Luggage inspection in airport by controlling the absorption of an X-ray beam are routinely used since several decades. In general, the d.c. current generated in a silicon photodiode coupled to a scintillator measures the absorption of the beam. More sophisticated systems have also appeared, for example based on single photon spectroscopic counting or dual energy arrangements.
In the present presentation, an alternative method is considered, still based on X-rays, but based on inelastic diffusion and single counting mode in advanced CdTe detectors. This method uses the rather large energy spectrum emitted by a X-ray tube, when impinging in a polycrystalline material. In this later, a family of crystalline planes (hi, ki, li) always finds the photon of a wavelength λi, such that the Bragg relation, expressed by:
d(hi, ki, li) = nλi / 2sinθi
can be fulfilled. This method is well known by the crystallographer, sometimes called wrongly "Laue Scattering". Here, single crystallinity is no longer necessary; and we can have a series of diffraction rings, without the any displacement of the sample, nore a change in the X-ray emission spectrum. A complete theoretical and analytical description will be given in the lecture.
Experimentally, it becomes possible to establish a library of crystalline powders spectra, specific for each molecular structure and, consequently, determine quite univocally the exact structure of a drug or an explosive. An illustration will be considered for a shoe analysis.