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.

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.

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( a device use to test the amount of radiation) 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.

X-ray diffraction.

X rays may be diffracted by passage through a CRYSTAL, (q.v.) 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 . X rays may also be diffracted by ruled gratings if the spacings are approximately equal to the wavelengths of the incident X rays.