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X-RAY CRYSTALLOGRAPHY

Study of crystalline structures using diffraction of X-rays. The Debye and Scherrer method uses diffraction of monochromatic rays from a powdered crystalline sample giving a range of angles of incidence with respect to crystallographic axes. The arrangement of atoms in space can be inferred from the distribution of intensity in the diffraction pattern. The unit cell determines the geometry of diffraction maxima while the distribution of atoms within the unit cell controls the intensity of each reflection. The Line method examines polychromatic diffraction from a single crystal. See Bragg equation

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X-RAY FLUORESCENCE SPECTROMETRY

A method of chemical analysis in which the sample is bombarded by very hard X-rays or gamma-rays, and secondary radiations, characteristic of the elements present, are studied spectro

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X-RAY FOCAL SPOT

That small area of the target anode of an X-ray tube on which the electron beam is incident, and from which emitted X-rays emerge. High- power tubes frequently have a line focus to minimize localization of the heat dissipated at the anode.

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X-RAY LASER

A laser with an output in the X-ray region of the spectrum. Using highly ionized selenium plasma as the lasing medium, laser output at 20-6 and 20-9 nm has been recently reported.

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X-RAY MICROSCOPE

Microscope using soft X-rays and a Fresnel zone plate as the focusing device; a resolution of 200 nm has been obtained using a scanning technique. The zone plate is made using a scanning transmission electron microscope, and synchrotron radiation is used as the source of X-rays. See panel on Electron microscope.

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X-RAY PHOTON

A quantum of X-ray energy given by hv, where v is the frequency and h is Planck's constant.

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X-RAY RADIOGRAPHY

The use of photographic film, often with an image intensifier screen, to record the intensities of X-rays which have passed through tissue in medicine or eg some mechanical structure in engineering. The X-rays are emitted by essentially a point source and the image is the shadow formed by the specimen.

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X-RAY SOURCE

One of several sources of cosmic X-rays: 1 the solar corona; 2 interacting binary stars, in which one member is a black hole or neutron star: 3 supernova remnants, such as the Crab Nebula; 4 some radio galaxies, such as Cygnus A. some Seyfert galaxies and some quasars. See panels on Black hole and Quasar.

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X-RAY SPECTROMETER

The name originally used for the X-ray diffractometer, but now abandoned in order to avoid confusion with X-ray fluorescent spectrometry/.

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X-RAY TELESCOPE

An instrument deployed at high altitude by balloon, rocket or spacecraft to detect and record cosmic X-rays from space. To focus the X-rays by mirrors, grazing incidence techniques are used. X-ray detectors include gas counters and scintillation counters. See panel on Astronomical telescope

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