Spectrum processing method for measuring low levels of specific activity of 137Cs with a NaI(Tl) detector in the presence of natural radionuclides

When measuring low levels of ¹³⁷Cs in soil samples using spectrometers with a NaI(Tl) detector, identification of the ¹³⁷mBa peak (the 662 keV gamma line of the daughter radionuclide) is difficult due to the influence of natural radionuclides, the peaks of which fall within the ¹³⁷mBa detection region. To identify and estimate the area under the peak of ¹³⁷mBa, it is necessary to know the contribution of the background component to this area and the contribution of each natural radionuclide. When using such approach, the accuracy of estimating the ¹³⁷mBa photopeak area depends on the accuracy with which the levels of natural radionuclides are determined. The article proposes a method of spectrum decomposition based on the taking into account individual parameters of the spectrometer and a more accurate determination of specific activities of natural radionuclides. The method involves the modeling the spectral regions for each natural radionuclide in the area of ¹³⁷mBa peak and subtracting the simulated regions and the background spectrum from the total spectrum under the peak of ¹³⁷mBa. The use of this approach allows to measure with a 63×63 NaI(Tl) detector the specific activity of ¹³⁷Cs at a level of 0.7–1 Bq/kg with an uncertainty of 30-50% in the Marinelly geometry with a volume of 1 liter and with the measurement time of 6 hours. 

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