X-ray

X-rays are a form of electromagnetic radiation with wavelengths much shorter than visible light and correspondingly higher photon energies. They occupy a region of the electromagnetic spectrum beyond ultraviolet radiation. Their ability to penetrate materials, combined with their interactions with matter, makes them useful for medical imaging, structural analysis, and astronomical observation. In everyday usage, “an X-ray” can also mean an examination or the image it produces, rather than the radiation itself. (science.nasa.gov)

Physical properties

Like other electromagnetic radiation, X-rays consist of photons that travel in a vacuum at the speed of light. Their energy is related to frequency and wavelength by $E=h\nu=hc/\lambda$, where $h$ is the Planck constant, $\nu$ is frequency, and $\lambda$ is wavelength. Consequently, shorter-wavelength X-rays carry more energy per photon. X-ray energies are commonly expressed in electron volts or kiloelectron volts rather than by wavelength. (science.nasa.gov)

“Soft” X-rays have relatively low energies and longer wavelengths; “hard” X-rays have higher energies and generally greater penetrating power. These labels describe portions of a continuous spectrum rather than fundamentally different radiation. Published wavelength boundaries vary with context; NASA’s introductory spectrum guide, for example, describes an X-ray range of approximately 0.03–3 nanometers. X-ray wavelengths can therefore be comparable to, or smaller than, the dimensions of an atom. (science.nasa.gov)

X-rays exhibit both particle-like interactions and wave phenomena. Diffraction and interference reveal their wave character, while the transfer of energy in individual photon interactions demonstrates their particle character. These complementary descriptions form part of the framework of quantum mechanics. (educationalgames-staging.nobelprize.org)

Discovery and early development

Wilhelm Conrad Röntgen discovered X-rays on November 8, 1895, while investigating cathode-ray tubes. He noticed that a fluorescent screen glowed even though the tube was covered. Subsequent experiments showed that the unfamiliar radiation could pass through materials opaque to visible light and produce images of internal structures. He used “X” to indicate its initially unknown nature. Röntgen received the first Nobel Prize in Physics in 1901 for the discovery. (educationalgames-staging.nobelprize.org)

In 1912, experiments by Max von Laue and collaborators demonstrated diffraction of X-rays by crystals, establishing their electromagnetic-wave character. William Henry Bragg and William Lawrence Bragg subsequently developed methods for determining crystal structures from diffraction patterns. These investigations transformed X-rays from a penetrating imaging medium into a quantitative probe of matter at atomic scales. (educationalgames-staging.nobelprize.org)

Production

A conventional X-ray tube accelerates electrons toward a target using an applied voltage. Two mechanisms contribute to its radiation. In bremsstrahlung, or “braking radiation,” electrons emit photons as their motion changes in the electric fields of target atoms, producing a continuous spectrum. Characteristic radiation arises when an inner-shell vacancy is filled by an electron from a higher energy level. The emitted photon has an energy determined by the difference between those levels, producing spectral lines associated with the target material. (pub.iaea.org)

Research facilities also generate X-rays through synchrotron radiation. High-energy electrons circulating in a storage ring emit electromagnetic radiation when magnetic fields deflect their trajectories. Bending magnets and arrays of alternating magnets called undulators supply beams to experimental stations. Such sources provide intense X-rays for measurements requiring small beams, high resolution, or selectable photon energies. (esrf.fr)

Interaction with matter

X-rays passing through matter may be absorbed, scattered, or transmitted. In photoelectric absorption, a photon transfers its energy to a bound electron, ejecting it from the atom. In Compton scattering, a photon transfers part of its energy to an electron and continues with lower energy and a changed direction. The relative importance of these processes depends on photon energy and material composition. (pub.iaea.org)

Attenuation—the reduction of a beam’s intensity—also depends on thickness, density, and atomic number. For a narrow, monoenergetic beam traversing a uniform material, the Beer–Lambert law gives $I=I_0e^{-\mu x}$, where $\mu$ is the linear attenuation coefficient and $x$ is thickness. Real imaging systems must additionally account for scattered radiation and beams containing multiple photon energies. Differences in attenuation provide the contrast used in transmission images. (pub.iaea.org)

Imaging and analytical applications

In radiology, projection imaging records the radiation transmitted through the body. Bones generally attenuate more X-rays than surrounding soft tissues and appear lighter in conventional image displays. X-ray computed tomography collects measurements from multiple directions and reconstructs cross-sectional images, reducing the overlap of structures present in projection images. Fluoroscopy produces sequences of images for observing movement and guiding procedures. Higher-energy X-rays are also used in radiation therapy, with doses substantially greater than those used for diagnostic imaging. (nibib.nih.gov)

X-ray crystallography uses diffraction patterns to investigate atomic arrangements in crystals. Bragg’s law, $n\lambda=2d\sin\theta$, relates constructive interference to wavelength, crystal-plane spacing, and angle. In X-ray fluorescence analysis, characteristic emitted radiation provides information about elemental composition. X-ray methods can also map the distribution of elements across a specimen without extensive preparation. (educationalgames-staging.nobelprize.org)

Astronomy and biological effects

In astronomy, X-rays reveal energetic environments invisible or inconspicuous at optical wavelengths. Hot gas in disks surrounding black holes can emit detectable X-rays as matter heats during accretion. Because Earth’s atmosphere blocks incoming cosmic X-rays, observations generally require instruments above the atmosphere. (science.nasa.gov)

X-rays can cause ionization and damage living tissue. Radiation exposure can damage DNA and increase cancer risk; biological effects depend on the exposure conditions and dose. This capacity for cellular damage underlies therapeutic applications as well as the radiation-protection requirements associated with diagnostic and research equipment. (nibib.nih.gov)