Decay, Radioactive

In the early days of chemical manipulation, medieval and Renaissance alchemists sought means by which they could transform, or transmute, base metals into gold and silver. In the 20th century, scientists discovered that the transmutation of elements was not an impossible dream. In nature, radioactivity results from the spontaneous decay or disintegration of certain kinds of atoms (with unstable nuclei); these unstable nuclei emit energy (i.e., radiation) as decay particles or electromagnetic waves. Such subatomic changes have occurred naturally throughout Earth's history. Nowadays [Page 281]nuclear physicists artificially alter the composition and behavior of chemical elements for industrial, medical, and military purposes. Because each radioactive isotope decays at a definable rate (known as a half-life), the decay processes of radioactive substances can serve as “clocks” and offer unique opportunities for the measurement of time.

The closing years of the 19th century and the opening decades of the 20th century witnessed remarkable advances in our understanding of matter. Indeed, many famous names from the history of science are associated with early research into atomic structure and the nature of radioactivity (e.g., Becquerel, the Curies, Thomson, Rutherford, Soddy, Chadwick, Bohr). Although some of the terminology has changed over the years, many fundamental concepts related to radioactive decay appear in a brief overview of the early investigations into this phenomenon. Soon after Wilhelm Röntgen's discovery of X-rays, A. H. Becquerel accidentally discovered the phenomenon of natural radioactivity, in 1896, as he conducted experiments on the phosphorescence of uranium salts. As a result, Marie Curie and Pierre Curie dedicated years to the study of radioactivity and discovered the elements polonium and radium through their painstaking analysis of pitchblende (uranium ore); the Curies introduced the term radioactive to describe the emanations of uranium and these other heavy elements, which were much more “active” than uranium. Meanwhile, J. J. Thomson's 1897 discovery of the electron, coupled with these other breakthroughs, negated John Dalton's earlier views on the indivisibility and stability of the atom. (Modern particle physics examines the components, forces, and behavior of a subatomic world that is more intricate than Dalton's early 19th-century model, when chemists knew only 33 elements.)

One of Thomson's most productive students, Ernest Rutherford (through his research and experiments in New Zealand, Cambridge, Montreal, and Manchester), collaborated with other pioneering physicists and examined a number of significant aspects of radioactive decay. As a pivotal participant in the golden age of physics, Rutherford, with his curiosity and tenacity, was nothing short of inspirational. In 1898, he discovered that radioactive atoms emitted at least two distinct types of rays (later designated as particles), which he called alpha and beta. Rutherford and his colleagues used these decay particles (later identified as equivalent to the nucleus of a helium atom [a cluster of two protons and two neutrons] and high-speed electrons, respectively) in much subsequent research on the nature of radioactive materials. He also suggested that a third type of radiation might exist; Paul Villard identified the existence of the gamma-ray (electromagnetic radiation) in 1900. These major types of radiation have different properties (e.g., velocity, reaction to magnetic fields, penetrating power).

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