X Ray

A

Fluorescence

X rays also cause fluorescence in certain materials, such as barium platinocyanide and zinc sulfide. If a screen coated with such fluorescent material is substituted for the photographic films, the structure of opaque objects may be observed directly. This technique is known as fluoroscopy. See Fluoroscope.

B

Ionization

Another important characteristic of X rays is their ionizing power, which depends upon their wavelength. The capacity of monochromatic X rays to ionize is directly proportional to their energy. This property provides a method for measuring the energy of X rays. When X rays are passed through an ionization chamber (see Particle Detectors), an electric current is produced that is proportional to the energy of the incident beam. In addition to ionization chambers, more sensitive devices, such as the Geiger-Müller counter and the scintillation counter, can measure the energy of X rays on the basis of ionization. In addition, the path of X rays, by virtue of their capacity to ionize, can be made visible in a cloud chamber.

C

X-Ray Diffraction

X rays may be diffracted by passage through a crystal or by reflection (scattering) from a crystal, which consists of regular lattices of atoms that serve as fine diffraction gratings (see Diffraction; Diffraction Grating). The resulting interference patterns may be photographed and analyzed to determine the wavelength of the incident X rays or the spacings between the crystal atoms, whichever is the unknown factor (see Interference). X rays may also be diffracted by ruled gratings if the spacings are approximately equal to the wavelengths of the incident X rays.

V

Interaction with Matter

In the interaction between matter and X rays, three mechanisms exist by which X rays are absorbed; all three mechanisms demonstrate the quantum nature of X radiation. See Quantum Theory.

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A

Photoelectric Effect

When a quantum of radiation, or a photon, in the X-ray portion of the electromagnetic spectrum strikes an atom, it may impinge on an electron within an inner shell and eject it from the atom. If the photon carries more energy than is necessary to eject the electron, it will transfer its residual energy to the ejected electron in the form of kinetic energy. This phenomenon, called the photoelectric effect, occurs primarily in the absorption of low-energy X rays. See Photoelectric Cell; Photoelectric Effect.