Basics
Anyone who has ever undergone a medical X-ray will recognise the principle of X-ray radiography: a sample is placed in an X-ray beam and the density of different biological features (bones, muscles, etc.) can be distinguished from the different amounts of absorption by the material. This same principle used with synchrotron X-rays, benefits from the extremely high intensity of the beam. Together with high-resolution detector systems, the technique allows for the investigation of very small samples at high spatial resolutions
X-ray microtomography is a related technique based on X-ray radiograms taken from many different angles of a sample. The computed reconstruction of these radiograms yields a detailed three-dimensional model of the sample. This principle, when used with a synchrotron source, is very similar to the well-known medical CT scan but with an approximately thousand times higher spatial resolution. In addition, the monochromaticity of the beam used in the experiment allows a much higher density contrast than a broad X-ray spectrum as used in laboratory devices
Different Contrast Modalities
In the case of weakly absorbing materials such as biological soft tissue, other X-ray imaging methods can provide a better contrast than X-ray imaging based on absorption. X-ray phase-contrast imaging uses the fact that the sample not only absorbs the X-rays but refracts them like a lens, slightly changing the X-ray propagation direction. By measuring these changes, different information can be obtained on the specimen, complementing the absorption technique.
Benefits
The high penetration of X-ray radiation allows the technique to effectively “see through” solid samples. The very intense synchrotron X-ray beam and the dedicated tomography experiment allow high resolution imaging of very small samples. X-ray radiography and tomography are particularly sensitive to density differences within a sample, making them very useful for detecting precipitates, voids and pores in solids - features that can have large effects on the mechanical properties of a material. The use of monochromatic X-rays at the synchrotron provides excellent density contrast, allowing the user to differentiate between materials of very similar composition.
Types of samples
Samples for X-ray radiography/tomography do not have to be homogeneous, ordered or crystalline to be suitable. The versatile synchrotron radiation experiment allows very different samples to be measured, such as porous materials, wood, bones, fossils and biological samples down to the micrometre scale. Furthermore, the technique is applicable to analysing samples in in situ environments, such as alloys in ovens, lightweight materials in corrosion environments or fibre-composites under tensile load. The achievable spatial resolution depends on the sample diameter – approx. 1/1000th of the diameter can be achieved in 3D. The ideal sample shape is cylindrical, but other shapes are also possible.
Applications of X-ray radiography and microtomography at DESY
- Investigation of the structural integrity of welds and other structural materials.
- Determination of the porosity in lightweight metallic foams or solid hydrogen/gas storage materials.
- Imaging of the internal structures of microscopic fossils.
- Imaging of specific parts of whole plants or animals.
