Analysis of materials by coherent x-ray scatter (CXRS)

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Wallace, G. 2004 Analysis of materials by coherent x-ray scatter (CXRS) . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2004/18 21 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 for materials analysis has been investigated in the work reported here. When the scattering materials have crystalline or defined molecular structures, energy-dispersive CXRS can provide a quick and simple method of chemical identification. Correspondence with angular-dispersive XRD is demonstrated. However, like XRD, measurements are influenced by sample forms, and scatter spectra can be distorted as a consequence. In XRD this is overcome by sample grinding. In energy-dispersive CXRS, sample movement can minimise the distortion, but not eliminate it entirely. Non-destructive analysis of crystalline materials, for instance geological rock specimens, may therefore be open to error. CXRS of amorphous samples is not subject to the same problems as the scatter responses tend to be broader. However, the broad responses and lack of ordered structure restrict the range of samples that can be measured. Nevertheless, CXRS can still be tailored to be useful in specific applications. In particular, CXRS can often be used for two-component analysis that is difficult with dual x-ray absorption attenuation (DEXA). This was demonstrated by analysing green wood samples for basic density and moisture content. CRXS requires the use of tight collimation to define scattering angles. This limits the scatter that is available for detection. The work reported here used a low power x-ray tube and a small area, electrically-cooled, high resolution CdZnTe detector, so the overall efficiency in measurement of the scattering process was very low. High power x-ray tubes and segmented, cryogenic Ge detectors provide a more efficient, and more expensive, option. Alternatively, simpler applications with amorphous materials, such as green wood, could possibly be realised using larger detectors of low resolution. While CXRS has the potential attraction of non-invasive analysis, and could be used to probe at variable depths within materials, there are qualifications preventing the automatic adoption of the technique. The practicality of any applications will need prior laboratory investigation to empirically determine these limitations, and develop optimum methods and parameters. (auth)