Due to the poor resolution of (76 mm×76 mm) NaI scintillation detector, ~7.5% for 137Cs peak (661.5 keV), mutual spectral interference of environmental samples between 137Cs and 214Bi (609 keV) in the decay series of uranium peaks, is a serious problem. This overlapped problem appears in low-level sample spectrum causes overestimation of the specific activity of both nuclides. The main purpose of this work is to resolve this problem and to present a methodology to correct the activity concentrations of 137Cs by NaI(Tl) spectroscopy. In Duhok governorate of Iraq, as a real NORM case, the accumulation of 137Cs is significantly brought by west-north wind and precipitation in the years following nuclear weapons testing and Chernobyl accident in 1986. The developed correlations for any ratio of the measured areas of 214Bi and 137Cs overcome the overestimation and show very good results. The method is approved by comparing the relative error of the measured and corrected activity of Bi with that measured Pb. The maximum, minimum and average relative errors decreased from 40%, 7.3% and 19.4% to 6.1%, -6.5% and 0.6% respectively.
