X-Ray
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III. X-Ray Production


The first X-ray tube was the Crookes tube, a partially evacuated glass bulb containing two electrodes, named after its designer, the British chemist and physicist Sir William Crookes. When an electric current passes through such a tube, the residual gas is ionized and positive ions, striking the cathode, eject electrons from it. These electrons, in the form of a beam of cathode rays, bombard the glass walls of the tube and produce X rays. Such tubes produce only soft X rays of low energy.

An early improvement in the X-ray tube was the introduction of a curved cathode to focus the beam of electrons on a heavy-metal target, called the anticathode, or anode. This type generates harder rays of shorter wavelengths and of greater energy than those produced by the original Crookes tube, but the operation of such tubes is erratic because the X-ray production depends on the gas pressure within the tube.

The next great improvement was made in 1913 by the American physicist William David Coolidge. The Coolidge tube is highly evacuated and contains a heated filament and a target. It is essentially a thermionic vacuum tube in which the cathode emits electrons because the cathode is heated by an auxiliary current and not because it is struck by ions as in the earlier types of tubes. The electrons emitted from the heated cathode are accelerated by the application of a high voltage across the tube. As the voltage is increased, the minimum wavelength of the radiation decreases.

Most of the X-ray tubes in present-day use are modified Coolidge tubes. The larger and more powerful tubes have water-cooled anticathodes to prevent melting under the impact of the electron bombardment. The widely used shockproof tube is a modification of the Coolidge tube with improved insulation of the envelope (by oil) and grounded power cables. Such devices as the betatron are used to produce extremely hard X rays, of shorter wavelength than the gamma rays emitted by naturally radioactive elements.

IV. Properties of X Rays


X rays affect a photographic emulsion in the same way light does . Absorption of X radiation by any substance depends upon its density and atomic weight. The lower the atomic weight of the material, the more transparent it is to X rays of given wavelengths. When the human body is X-rayed, the bones, which are composed of elements of higher atomic weight than the surrounding flesh, absorb the radiation more effectively and therefore cast darker shadows on a photographic plate. Another type of radiation, which is known as neutron radiation and is now used in some types of radiography, produces almost opposite results. Objects that cast dark shadows in an X-ray picture are almost always light in a neutron radiograph.


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.


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 , 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 . 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.