What Are X-rays?

X-rays are a form of electromagnetic radiation with photon energies ranging from approximately 100 electron volts (eV) to 100 kiloelectron volts (keV), corresponding to wavelengths between about 0.01 and 10 nanometers. They occupy the portion of the electromagnetic spectrum between ultraviolet radiation and gamma rays, and their wavelengths are on the order of atomic bond lengths, which gives them the ability to penetrate solid matter and to diffract from crystalline structures. Wilhelm Conrad Röntgen discovered X-rays in 1895 at the University of Würzburg while experimenting with cathode ray tubes, an observation for which he received the first Nobel Prize in Physics in 1901.

X-rays are produced when high-energy electrons decelerate rapidly in the vicinity of atomic nuclei, emitting bremsstrahlung ("braking radiation"), or when electrons transition between inner shell orbitals of an atom, releasing characteristic X-rays at energies specific to the target element.

Properties and Generation

In conventional X-ray tubes, electrons emitted from a heated filament are accelerated across a potential difference of tens to hundreds of kilovolts before striking an anode target, typically tungsten or molybdenum. The resulting X-ray spectrum consists of a continuous bremsstrahlung background superimposed on discrete characteristic emission lines. Beam collimators, aperture devices made of high-atomic-number materials such as lead or tungsten, shape the diverging beam into well-defined field geometries. The penetration depth of X-rays depends on photon energy and the density and atomic number of the traversed material, described quantitatively by the linear attenuation coefficient. The production and physics of X-rays, including the role of tube voltage, filtration, and target material, are described in a StatPearls review of X-ray production hosted by the National Institutes of Health.

Synchrotron Radiation

Synchrotron storage rings produce X-rays of exceptional brightness by steering relativistic electrons through bending magnets and insertion devices such as wigglers and undulators. The resulting beams are many orders of magnitude more intense than laboratory sources, are highly collimated, and span a tunable energy range from the soft X-ray regime into the hard X-ray regime above 10 keV. These properties underpin techniques including protein crystallography, X-ray absorption fine structure (XAFS) spectroscopy, and phase-contrast imaging that would be impractical with tube sources. A review in Reviews of Modern Physics covering synchrotron X-ray microprobes and nanoprobes documents how synchrotron-based X-ray methods transformed materials characterization across physics, chemistry, biology, and cultural heritage science.

X-ray Lasers and Coherent Sources

X-ray free-electron lasers (XFELs), such as the Linac Coherent Light Source at SLAC National Accelerator Laboratory and the European XFEL, produce femtosecond pulses of coherent X-rays with peak brightness more than a billion times higher than third-generation synchrotrons. This combination of spatial coherence and ultrashort pulse duration allows imaging of individual molecules, tracking of chemical reaction dynamics on timescales of femtoseconds, and single-particle diffractive imaging of non-crystalline specimens. The National Institutes of Biomedical Imaging and Bioengineering provides an accessible explanation of medical X-ray physics and imaging principles applicable to understanding how X-ray properties are exploited across the full range of diagnostic and research applications.

Applications

X-rays have applications across a wide range of disciplines, including:

• Medical diagnostic imaging, radiotherapy, and fluoroscopy
• Crystallographic structure determination of proteins, minerals, and engineered materials
• Non-destructive testing and industrial inspection of welds, castings, and electronic assemblies
• Security screening of baggage, cargo, and mail
• Lithographic patterning in semiconductor and nanofabrication processes
• Scientific research at synchrotron and free-electron laser facilities