RADIOACTIVITY

property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei. Radioactivity was first reported in 1896 by the French physicist Henri Becquerel for a double salt of uranium and potassium. Soon thereafter it was found that all uranium compounds and the metal itself were similarly radioactive. Intensity of activity was proportional to the amount of uranium present, chemical combination having no effect. In 1898 the noted French physicists Pierre and Marie Curie discovered two other strongly radioactive elements, radium and polonium, that occur in nature. The early study of the radioactivity of the heavy elements led to revolutionary changes in ideas of the structure of matter. At the beginning of the 20th century the theory that matter consists of atoms was generally accepted by scientists; notions of the inner structure of atoms, however, were entirely speculative. By 1903 research on radioactive processes and radiations led to the realization that atoms are not of necessity permanently stable. The conclusion by 1911 was that nearly all of the mass of the atom is concentrated in a nucleus occupying only a minute portion of the total volume. Next came the important concept of isotopes (1913); and transmutation, the modification of an atomic nucleus, was achieved in a laboratory experiment six years later. Finally, in 1934, it was discovered that radioactivity could be induced in ordinary matter by transmutation in an artificially contrived arrangement. In these first experiments radioactive varieties of nitrogen, aluminum, and phosphorus were identified. Within a few months it had been shown that neutrons (uncharged nuclear particles) could effect transmutation, and the list of newly discovered radioactive isotopes covered the whole range of known elements from hydrogen to uranium. At this time there were indications that radioactive isotopes of transuranium elements (i.e., those of atomic number greater than that of uranium) might be obtained through transmutation, but it was not until 1940 that the first clear identification of such an elementneptuniumwas made. Of the various processes resulting in the production of radioactive species, neutron-induced nuclear fission, achieved in 1939, has been the most fruitful. In 1941 it was learned that fission may also occur spontaneously. In this case, certain unstable nuclei of heavier elements split into nearly equal fragments without the introduction of outside energy. With such discoveries, modern theories of nuclear structure became possible, and the large-scale release of nuclear energy was achieved in 1942. Radioactive substances emit energy in the form of ionizing radiations. Such radiations dissipate their energy in passing through matter by producing ionization and other effects. The radiated energy is either kinetic energy of particles or quantum energy of photons; these are eventually degraded into heat. If the radioactive source is a compact portion of matter, some of the energy of radiations is dissipated in the source itself. The source then tends to maintain a temperature higher than that of its surroundings. The emission is spontaneous, and its rate is uninfluenced by changes of pressure and temperature available to laboratory study. It is, however, not inexhaustible. For each source the rate of emission of energy continually decreases, as measured by its half-life. (Half-life is defined as the period in which the rate of radioactive emission by a pure sample falls by a factor of two.) Among known radioactive isotopes, half-lives range from about 10-7 second to 1016 years. See also nuclear fission. property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei. An unstable nucleus will decompose spontaneously, or decay, into a more stable configuration but will do so only in a few specific ways by emitting certain particles or certain forms of electromagnetic energy. Radioactive decay is a property of several naturally occurring elements as well as of artificially produced isotopes of the elements. The rate at which a radioactive element decays is expressed in terms of its half-life; i.e., the time required for one-half of any given quantity of the isotope to decay. Half-lives range from more than 1,000,000,000 years for some nuclei to less than 10-9 second (see below Rates of radioactive transitions). The product of a radioactive decay processcalled the daughter of the parent isotopemay itself be unstable, in which case it, too, will decay. The process continues until a stable nuclide has been formed. Additional reading Bernard G. Harvey, Introduction to Nuclear Physics and Chemistry, 2nd ed. (1969), an excellent introductory text on nuclear phenomena; Aage Bohr and Ben R. Mottelson, Nuclear Structure, 2 vol. (1969); C. Michael Lederer and Virginia S. Shirley, Table of Isotopes, 7th ed. (1978), a comprehensive table that lists all the known radioactive and stable isotopes and their properties; and Alfred Romer, The Restless Atom: The Awakening of Nuclear Physics (1960, reprinted 1982), a popular account of the discovery of radioactivity and research in that field. Collections of articles and reports are Frederick Soddy, Radioactivity and Atomic Theory (1975); and Alfred Romer (ed.), The Discovery of Radioactivity and Transmutation (1964). Applications of radiation are discussed in International Atomic Energy Agency, Industrial Application of Radioisotopes and Radiation Technology (1982); and Howard J. Glenn (ed.), Biologic Applications of Radiotracers (1982), on the use of small animals in radiotracer research. Ellis P. Steinberg