X-radiation is a form of electromagnetic radiation. Individual packets of this radiation are commonly called X-rays and behave like waves that have a very short wavelength and therefore carry more energy than ultraviolet light. In the overall electromagnetic spectrum, rays with still shorter wavelengths and higher energies are classified as Gamma radiation.
Wavelength, frequency and energy
The properties of X-rays span a broad range. Typical wavelengths lie between about 0.01 and 10 nanometres, which correspond to frequencies on the order of 30 petahertz up to 30 exahertz. Their photon energies are commonly given in electronvolts, roughly from 100 eV up to 100 keV for many laboratory and medical sources.
How X-rays interact with matter
X-rays can penetrate many solid materials, but the degree of transmission depends on the material's composition and thickness. Because of this penetrating power, X-ray images (often called photograms) are widely used in medicine to visualize internal structures such as bones. The term "X-ray" is frequently used for these photographs as well as for the radiation itself.
The visible contrasts on an X-ray image arise from several physical processes, including Rayleigh scattering, Compton scattering and photoabsorption. Dense tissues and materials, being more dense, absorb or deflect X-rays to a greater extent, so bones appear clearly because the rays are either absorbed or scattered. In contrast, soft tissues such as skin and muscle are relatively transparent to diagnostic X-ray energies and therefore show less contrast.
Imaging devices and alternatives
To detect features that are not easily seen on standard X-rays, other scanning technologies are used. For soft-tissue detail a common choice is magnetic resonance imaging. A computed tomography scanner uses multiple X-ray projections and a computer to build a three-dimensional (3D) representation, which can reveal structures that a single X-ray projection cannot.
Production and classification of X-rays
Most X-ray tubes generate radiation by accelerating electrons and striking a metal target; the sudden deceleration produces X-ray photons. These quanta are forms of electromagnetic energy and are often described as photons.
Energy-dependent effects and uses
X-rays are a type of ionizing radiation, which means they can remove electrons from atoms and molecules. The interactions they produce depend strongly on photon energy. Lower-energy ("soft") X-rays tend to cause the photoelectric effect, intermediate energies commonly undergo Compton scattering, and very high energies can lead to pair production. X-rays used for standard radiography are generally low to medium in energy. In contrast, radiation therapy for treating cancer uses higher-energy beams that exploit Compton scattering and, in some cases, other high-energy processes to damage tumour cells.
Background exposure and biological risks
Small amounts of X-rays occur naturally in the environment and, like other forms of ionizing radiation, they can alter biological molecules. Prolonged or high-dose exposure increases the risk of developing cancer. At the same time, because rapidly dividing tumour cells are often more sensitive to ionizing damage than healthy tissue, controlled doses of X-rays are deliberately used in radiation therapy to destroy malignant cells.