Many different kinds of sophisticated analyses, on a vast array of samples, can benefit from synchrotron radiation. These synchrotron techniques can be broadly broken down into three categories: diffraction (and scattering), spectroscopy and imaging.
One of the most established synchrotron techniques is X-ray diffraction. When X-rays pass through a crystal they are reflected off the regular arrangement of planes of atoms that make up the crystal, and onto a detector. From the patterns that these reflected X-rays create, researchers can work backwards to discern the atomic structure that scattered the X-rays. Diffraction can be used to look at the structure of chemical compounds and composite materials, such as minerals, ceramics, biological samples and electronic and magnetic materials. By contrast, scattering provides essential information about 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.
Another prominent technique is spectroscopy, which allows 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 can be 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. By subjecting samples to a variety of X-ray wavelengths, and detecting which X-ray wavelengths do not pass through the sample, element-specific information can be obtained. In the infrared wavelength range, characteristic vibrational modes of condensed matter or biomedical samples are excited to disclose the molecular structure, which is eventually probed by a microscope, for a 2-D image. Scientists use X-rays, infrared, UV and visible light to reveal different characteristics of samples, which range from biomedical samples and condensed matter, to engineering materials and magnetic materials.
Imaging is a wide area of synchrotron research, which records images of the object under study. These images are, however, significantly enhanced by the unique nature of the light generated by synchrotrons.
Absorption contrast imaging, for example, functions in much the same way as a hospital X-ray machine, where X-rays are shone on the body, and X-rays pass more easily through denser materials (like bone), than through surrounding tissue. The result is a shadow-like X-ray. In a very similar way, in absorption contrast imaging, X-rays are shone on a sample, and a detector behind the sample measures the residual X-rays which pass through. This forms a shadowgraph from varying strengths of X-rays, which provides data about the object’s varying densities. Synchrotron light is, however, up to 100 billion times brighter than a standard hospital X-ray machine, and provides much finer detail than the resolution available in hospitals. This particular technique is used in a wide range of applications, from bio-medicine to materials science, engineering, environmental science and technology.
Microscopy makes use of the tuneability of synchrotron light to carry out imaging with varying X-ray and infrared frequencies, which interact with each element in the sample through its particular absorption frequency (see ‘Spectroscopy’, above). As a result, not only does microscopy give imaging results on a nm scale about the structure of a sample’s surface, but these results contain data about the elements in the sample, too. This enables users to link a sample’s structure to its function more easily. Microscopy can be used for studying the structure of micro-and nano-objects.
Tomography develops upon standard X-ray techniques by constructing a three-dimensional image from a series of two-dimensional images, which are taken from a sample in several different orientations – much in the same way as a CT scan in a hospital scans patients from various angles to create a three-dimensional image of the body. In combination with recording data about a sample’s ‘refractive index’ (the way in which X-rays bend through an object, like light through a prism), tomography can be used to construct a three-dimensional image of the sample’s interior. In other words, tomography enables us to look at different slices of the sample, without actually physically cutting it. Its non-destructive nature is beneficial to many applications in the materials science, engineering and biomedical fields. For example, tomography can determine internal stresses and cracks inside components such as aircraft wings, which later could, if undetected, have catastrophic results. Furthermore, just like for hospital CT scans, X-ray tomography is also very useful for studying the internal structure of biological tissues such as human dental tissue and brain tumours.