Publications

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    ABSTRACT: In recent times, there has been a drive to develop non-destructive X-ray imaging techniques that provide chemical or physica insight. To date, these methods have generally been limited; either requiring raster scanning of pencil beams, using narro bandwidth radiation and/or limited to small samples. We have developed a novel full-field radiographic imaging technique tha enables the entire physio-chemical state of an object to be imaged in a single snapshot. The method is sensitive to emitte and scattered radiation, using a spectral imaging detector and polychromatic hard X-radiation, making it particularly usefu for studying large dense samples for materials science and engineering applications. The method and its extension to three-dimensiona imaging is validated with a series of test objects and demonstrated to directly image the crystallographic preferred orientatio and formed precipitates across an aluminium alloy friction stir weld section.
    Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences 05/2014; 470(2165). · 2.38 Impact Factor
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    ABSTRACT: By imaging the X-ray spectral properties or ‘colours’ we have shown how material specific imaging can be performed. Using a pixelated energy-dispersive X-ray detector we record the absorbed and emitted hard X-radiation and measure the energy (colour) and intensity of the photons. Using this technology, we are not only able to obtain attenuation contrast but also to image chemical (elemental) variations inside objects, potentially opening up a very wide range of applications from materials science to medical diagnostics.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2014; 324:25–28. · 1.27 Impact Factor
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    ABSTRACT: The internal crystalline structure of a human molar tooth has been non-destructively imaged in cross section using X-ray diffraction computed tomography. Diffraction signals from high energy X-rays which have large attenuation lengths for hard biomaterials have been collected in a transmission geometry. Coupling this with a computed tomography data acquisition and mathematically reconstructing their spatial origins, diffraction patterns from every voxel within the tooth can be obtained. Using this method we have observed the spatial variations of some key material parameters including nanocrystallite size, organic content, lattice parameters, crystallographic preferred orientation and degree of orientation. We have also made the link between the spatial variations of the unit cell lattice parameters and the chemical make-up of the tooth. In addition, we have determined how the onset of tooth decay occurs through clear amorphisation of the hydroxyapatite crystal, and we have been able to map the extent of decay within the tooth. The described method has strong prospects for non-destructive probing of mineralised biomaterials.
    Acta biomaterialia 06/2013; · 5.09 Impact Factor
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    ABSTRACT: A pixellated CdTe detector system comprising 2×2 detector modules has been developed for high energy spectroscopic X-ray imaging applications and has an active area of 16 cm2 . The detector modules are made from the HEXITEC 80×80 ASIC and 1 mm thick CdTe with Al-Schottky contacts. The CdTe has 250 μm pitch pixels with an outer guard ring on the same pitch. The single HEXITEC 80×80 detectors have an average energy resolution (FWHM) of 800 eV at 59.9 keV. Limitations in the multiple module DAQ system mean that the energy resolution of the pixels in the 2×2 detector array is 2.0 keV at 59.9 keV. The spacing between the tiled detector modules is 150 μm which results in an inactive area equivalent to 3 pixels, including the guard ring on the edge of the detectors. The modular detector configuration demonstrates the potential to create large area detector arrays in the future.
    IEEE Transactions on Nuclear Science 01/2013; 60(2):1197-1200. · 1.22 Impact Factor
  • Christopher K Egan, Simon DM Jacques, Robert J Cernik
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    ABSTRACT: This article describes methods to analyse and process hyperspectral hard X‐ray imaging data. We focus on the use of multivariate techniques that exploit the spectral information to make informed decisions on the material content within each pixel of an X‐ray image. These analysis methods have the ability to auto‐segment data without prior knowledge of the sample composition or structure, and are particularly useful for studying completely unknown, diluted or complex specimens. We demonstrate the methods on a variety of hard X‐ray images including X‐ray fluorescence and absorption data recorded using a hard X‐ray imaging spectrometer. The multivariate methods described are very powerful with the ability to segment, distinguish and, in some cases, identify different materials within a single X‐ray image. Potential uses of hyperspectral X‐ray imaging are discussed varying from materials science to industrial or security applications. Copyright © 2013 John Wiley & Sons, Ltd.
    X-Ray Spectrometry 01/2013; · 1.55 Impact Factor
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    ABSTRACT: X-ray tomography is a ubiquitous tool used, for example, in medical diagnosis, explosives detection or to check structural integrity of complex engineered components. Conventional tomographic images are formed by measuring many transmitted X-rays and later mathematically reconstructing the object, however the structural and chemical information carried by scattered X-rays of different wavelengths is not utilised in any way. We show how a very simple; laboratory-based; high energy X-ray system can capture these scattered X-rays to deliver 3D images with structural or chemical information in each voxel. This type of imaging can be used to separate and identify chemical species in bulk objects with no special sample preparation. We demonstrate the capability of hyperspectral imaging by examining an electronic device where we can clearly distinguish the atomic composition of the circuit board components in both fluorescence and transmission geometries. We are not only able to obtain attenuation contrast but also to image chemical variations in the object, potentially opening up a very wide range of applications from security to medical diagnostics.
    The Analyst 11/2012; · 4.23 Impact Factor
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    ABSTRACT: A new data collection strategy for performing synchrotron energy-dispersive X-ray diffraction computed tomography has been devised. This method is analogous to angle-dispersive X-ray diffraction whose diffraction signal originates from a line formed by intersection of the incident X-ray beam and the sample. Energy resolution is preserved by using a collimator which defines a small sampling voxel. This voxel is translated in a series of parallel straight lines covering the whole sample and the operation is repeated at different rotation angles, thus generating one diffraction pattern per translation and rotation step. The method has been tested by imaging a specially designed phantom object, devised to be a demanding validator for X-ray diffraction imaging. The relative strengths and weaknesses of the method have been analysed with respect to the classic angle-dispersive technique. The reconstruction accuracy of the method is good, although an absorption correction is required for lower energy diffraction because of the large path lengths involved. The spatial resolution is only limited to the width of the scanning beam owing to the novel collection strategy. The current temporal resolution is poor, with a scan taking several hours. The method is best suited to studying large objects (e.g. for engineering and materials science applications) because it does not suffer from diffraction peak broadening effects irrespective of the sample size, in contrast to the angle-dispersive case.
    Journal of Synchrotron Radiation 07/2012; 19(Pt 4):471-7. · 2.19 Impact Factor
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    ABSTRACT: Synchrotron white beam X-ray topography has been used to characterise bulk crystal defects of thick vapour grown CdZnTe crystals. Whole 50 mm diameter wafers with thicknesses in the range of 2–3 mm were sliced from boules grown by the multi-tube physical vapour transport method and analysed by diffraction topography in a transmission geometry. A variety of defects were observed including cracks, voids and grain boundaries. The largest quantity of defects observed were sub-grains appearing as localised increased intensity in the topographs. The periphery of the wafers showed the highest number of defects, whereas central regions where largely defect-free. We failed to observe any inclusions or precipitates within these crystals. Surface damage from wire-saw cutting was also observed on poorly processed wafers; these defects were otherwise invisible to standard characterisation methods. X-ray topography has proven to be a useful tool for non-destructively investigating bulk extended defects in CdZnTe crystals for radiation detector applications.
    Journal of Crystal Growth 03/2012; 343(1):1–6. · 1.55 Impact Factor
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    ABSTRACT: We have developed a pixellated high energy X-ray detector instrument to be used in a variety of imaging applications. The instrument consists of either a Cadmium Zinc Telluride or Cadmium Telluride (Cd(Zn)Te) detector bump-bonded to a large area ASIC and packaged with a high performance data acquisition system. The 80 by 80 pixels each of 250 μm by 250 μm give better than 1 keV FWHM energy resolution at 59.5 keV and 1.5 keV FWHM at 141 keV, at the same time providing a high speed imaging performance. This system uses a relatively simple wire-bonded interconnection scheme but this is being upgraded to allow multiple modules to be used with very small dead space. The readout system and the novel interconnect technology is described and how the system is performing in several target applications.
    Journal of Instrumentation 12/2011; 6(12). · 1.66 Impact Factor
  • Christopher K Egan, QZ Jiang, AW Brinkman
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    ABSTRACT: The morphology and reconstructions of clean CdTe(111)A and CdTe(111)B surfaces have been studied by scanning tunneling microscopy. The (111)A surface is highly ordered and has a (2×2) reconstruction with a cadmium vacancy structure. The CdTe(111)B surface shows a large amount of disorder with small ordered domains having a c(8×4) reconstruction. This consists of chains of atoms in a two layer structure sitting upon the bulk terminated surface. These chains extend in any of the surface directions. A number of other surface structures were observed, but these showed little long-range ordering. For both surfaces, a certain amount of faceting on {311} planes was observed. This faceting suggests that the surface energy for the {111} surfaces is quite large.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2011; 29(1):011021-011021-8. · 1.43 Impact Factor
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    C. K. Egan, A. Choubey, A. W. Brinkman
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    ABSTRACT: The morphology and electronic structure of chemically treated CdZnTe surfaces has been studied using atomic force microscopy and scanning tunneling microscopy/spectroscopy. Studies have been performed on wet chemically etched and passivated surfaces. Etching in bromine methanol solutions effectively removes surface damage after polishing and produces a hillock like structure. A number of surface electronic states are introduced, both donor-like and acceptor-like, and are attributed to various surface defects. Because of the amorphous nature of the surface, these states have tails that extend into the band gap and produces a very narrow surface band. Charge hopping across the narrow band gap is proposed to explain the high surface conductivity. Surfaces passivated in hydrogen peroxide show increased roughness and using atomic force microscopy, the evolution of the formation of an oxide layer has been observed. Additional acceptor-like surface states of higher energy are introduced and the surface band gap is found to increase, reducing the surface conductivity.
    Journal of Applied Physics 07/2010; 108(2):024310-024310-6. · 2.21 Impact Factor
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    ABSTRACT: The morphology and electronic structure of the (110) surface of semi-insulating CdZnTe has been studied by scanning tunneling microscopy and spectroscopy. The surface shows a 1 × 1 reconstruction whilst the tunneling spectra are highly rectified implying that imaging could only be performed at negative sample bias. Theoretical computations of the tunnel current have been used to fit to experiment to reveal the origin of each tunneling component and explain the rectification observed. The implications of various surface defects and surface states are considered. A discussion on scanning tunneling microscopy and spectroscopy on semi-insulating materials in general is also given.
    Surface Science 01/2010; 604:1825-1831. · 1.84 Impact Factor
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    ABSTRACT: The effects of several ex vacuo methods used in the surface preparation of Cd1−x Znx Te (CZT) have been studied using noncontact atomic force microscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Preparation techniques include mechanical lapping, hydroplane bromine-methanol polishing, and in vacuo annealing. The morphology, electrical homogeneity, and local density of states (LDOS) have been studied for each preparation method. Impurities and oxides quickly form on the surface after each preparation method. Annealing in ultrahigh vacuum causes the surface electronic structure to become inhomogeneous whilst the LDOS suggests a compositional change from an oxide surface to p-type CZT.
    Journal of Electronic Materials 01/2009; 38(8):1528-1532. · 1.64 Impact Factor

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