II.

Nature of X Rays

X rays are electromagnetic radiation ranging in wavelength from about 100 A to 0.01 A (1 A is equivalent to about 10^-8 cm/about 4 billionths of an in.; see Wave Motion). The shorter the wavelength of the X ray, the greater is its energy and its penetrating power. Longer wavelengths, near the ultraviolet-ray band of the electromagnetic spectrum, are known as soft X rays (see Spectrum). The shorter wavelengths, closer to and overlapping the gamma-ray range, are called hard X rays (see Radioactivity). A mixture of many different wavelengths is known as “white” X rays, as opposed to “monochromatic” X rays, which represent only a single wavelength. Both light and X rays are produced by transitions of electrons that orbit atoms, light by the transitions of outer electrons and X rays by the transitions of inner electrons. In the case of bremsstrahlung radiation (see below), X rays are produced by the retardation or deflection of free electrons passing through a strong electrical field. Gamma rays, which are identical to X rays in their effect, are produced by energy transitions within excited nuclei. See Atom.

X rays are produced whenever high-velocity electrons strike a material object. Much of the energy of the electrons is lost in heat; the remainder produces X rays by causing changes in the target's atoms as a result of the impact. The X rays emitted can have no more energy than the kinetic energy of the electrons that produce them (see Energy). Moreover, the emitted radiation is not monochromatic but is composed of a wide range of wavelengths with a sharp, lower wavelength limit corresponding to the maximum energy of the bombarding electrons. This continuous spectrum is referred to by the German name bremsstrahlung, which means “braking,” or slowing down, radiation, and is independent of the nature of the target. If the emitted X rays are passed through an X-ray spectrometer, certain distinct lines are found superimposed on the continuous spectrum; these lines, known as the characteristic X rays, represent wavelengths that depend only on the structure of the target atoms. In other words, a fast-moving electron striking the target can do two things: It can excite X rays of any energy up to its own energy; or it can excite X rays of particular energies, dependent on the nature of the target atom.