When an incident beam of X-rays interacts with a target material scattering of those X-rays occurs within the target material. An X-ray diffraction pattern is the variation in intensity observed when the scattered X-rays undergo constructive and destructive interference as a result of their interaction within the material. The angular directions of possible diffraction peaks depend on the size and shape of the unit cell of the material. The intensities of the diffracted waves depend on the kind and arrangement of atoms in the crystal structure. A primary use of the technique is the identification and characterization of compounds based on their diffraction pattern.
The principle of the Energy Dispersive EXAFS (EDE) is based on the diffraction of non monochromatic X-rays by a bent crystal (polychromator crystal). The polychromatic beam is focused on the sample and then perges towards a position sensitive detector where beam position is correlated to energy. The dispersive configuration of XAS has two main advantages that make it scientifically attractive. First, the whole x-ray absorption spectrum is collected simultaneously which makes the technique especially useful for the study of fast processes. Second, the size of the focussed beam at the sample position is small and very stable due to the fact that no movement of optical elements is required to collect the spectral data.
Organic or inorganic heterogeneous matter attenuates and scatters X-rays in different manner, which is used to characterize the morphology of the specimen using a variety of contrast techniques.
A range of Imaging techniques are available at Diamond and the spectroscopy group supports a number of specific imaging modes.
Due to their interaction with matter, X-rays traversing a sample are deviated (scattered) from their path. The exit angle of the scattered photon with respect to its original path depends on the size and arrangement of the deviating structure, with larger structures deviating photons at smallerangles (this is described by Bragg's Law). Therefore, Small Angle X-ray Scattering (SAXS) provides information on long-range order, such as the shape and size distribution of nanoparticles and their possible interaction, the size and arrangement of nanometre-sized pores in a material, the diameter and orientation of fibers in tissues and composites or the outer envelope of a protein in solution, to name but just a few.
Because the beam footprint is typically much larger than the size of the structures it illuminates, SAXS analysis yields quantitative average values over the whole irradiated volume with high statistical significance.
A range of X-ray Absorption Spectroscopy 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).
X-ray emission spectroscopy (XES) is one of the so-called photon-in - photon-out spectroscopies in which a core electron is excited by an incident x-ray photon and then this excited state decays by emitting an x-ray photon to fill the core hole. The energy of the emitted photon is the energy difference between the involved electronic levels. The analysis of the energy dependence of the emitted photons is the aim of the X-ray emission spectroscopy.
X-ray Fluorescence (XRF) occurs when the inner shell electrons of atoms in the sample get excited by the incident X-ray photons (synchrotron beam) and subsequently release X-ray photon when the system relaxes, that is when the electrons transition from the higher energy levels of the atom to the vacant inner shell. The beauty of this process is that each secondary X-ray photon (sometimes called characteristic radiation) emitted from the sample has a specific energy which is a fingerprint of the atom from which it has originated. By measuring the energy of the secondary photons it is possible to establish the elemental composition of the sample at the point where the X-ray beam hits the sample. Typically a special type of detector called energy-dispersive detector is used to precisely measure the energy of each photon. The plot of the number of photon counts versus their energy, the X-ray spectrum, typicallly shows a number of peaks which are directly associated with specific elements, so by just glancing at the spectrum it is possible to quickly deduce which elements are present in the sample.
By use of calibrated standards it is possible to make synchrotron XRF a quantitative technique.
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