On the efficiency calibration of a drum waste assay system

The present study was focused on the efficiency calibration of the gamma spectroscopy system achieved in IFIN-HH Bucharest for drum waste assay for efficiency calibration we used a linear source of 152 Eu placed in a 220 l container filled Portland cement, provided with 7 channels of 20 mm diameters. In order to minimize the effect of non-homogeneities (in matrix density, radioactive source distribution, etc.) and to obtained a complete covering of the surface of the drum, the system is provided with an electromechanical system, which ensure simultaneously the rotation and horizontal translation of the drum. The linear source was approximated by six 152 Eu sources of equal activity placed at equal distances in a plastic tube having the length of the drum's active area (620 mm). The total activity of the linear source is 91.02 MBq ± 4.55 MBq. The linear calibration source was introduced successively in the 7 tubes and the gamma spectrum was registered for about 10 minutes each time. Using ASPERA 8k software, the gamma spectra were analyzed and the net peak areas for six energies (122, 344, 779. 964, 1112, and 1408 keV) were obtained. The values obtained were integrated over the entire volume of the drum by using an appropriate mathematical model to obtain the detector response. The system efficiency was calculated by using the equation Ef = Atot/ykΛ, where, Λ is the activity of the linear 152 Eu source and yk is the yield of each of the energy lines, selected for efficiency determination. The calibration curves for cylinders of different radii, containing the radioactive waste approximately uniformly distributed are shown. Differences in the pattern of the efficiency curves are discussed in terms of the source volume and gamma attenuation in the cement layer. Also determined was the minimum detectable activity (MDA) using the formula MDA = (2.71 + 4.65σB)/TEfy, where σB=√B is standard deviation and B, the counts in the peak region obtained in the absence of radioactive source and T is measuring time for 137 Cs and 60 Co and T = 10 min MDA is approximately 45 Dq/kg. the error source were identified and the total measurement uncertainty was found ranging from - 70% to + 40%, for gamma ray energies less then 500 keV. (authors)