Hard X-ray imaging allow detailed information to be gathered from below the surface of a material through either full-field imaging, where the whole sample is illuminated, or through scanning, where the beam is focused to a small spot which is scanned across the sample. The high intensity and energy of synchrotron X-rays makes it possible to image a much larger range of materials and sample thicknesses than conventional X-ray sources, and the brilliance of the synchrotron source produces very high resolution images.
Absorption contrast imaging is the most common imaging technique, and is the technique used in hospital X-ray imaging. An absorption contrast image is essentially a shadowgraph, the contrast being generated by the different attenuating power of materials in the sample. The small spot size and high intensity of synchrotron X-rays also make it possible to scan samples and provide a composite image in much finer detail than from conventional sources.
Phase contrast imaging takes advantage of the fact that different materials have different refractive indices. This produces a phase shift in the X-rays passing through the sample. By placing the imaging detector at a specific distance from the sample, interference between waves can be used to enhance contrast in the image.
Coherent X-ray diffraction is an imaging technique which overcomes some of the limitations encountered with using lenses. Instead, a series of X-ray diffraction patterns are combined to mathematically reconstruct a three dimensional image of the structure being studied. With highly coherent synchrotron X-rays, this approach can provide spatial resolution on a nm scale. In the past this technique has been applied to model repeating crystal structures, but it is now being used to examine small, non-periodic samples.
Spectroscopic experiments allow researchers to reveal elemental composition, chemical state and physical properties of both inorganic material and biological systems. By sweeping through a range of photon energies the absorption, reflectivity or fluorescence of the sample is measured. In the X-ray region all atoms absorb X-rays sharply at certain wavelengths (called absorption edges) that are characteristic of that atomic species, so element-specific information can be obtained.
A range of X-ray Absorption Spectroscopy (XAS) techniques are available at Diamond, including X-ray Absorption Near-Edge Structure (XANES), Extended X-Ray Absorption Fine Structure (EXAFS), Resonant Inelastic X-Ray Scattering (RIXS) and X-Ray Emission Spectroscopy (XES).
Diffraction and scattering techniques analyse the patterns produced when a sample is illuminated by X-rays and causes deflections. Diffraction patterns provide the atomic structure of molecules such as powders, small molecules or larger ordered molecules like protein crystals.
Small molecule diffraction is the most widely used technique for obtaining full three-dimensional structural information of solid-state crystalline materials. This allows characterisation of, for example, molecular packing, guests in a framework structure, the nature of intra- and intermolecular interactions, molecular conformation and static and dynamic disorder. Environmental cells can be used to probe materials under a range of non-ambient conditions.
X-ray Scattering provides essential information on the structure and dynamics of large molecular assemblies in low ordered environments. These are characteristic of living organisms and many complex materials such as polymers and colloids. Wide Angle X-ray Scattering (WAXS) typically covers the angular range 5 - 60°. The capability of investigating materials under extreme environments and the ability to mimic industrial processes (e.g. high pressure, temperature, and or rheology), whilst simultaneously recording X-ray measurements in both small and wide angle X-ray scattering opens up new avenues for addressing questions regarding phase-space identification and the relationship between microscopic structure and macroscopic properties.
X-ray reflectivity (XRR) is a technique for studying the detailed surface properties of materials. Specifically, x-rays are used to probe the electron density perpendicular to the surface and thereby obtain information about the surface roughness, thin film thickness and density.
X-ray tomography is the construction of a three dimensional image from two dimensional projections taken at different orientations (usually with phase contrast or absorption contrast imaging). The tuneability of synchrotron X-rays make it possible to provide increased contrast images, and the coherent nature and high intensity of synchrotron X-rays have led to significant developments in this field, particularly in phase tomography which in the past has required extremely complex instrumentation.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
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