal-to-noise ratio is smaller than −30 dB. This study is useful for the determination of radioactive specific activity of 137Cs by in situ spectrometry.