X ray

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X ray,

invisible, highly penetrating electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
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 of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10−8 m to about 10−11 m, or from less than a billionth of an inch to less than a trillionth of an inch; the corresponding frequency range is from about 3 × 1016 Hz to about 3 × 1019 Hz (1 Hz = 1 cps).

Production of X Rays

An important source of X rays is synchrotron radiationsynchrotron radiation,
in physics, electromagnetic radiation emitted by high-speed electrons spiraling along the lines of force of a magnetic field (see magnetism). Depending on the electron's energy and the strength of the magnetic field, the maximum intensity will occur as
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. X rays are also produced in a highly evacuated glass bulb, called an X-ray tube, that contains essentially two electrodes—an anode made of platinum, tungsten, or another heavy metal of high melting point, and a cathode. When a high voltage is applied between the electrodes, streams of electrons (cathode rays) are accelerated from the cathode to the anode and produce X rays as they strike the anode.

Two different processes give rise to radiation of X-ray frequency. In one process radiation is emitted by the high-speed electrons themselves as they are slowed or even stopped in passing near the positively charged nuclei of the anode material. This radiation is often called brehmsstrahlung [Ger.,=braking radiation]. In a second process radiation is emitted by the electrons of the anode atoms when incoming electrons from the cathode knock electrons near the nuclei out of orbit and they are replaced by other electrons from outer orbits. The spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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 of frequencies given off with any particular anode material thus consists of a continuous range of frequencies emitted in the first process, and superimposed on it a number of sharp peaks of intensity corresponding to discrete frequencies at which X rays are emitted in the second process. The sharp peaks constitute the X-ray line spectrum for the anode material and will differ for different materials.

Applications of X Rays

Most applications of X rays are based on their ability to pass through matter. This ability varies with different substances; e.g., wood and flesh are easily penetrated, but denser substances such as lead and bone are more opaque. The penetrating power of X rays also depends on their energy. The more penetrating X rays, known as hard X rays, are of higher frequency and are thus more energetic, while the less penetrating X rays, called soft X rays, have lower energies. X rays that have passed through a body provide a visual image of its interior structure when they strike a photographic plate or a fluorescent screen; the darkness of the shadows produced on the plate or screen depends on the relative opacity of different parts of the body.

Photographs made with X rays are known as radiographs or skiagraphs. Radiography has applications in both medicine and industry, where it is valuable for diagnosis and nondestructive testing of products for defects. Fluoroscopy is based on the same techniques, with the photographic plate replaced by a fluorescent screen (see fluorescencefluorescence
, luminescence in which light of a visible color is emitted from a substance under stimulation or excitation by light or other forms of electromagnetic radiation or by certain other means.
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; fluoroscopefluoroscope
, instrument consisting of an X-ray machine (see X ray) and a fluorescent screen that may be used by physicians to view the internal organs of the body. During medical diagnosis the patient stands between the X-ray machine, or other radiation source, and the
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); its advantages over radiography in time and cost are balanced by some loss in sharpness of the image. X rays are also used with computers in CAT (computerized axial tomography) scans to produce cross-sectional images of the inside of the body.

Another use of radiography is in the examination and analysis of paintings, where studies can reveal such details as the age of a painting and underlying brushstroke techniques that help to identify or verify the artist. X rays are used in several techniques that can provide enlarged images of the structure of opaque objects. These techniques, collectively referred to as X-ray microscopy or microradiography, can also be used in the quantitative analysis of many materials. One of the dangers in the use of X rays is that they can destroy living tissue and can cause severe skin burns on human flesh exposed for too long a time. This destructive power is used in X-ray therapy to destroy diseased cells.

Discovery and Early Scientific Use

X rays were discovered in 1895 by W. C. Roentgen, who called them X rays because their nature was at first unknown; they are sometimes also called Roentgen, or Röntgen, rays. X-ray line spectra were used by H. G. J. Moseley in his important work on atomic numbers (1913) and also provided further confirmation of the quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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 of atomic structure. Also important historically is the discovery of X-ray diffractiondiffraction,
bending of waves around the edge of an obstacle. When light strikes an opaque body, for instance, a shadow forms on the side of the body that is shielded from the light source.
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 by Max von Laue (1912) and its subsequent application by W. H. and W. L. Bragg to the study of crystal structure.


See D. Graham and T. Eddie, X-ray Techniques in Art Galleries and Museums (1985); B. H. Kevles, Naked to the Bone: Medical Imaging in the Twentieth Century (1997).

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