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

Abstract

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.

Keywords

• gamma spectrometry
• spectrum decomposition
• cesium-137
• natural radionuclides
• spectrometer calibration