Atomic number reconstruction using multi-MeV X-ray DECT in concrete drums

The DECT technique consists in two distinct X-ray tomography measurements using sources with different energy spectra, and allows an estimation of both the density and the effective atomic number (Z eff ) of the tested material. The present paper develops a methodology to extract Z eff of objects inside large concrete drums using DECT technique with multi-MeV X-ray, for the needs of nuclear waste drums inspection. The X-ray source is a LINAC which can deliver output electron energy from 6 MeV to 20 MeV. An appropriate definition of the energy-dependent effective atomic number of a compound material is established for this particular measurement, it is determined via tabulated X-ray cross-sections. The choice of the two energy sources as well as the effect of the source filtering on the Z eff reconstruction accuracy is discussed. The proposed algorithm of Z eff extraction is based on the ratio of the two tomography images. It consists in an interpolation of the theoretical relationship between Z eff and this ratio, calculated using tabulated X-ray cross-sections, knowing the energy of both sources. An effective energy for each of the two sources is estimated as a function of the radial position of the object inside the drum, based on the experimental profile of concrete. This algorithm was tested by simulation, using the CEA code MODHERATO. The code generates tomography images of an object inspected, from the description of the object material, source (especially its energy spectrum) and detector response as a function of energy. The simulation accounted for materials of different categories (solvents, plastics, metals) blocked into a concrete matrix. Very good Z eff reconstruction is achieved (discrepancy below one) for materials with Z eff <40. However, the method becomes relatively imprecise for heavy materials (metals). Based on the present results, this method could help classify inspected objects into pre-defined categories (solvents, metals, etc.).