What are X-rays?

The ESRF produces so-called “hard” X-rays at wavelengths of 0.10 to 0.01 nanometres.

X-rays are electromagnetic waves like visible light but situated at the high energy/short wavelength end of the electromagnetic spectrum, between ultraviolet light and gamma rays. Their wavelength, at around a tenth of a nanometre (a nanometre is one billionth of a metre, i.e. 10-9 m), is comparable to interatomic distances, which makes X-rays suitable for the study of atoms and bonds.

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The electromagnetic spectrum comparing the size of objects that can be studied with various techniques.

X-rays were discovered by Wilhelm Röntgen in 1895. Today, they are used extensively in medical imaging because they have a high penetration depth through materials and are selectively absorbed by the parts of the body with the highest electron density such as bones. The principal difference between synchrotron light and the X-rays used in hospitals is the brilliance: a synchrotron source is one hundred billion times brighter than a hospital X-ray source.

What are “hard” X-rays?

The ESRF produces X-rays of high energy, called “hard” X-rays, which have wavelengths of 0.10 to 0.01 nm or energies in the range 10 to 120 keV. (In synchrotron science, it is more usual to speak of energies.) Because of their higher energies, hard X-rays penetrate deeper into matter than soft X-rays, those with energies below 10 keV. The synchrotron X-ray beam can have other valuable properties, including time structure (a flashing beam), coherence (a parallel beam) and polarisation.

In addition to being absorbed by a material, X-rays can also interact with the atoms, giving rise to diffraction or scattering of the X-rays. X-ray absorption can also be followed by re-emission of the energy absorbed, for example as fluorescence. These interactions with matter are used to gain information about the composition of a sample, including the type and location of individual atoms within it.

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