Estimation of the matrix attenuation in heterogeneous radioactive waste drums using dual-energy computed tomography

Gamma spectroscopy measurements of the activity of radionuclides in nuclear waste drums must be corrected for the attenuation due to the non-homogeneous waste matrix. The attenuation factors depend on the matrix local density and effective atomic number, and on the energy of the gamma rays emitted by the radionuclides. The requirements for the system presented in this paper are to estimate the attenuation in low-density (<0.4 g/cm3), 120 l drums containing radionuclides emitting in the (59.5 keV, 1.4 MeV) energy range. A series of three-dimensional (3D) attenuation maps of the drum are computed using a dual-energy computerized tomography (DE-CT) system with an external, polychromatic X-ray source. The system successively records low-energy (mean energy about 62 keV) and high-energy (about 300 keV) projections using different tube voltages, anode current, and filtration. Each projection is acquired by 22 BGO scintillators – PM detectors in fan-beam geometry. The drum is rotated and elevated in a helical scan. A DE calibration transforms pairs of DE projections into pairs of “equivalent basis materials (BM)” projections. This non-linear transformation allows to correct for polychromaticity. After reconstruction, the two “equivalent BM” 3D maps are used, together with tabulated attenuation data of the BMs, in order to extrapolate the 3D attenuation map at any energy peak. Maps of the mass density and of the effective atomic number can also be computed. The total examination time is less than 5 min. Experimental images are shown.