Wide field x-ray spectral imaging using bent Laue monochromators
Date
2020-08-18
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0001-6612-9550
Type
Thesis
Degree Level
Doctoral
Abstract
X-ray Absorption Spectroscopy (XAS) is a widely used method for determining the local environment and/or speciation of elements. Conventionally, synchrotron based XAS requires mechanically scanning through the desired energy range using a flat double-crystal monochromator. Energy Dispersive XAS (EDXAS) is a variant of the XAS technique, which typically uses a bent crystal monochromator to provide a focused x-ray beam that contains all the energies required to make an XAS measurement. An obvious advantage of EDXAS over conventional XAS is the high efficiency and stability, because mechanical energy scanning is not needed. With a high brilliance x-ray source and high speed detection system, EDXAS enables XAS studies associated with extreme conditions, femtosecond time resolution and atomic level precision.
The thesis describes the development of a Wide Field EDXAS imaging system, employing a specially designed bent Laue crystal monochromator that matches the so-called 'magic condition' which was previously described for spectral K-edge subtraction imaging. Compared to a classic EDXAS system, the Wide Field EDXAS system has the following advantages. (i) The diffracted beam from a bend magnet synchrotron source is focused in the vertical dimension and unaltered in the horizontal dimension. This focused line beam takes advantage of the width of a bend magnet x-ray beam and records spatial information across the beam. Combining the information from the energy dispersion dimension and the spatial dimension, the Wide Field EDXAS system can acquire several thousand spectra per exposure. (ii) A bent crystal that matches the 'magic condition' provides a good energy resolution in addition to the optimized focal size. The improved energy resolution enables the system to investigate the fine structure of x-ray absorption spectra with good spatial resolution using a single x-ray optic.
To make the use of the Wide Field EDXAS system more accessible, user-friendly software has been developed to process and extract concentration information from images acquired in either projection (2D) or computed tomography (3D) mode of operation. This software is made available at the Canadian Light Source BMIT-BM beamline and is compatible with another contrast element imaging method - spectral K-edge subtraction imaging.
The energy resolution of a bent crystal at the 'magic condition' is still not as good as that of a flat double-crystal monochromator. To pursue the ultimate energy resolution of a bent Laue crystal, a new approach has been developed for describing the general behavior of bent Laue crystals from a ray-tracing point of view. The so-called quasi-mono beam approach provides an intuitive view of bent crystal diffraction and leads to deeper insights. It explains the energy and spatial properties of common and special cases of bent Laue optics, predicts phenomena that can improve energy dispersion related x-ray imaging techniques and provides a theory framework that makes ray-tracing simulation easier to realize.
Description
Keywords
energy dispersive x-ray absorption spectroscopy, wide field EDXAS, chemical computed tomography, x-ray optics, bent Laue crystal, magic condition, monochromatic focusing
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Biomedical Engineering
Program
Biomedical Engineering