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X-rays are part of the electromagnetic spectrum, with wavelengths shorter than visible light. Different applications use different parts of the X-ray spectrum.

X-radiation (composed of X-rays) is a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after Wilhelm Röntgen,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> who is usually credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation.<ref name="squires">Novelline, Robert (1997). Squire's Fundamentals of Radiology. Harvard University Press. 5th edition. ISBN 0-674-83339-2.</ref> Spelling of X-ray(s) in the English language includes the variants x-ray(s), xray(s), and X ray(s).<ref></ref>

X-rays with photon energies above 5–10 keV (below 0.2–0.1 nm wavelength) are called hard X-rays, while those with lower energy are called soft X-rays.<ref name="Attwood">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Due to their penetrating ability, hard X-rays are widely used to image the inside of objects, e.g., in medical radiography and airport security. As a result, the term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. Since the wavelengths of hard X-rays are similar to the size of atoms they are also useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air; the attenuation length of 600 eV (~2 nm) X-rays in water is less than 1 micrometer.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

There is no consensus for a definition distinguishing between X-rays and gamma rays. One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus.<ref name="Dendy">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> This definition has several problems: other processes also can generate these high-energy photons, or sometimes the method of generation is not known. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength (or, equivalently, frequency or photon energy), with radiation shorter than some arbitrary wavelength, such as 10−11 m (0.1 Å), defined as gamma radiation.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> This criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. (Some measurement techniques do not distinguish between detected wavelengths.) However, these two definitions often coincide since the electromagnetic radiation emitted by X-ray tubes generally has a longer wavelength and lower photon energy than the radiation emitted by radioactive nuclei.<ref name="Dendy" /> Occasionally, one term or the other is used in specific contexts due to historical precedent, based on measurement (detection) technique, or based on their intended use rather than their wavelength or source. Thus, gamma-rays generated for medical and industrial uses, for example radiotherapy, in the ranges of 6–20 MeV, can in this context also be referred to as X-rays.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}


X-ray sections
Intro  Properties  Interaction with matter  Production  Detectors  Medical uses  Adverse effects  Other uses  History  Visibility  Units of measure and exposure  See also  References  External links  

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