- General considerations
- Forms of electromagnetic radiation
- Historical survey
The German physicist Wilhelm Conrad Röntgen discovered X rays in 1895 by accident while studying cathode rays in a low-pressure gas discharge tube. (A few years later J.J. Thomson of England showed that cathode rays were electrons emitted from the negative electrode [cathode] of the discharge tube.) Röntgen noticed the fluorescence of a barium platinocyanide screen that happened to lie near the discharge tube. He traced the source of the hitherto undetected form of radiation to the point where the cathode rays hit the wall of the discharge tube, and mistakenly concluded from his inability to observe reflection or refraction that his new rays were unrelated to light. Because of his uncertainty about their nature, he called them X-radiation. This early failure can be attributed to the very short wavelengths of X rays (10-8 to 10-11 centimetre), which correspond to photon energies from 200 to 100,000 eV. In 1912 another German physicist, Max von Laue, realized that the regular arrangement of atoms in crystals should provide a natural grating of the right spacing (about 10-8 centimetre) to produce an interference pattern on a photographic plate when X rays pass through such a crystal. The success of this experiment, carried out by Walter Friedrich and Paul Knipping, not only identified X rays with electromagnetic radiation but also initiated the use of X rays for studying the detailed atomic structure of crystals. The interference of X rays diffracted in certain directions from crystals in so-called X-ray diffractometers, in turn, permits the dissection of X-radiation into its different frequencies, just as a prism diffracts and spreads the various colours of light. The spectral composition and characteristic frequencies of X rays emitted by a given X-ray source can thus be measured. As in optical spectroscopy, the X-ray photons emitted correspond to the differences of the internal electronic energies in atoms and molecules. Because of their much higher energies, however, X-ray photons are associated with the inner-shell electrons close to the atomic nuclei, whereas optical absorption and emission are related to the outermost electrons in atoms or in materials in general. Since the outer electrons are used for chemical bonding while the energies of inner-shell electrons remain essentially unaffected by atomic bonding, the identity and quantity of elements that make up a material are more accurately determined by the emission, absorption, or fluorescence of X rays than of photons of visible or ultraviolet light.
The contrast between body parts in medical X-ray photographs (radiographs) is produced by the different scattering and absorption of X rays by bones and tissues. Within months of Röntgen’s discovery of X rays and his first X-ray photograph of his wife’s hand, this form of electromagnetic radiation became indispensable in orthopedic and dental medicine. The use of X rays for obtaining images of the body’s interior has undergone considerable development over the years and has culminated in the highly sophisticated procedure known as computerized axial tomography (CAT; see radiation).
Notwithstanding their usefulness in medical diagnosis, the ability of X rays to ionize atoms and molecules and their penetrating power make them a potential health hazard. Exposure of body cells and tissue to large doses of such ionizing radiation can result in abnormalities in DNA that may lead to cancer and birth defects. (For a detailed treatment of the effects of X rays and other forms of ionizing radiation on human health and the levels of such radiation encountered in daily life, see radiation: biologic effects of ionizing radiation.)
X rays are produced in X-ray tubes by the deceleration of energetic electrons (bremsstrahlung) as they hit a metal target or by accelerating electrons moving at relativistic velocities in circular orbits (synchrotron radiation; see above Continuous spectra of electromagnetic radiation). They are detected by their photochemical action in photographic emulsions or by their ability to ionize gas atoms: every X-ray photon produces a burst of electrons and ions, resulting in a current pulse. By counting the rate of such current pulses per second, the intensity of a flux of X rays can be measured. Instruments used for this purpose are called Geiger counters.
X-ray astronomy has revealed very strong sources of X rays in deep space. In the Milky Way Galaxy, of which the solar system is a part, the most intense sources are certain double star systems in which one of the two stars is thought to be either a compact neutron star or a black hole. The ionized gas of the circling companion star falls by gravitation into the compact star, generating X rays that may be more than 1,000 times as intense as the total amount of light emitted by the Sun. At the moment of their explosion, supernovae emit a good fraction of their energy in a burst of X rays.