Determination of radioactivity concentrations of several manmade radionuclides in soil samples by gamma spectrometry

In a radioactive contaminated area, by sampling soil, some radionuclides are found: 137Cs, 60Co, 152Eu, 154Eu, 155Eu, 241Am and others. For determination of these radionuclides with a HPGe gamma-ray spectrometer, the method of selection of gamma peak energy, preparation of samples, calibration of the spectrometer, and self-absorption correction is discussed. Selection of gamma peak energy has been considered for spectrum analysis. Using a 100% n-type detector, the overlapped peaks for 122 keV(152Eu) and 123 keV(154Eu) cannot been deconvoluted in analysis results. The 344 keV peak is chosen to determine the activity of 152Eu. For 86 keV peak (155Eu), the interference from natural radionuclides is severe in most cases, so that 105 keV peak is suitable for 155Eu. Similar check-up has been done for 154Eu and 60Co. The sample container is cylindrical plastic boxes (φ75 mm x 50 mm). To study the influence of grain size of soil on deviation of measured counts, soil is ground by machine and then sieved with three different sifters, which are mesh 35, 60 and 100, respectively. Each group of different grain sizes has five parallel samples. For samples from one location, there is no remarkable distinction among the three groups. But there are two samples from two groups for another location that are very abnormal with high concentration of 241Am. To ensure the reliability of efficiency determination, relative method is used with certificated standard sources (soil or simulated soil). Meanwhile the efficiency curve is calibrated by using source of solution with eight single-line gamma emitter nuclides. The factor of self-absorption is calculated by the expression of constant efficiency parallel beam. By comparing the efficiency of 60 keV (241Am), 105 keV (155Eu) and 662 keV (137Cs) derived from these two methods, the uncertainties of self-absorption correction have been estimated, which are about 5%-6% in low energy region and 1%-3% in high energy region. (authors)