X-Ray

I

Introduction

X-Ray, penetrating electromagnetic radiation, having a shorter wavelength than light, and produced by bombarding a target, usually made of tungsten, with high-speed electrons. X-rays were discovered accidentally in 1895 by the German physicist Wilhelm Conrad Roentgen while he was studying cathode rays in a high-voltage gaseous-discharge tube. Despite the fact that the tube was encased in a black cardboard box, Roentgen noticed that a barium platinocyanide screen, placed nearby by chance, emitted fluorescent light whenever the tube was in operation. After conducting further experiments, he determined that the fluorescence was caused by invisible radiation of a more penetrating nature than ultraviolet radiation (see Luminescence). He named the invisible rays “X-rays” because of their unknown nature. Subsequently, X-rays were known also as Roentgen rays in his honour.

II

Nature of X-Rays

X-rays are electromagnetic radiation ranging in wavelength from about 10 nm to 0.001 nm (1 nm, or nanometre, is 10^-6 mm, or 40 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. The shorter wavelengths, closer to or overlapping the gamma-ray range, are called hard X-rays (see Radioactivity). X-rays forming a mixture of many different wavelengths are 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 from orbit to orbit of electrons in 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 similar to X-rays in their effects, 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. 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, or it can excite X-rays of particular energies, which are dependent on the nature of the target atom.

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. See Ion; Ionization.

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 it 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 (see Particle Accelerators) 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 that light does (see Photographic Techniques). 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. Radiation consisting of neutrons is now used in some types of radiography and produces almost opposite results. Objects that cast dark shadows in an X-ray picture are almost always light in a neutron radiograph.