Measuring with X-Rays

Since their discovery in 1885, x-rays have found numerous applications in fields such as medicine, pharmaceutics, physics, materials science, chemistry, and biology. X-rays are in many ways complemetary to light due to the low refractive-index contrast, different absorption characteristics, short wavelength, and a photon energy well beyond chemical binding energies. Methods can be categorized into 1) imaging, 2) scattering/diffraction, and 3) spectroscopy. First to imaging: The refractive index contrast of matter to x-rays is generally very weak. Therefore, multiple scattering is often negligible, and imaging of highly inhomogeneous samples (such as human tissue) is possible. The absorption properties of matter for x-rays differ considerably from those for light: Whereas light is absorbed in the band structure or in chemical bonds, x-rays interact more with inner-shell atoms.
Thus, for example, metallic samples may be transparent for x-rays, but not for light while the opposite is the case with lead glass. The simplest type of imaging is based on absorption contrast but phase contrast and interferometry are also possible. X-ray microscopy is a standard technique, but due to limitations of x-ray optics the resolution is much coarser than the wavelength. Currently, the limit is at ca. 20 nm. In contrast to imaging, which works in real space, scattering and diffraction yield inormation in reciprocal (Fourier) space. The real-space structure can be Fourier-reconstructed from the diffraction patterns. Due to the short wavelength (typ. 0.1 nm), structures down to a fraction of atomic diameters can be resolved. By exploiting refractive and absorptive sample properties, the depth inside a sample from which the information is obtained can be chosen rather freely between nanometers and centimeters. After medical imaging, the oldest application of x-rays is in crystallography (using diffraction). It is still very relevant, mostly in the study of protein structures. Spectroscopic applications make use of the element specificity of x-ray absorption and scattering at wavelengths near absorption edges for inner-shell electrons. As with diffraction, the information depth can be chosen within wide boundaries. Altough x-ray photons have energies of, typically, several kiloelectron-Volt (keV), it is possible to resolve to eV in the study of chemistry, or meV for molecular vibrations and phonons. Nuclear-resonant scattering (M?osbauer spectroskopy) reaches even 10-9eV, and is capable of resolving slow processes like diffusion in the energy domain (energy is reciprocal to time). This is useful in cases where direct time-domain observation is impossible.
Moosbauer spectroscopy also yields chemical and magnetic information. The technique is so powerful that the Mars rovers "Spirit" and "Opportunity" were equipped with onboard M?ossbauer spectrometers. Beyond the applications mentioned above, a few more, almost randomly selected ones shall be listed here. The field is, of course, much too wide for complete coverage: 1) Crystal defects such as grain boundaries, dislocations, interstitial atoms, 2) phase transition kinetics, 3) chemical reaction kinetics, catalysis 4) electronic structure of semiconductors, superconductors, magnetism 5) trace element analysis using x-ray fluorescence 6) fundamental physics, such as quantum optics. The application areas are closely related to the available sources. Besides x-ray tubes, the most important contemporary sources are synchrotrons/storage rings. Laser-plasma sources are becoming increasingly popular, and new sources, such as free-electron lasers or energy-recovery linacs will open many future application areas.