X-ray tomography is a technique to get three-dimensional information about a sample. It is based on the idea of recording photons after interaction of a primary beam with the sample (transmitted-, scattered- or fluorescence photons) under various angles and subsequent (computer-assisted) decomposition of the information overlay.
While within conventional X-ray absorption tomography the transmitted beam is measured to determine the spatial distribution of the absorption coefficient inside a sample, X-ray fluorescence tomography is based on the detection of fluorescence photons emitted from the sample. From such measurements the internal elemental composition of the sample can be obtained. However, the reconstruction problem for this technique is much more difficult than that of transmission tomography, mainly due to self absorption effects in the sample.
The setup is as follows: an absorption detector is placed in the forward direction of the strongly collimated parallel Xray beam for measuring the photons transmitted through the sample. A second (energy dispersive) detector is placed perpendicular to the incident beam and measures the fluorescence photons emitted from the sample. For performing translation and rotation scans the sample is placed on a micrometer driven stage which allows to scan along the direction perpendicular (s-direction) to the X-ray beam and to rotate the sample around an axis perpendicular to the incident plane of the beam.