Coherent x-ray scatter

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Wallace, G. 2002 Coherent x-ray scatter . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2002/19 18 p.

Abstract: In the range of small momentum transfer, the coherent scatter of x-rays is dependent on the atomic form factors of the scattering atoms, and measurement of this scattering can allow the non-invasive identification of the components of materials. This is different from the more common measurement of x-ray transmission through a material. The latter technique allows estimates of densities to be made from the attenuation of the x-rays. With coherent x-ray scatter (CXRS), measurements can provide non-invasive chemical analysis of materials. CXRS includes x-ray diffraction (XRD) in which low-angle scattering of x-rays from the surface of materials enables identification of compounds by recognition of the corresponding crystalline structure. The technique of CXRS has been demonstrated in the work reported here. It can be exploited either by detecting the scatter of monochromatic x-ray beams at varying low angles, or by scattering a polychromatic beam through a fixed angle, and analysing the selected x-ray energies. Both methods have been investigated, and each has advantages and disadvantages. The atomic form factors are very difficult to predict, and an empirical approach is recommended for optimisation of specific applications. When the scattering materials have crystalline or defined molecular structures, energy-dispersive CXRS can provide a quick and simple method of chemical identification. This applies to many drugs and explosive materials. Identification is not so definitive with amorphous materials; however, CXRS can still be tailored to be useful in specific applications. CRXS requires the use of tight collimation to define scattering angles. While this restricts the overall efficiency of the scattering process, it also enables depth profiling to be achieved by translating the small scattering volume through the material. There would be some differential absorption to be taken into account, but as the scatter angle is slight, corrections can be made. Thus, the potential exists for 3D investigations by movement of the scattering material.The next step beyond this proof of principle is the application of the technique to appropriate industrial problems. Beyond the identification of unknown materials, these could involve the quantitative analysis of two-component mixes, or investigations where there are chemical changes underway. The attraction of CXRS is that it is non-invasive, and can be used to probe at variable depths within materials. However, the practicality of any applications will need prior laboratory investigation to empirically determine optimum methods and parameters. (auth)