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Title: Optimising, understanding and quantifying Itrax XRF data
Author: Jarvis, Stuart
ISNI:       0000 0004 2741 2929
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2012
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The Itrax core scanner provides rapid, high resolution, non-destructive sediment core analysis using x-ray fluorescence (XRF) spectrometry and x-radiography. The effect of varying instrument settings are explored. Effects of sample properties on XRF data are tested and the uncertainty in XRF data is considered. Existing methods of quantifying XRF data are evaluated. Comparisons are made with other XRF micro-scanners. Finally, the x-radiographic capability of the Itrax core scanner is compared to x-ray computed tomography. Itrax XRF data is generally optimised by use of a 30kV x-ray tube voltage. Current should be set as high as possible without generation of sum peaks (30mA is often a good value). A chromium anode tube is suitable for use with most samples. Water content has a diluting effect on detected peak areas, but it is shown that the effect can be corrected, removing an obstacle to quantification of Itrax data. Water content can be determined non-destructively from the ratio of incoherent to coherent scatter of characteristic radiation from the x-ray tube anode. Surface slope can change recorded peak areas, but a simple model is developed to correct for this effect. Surface roughness increases variability in data and, if the scale of roughness is similar to the beam size, elemental peak areas may be reduced. The presence of a mixture of grain sizes greatly reduces peak areas for elements in the larger grains. The uncertainty in Itrax data is found to be higher than suggested by the conventional estimate that uncertainty is equal to the square root of the peak area. This information is vital for researchers to decide what significance they should attach to variations in Itrax elemental profiles. Quantification methods for core scanner XRF data are compared and an approach using log-ratio transformations determined to be the best. Additionally, an improved entirely non-destructive, quantification approach is presented in which explicit corrections are made for the diluting effect of water (water content may determined from the ratio of incoherent and coherent scatter of the tube anode characteristic radiation) . Compared to similar instruments, the Itrax core scanner is more tolerant of surface imperfections. Its x-radiographic scanning helps to determine the significance and extent of features revealed in XRF data. Itrax x-radiography provides no match for the level of detail that can be obtained using x-ray computed tomography and is not readily quantified. It does however provide information on features below the sample surface and, in masking small variations, can make the main core features more apparent. Users of the Itrax core scanner are provided with quantification of known effects (water content, surface slope; x-ray tube, current and voltage) and are alerted to issues that were not previously widely known (mixing of grain sizes, size of uncertainty in data). The areas of effective use and limitations of the Itrax core scanner are set out and recommendations made for optimising results. An optimal quantification method is identified. Many of the conclusions may have relevance to other XRF core scanners.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available