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The detection of X-ray photons scattered through a sample by the Rayleigh and Compton processes is used to perform tomographic images. A map can thus be obtained, which emphasizes small changes in atomic number Z within the sample. In such a way to distinguish between photons scattered through Rayleigh and Compton processes, a monochromatic photon beam must be used. Choosing a 60 keV photon energy, difference between polyethylene and an aqueous solution containing a low concentration of iodine (0.5 mg.cm-3) is easily obtained. The most common experimental device involves a collimator with an unique slit. The scanning throughout the slice is performed point by point and the corresponding image can directly be drawn up. Beside the point by point method, the present paper describes a new experimental arrangement and the corresponding reconstruction method. The scanning method is similar to the one used for first generation tomographs. A standard reconstruction algorithm delivers two intermediate images, corresponding to the Compton and Rayleigh contributions. On both images artifacts are present, due to photon attenuation inside the sample. Computation of the ratio between those two images gives a Z map of the sample, free of artifacts. The experiment was performed at the European Synchrotron Radiation Facility (ESRF), in Grenoble (France), on line ID15 B. Due to the very high photon intensity, short measurement times are allowed (around five seconds by point), as well as a good spatial resolution. The voxel size is 1 mm X 1 mm in the plane of the slice, and 0.3 mm in the third direction.
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