Ajeer, Ashkan; (2024) Development of a rapid lab-based X-Ray diffraction computed tomography technique. Doctoral thesis (Ph.D), UCL (University College London).

Text Ashkan_Ajeer_Thesis_Final.pdf - Accepted Version Access restricted to UCL open access staff until 1 June 2025. Download (9MB)

Abstract

This thesis primarily centres on development and evaluation of a high-speed, lab-based X-ray diffraction computed tomography (XRD-CT) system. Built around the HEXITEC pixelated energy-resolving cadmium telluride (CdTe) detector, it employs a hybrid approach that combines angular and energy-dispersive XRD techniques with a polychromatic pencil beam. This approach eliminates the need for monochromators, filtration, or detector collimation, which significantly enhances source efficiency and enables full spectrum utilisation. Samples containing several different crystalline materials were scanned to produce projection data sets. Tomographic reconstruction was applied to recover diffraction patterns as a function of spatial position. The reconstructed patterns were subject to a machine learning material classification algorithm to produce 3D material maps. A drawback of this technique is the effect of attenuation on the reconstructed diffraction profiles. To correct for this, a novel synchronous XRD-CT and energy resolving (colour) transmission CT was developed using a second energy dispersive detector. The further implementation of a ‘fly-scanning’ approach to data collection led to a scan time of 9.3 hours for a 25 mm diameter samples. The spatial resolution of the reconstructed images was determined by measuring the full width at half maximum (FWHM) of the line spread function (LSF) acquired from the edge response function (ERF), yielding a value of 1.26 mm. Most of the results were collected using a low power (80 W) monoblock X-ray source. Some preliminary measurements were made with a higher power source and this demonstrate the potential for significant reduction in scan time, paving the way for subhour XRD-CT scans: something that has not been demonstrated outside of a synchrotron facility. Taking the low power source into consideration, the presented technique demonstrates a 160-fold increase in scan speed when compared to other lab-based XRDCT systems. A notable advantage of this approach lies in its adaptability, allowing for straightforward adjustments to the spatial and spectral resolutions through alterations in system geometry. A simulation tool was developed that accurately models the system performance and response for changing operating parameters, such as the source-sample-detector configuration, X-ray tube voltage, aperture size, detector pixel pitch, etc. By knowing some prior information about the sample to be evaluated (e.g. the materials that are likely to be present or most important), it is possible to pinpoint the most favourable system configuration. The model was also used to explore how detector attributes, including phenomena such as charge sharing among pixels, affect the measured diffraction profile. In summary, this thesis reports on the successful development of a rapid, XRD-CT system. This is the first work to demonstrate synchrotron-like capability in a compact labbased system, having addressed key challenges in data collection methods and speed, and developed a greater understanding of the system configuration and the capability. While there is still work to be done to make this technology openly available to researchers in standard laboratories, excellent progress has been made and the pathway toward exploitation is clearer than ever.

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