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 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 Dalíton’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 Dalíton’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).
After discovering radon, a radioactive gas, Rutherford worked with Frederick Soddy and demonstrated that certain heavy radioactive atoms seek stability through disintegration (subatomic change). Rutherford, Soddy, and Otto Hahn conducted research on thorium and other elements that demonstrated their change from one form to another. This research into so-called decay chains resulted in a general, initially shocking, understanding of radioactive disintegration, namely, that elements are not immutable and that parent atoms of certain elements decay to daughter (and granddaughter) products through the loss of particles. For example, the radioactive decay series that begins with unstable uranium-238 ends with lead-206, a stable nuclide. In his study of the transformations that took place in these disintegration series, Soddy referred to atoms that had the same atomic number but different atomic masses as isotopes (also called nuclides nowadays).
Along with Hans Geiger and Ernest Marsden, Rutherford helped create a more accurate model of the atom, as their experiments directed alpha- and beta particles toward sheets of metal foil and observed patterns of divergence and deflection (as these particles reacted to an atom’s positively charged nucleus). With Geiger, Rutherford developed an instrument to detect radioactive particles, a noteworthy advance in a field that depends on sensitive instrumentation. He also had a hand in the development of C. T. R. Wilson’s cloud chamber, a relatively simple but effective detector that played a major role in the analysis of charged particles, including cosmic rays. (Physicists continue to develop hardware by which they can probe the subatomic cosmos, and much of this research focuses on high-energy particles and radioactive isotopes, e.g., reactors, accelerators, and cyclotrons.) Rutherford also used the versatile alpha particles to bombard light elements and brought about the first artificial transmutation of one element to another (the alchemist’s dream) by disintegrating nitrogen nuclei and changing them into oxygen (“playing with marbles,” as he described it). As representatives from a slightly later period of research on radioactive decay, Frederic Joliot- Curie and Irene Joliot-Curie (the daughter of Madame Curie) worked on natural and artificial radioactivity and the transmutation of metals. Later, the Manhattan Project and the subsequent worldwide development of nuclear weapons generated a vast amount of research on fission and radioactivity, research that continues today for nonnuclear scientific and applied purposes.
Every radioactive element has a known rate of decay, or half-life, which is defined as the amount of time required for half of a sample’s radioactive atoms to undergo decay. The half-life of some elements is extremely short while the half-life of other elements is extremely long, from a fraction of a second to several billion years. Isotopes whose half-lives are brief are quite radioactive, but elements with longer half-lives cause contamination for a longer period of time. Whereas the decay is random and spontaneous, the rate of decay (or activity) for a sample that contains a large number of the same atoms is predictable. As a result of their investigation of the radioactive decay of thorium, Rutherford and Soddy developed the exponential equation and the so-called decay constant used to calculate this decay rate. The international standard for measuring this disintegration or decay is the becquerel (= 1 disintegration per second).
Various nuclides are useful in radiometric dating, and, as discussed elsewhere in this encyclopedia, different techniques are applied to date archaeological remains and geological samples, from Earth and extraterrestrial sources. There are two kinds of radiometric dating techniques:
- those based on the known parent-to-daughter decay rate of radioactive isotopes (e.g., radiocarbon [limited to organic materials], uraniumthorium, potassium-argon, uranium-lead), and
- those based on the measurement of damage caused by the decay of certain radioactive elements (e.g., thermoluminescence, electron spin resonance, fission track). Of course, these methods provide approximate dates that include a range of error. Results from several methods of radiometric dating, applied to numerous samples (meteorites and lunar rocks), suggest that Earth and our solar system are some 4 1/2 billion years old.
Gerald L. Mattingly
See also Clocks, Atomic; Dating Techniques; Geologic Timescale; K-T Boundary
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