There is a broad spectrum (approximately 1700) of radioactive isotopes (or radionuclides) that are useful tools for measuring rates of processes on Earth. The term nuclide is commonly used interchangeably with atom. The major sources of radionuclides are: (1) primordial (e.g., 238U, 235U, and 234Th-series radionuclides); (2) anthropogenic or transient (e.g., 137Cs, 90Sr, 239Pu); and (3) cosmogenic (e.g., 7Be, 14C, 32P). These isotopes can be further divided into two general groups, the particle-reactive and non-particle-reactive radionuclides. Transport pathways of non-particle-reactive radionuclides in aquatic systems are more simplistic and primarily controlled by water masses. Conversely, particle-reactive radionuclides adsorb onto particles, making their fate inextricably linked with the particle. Consequently, these particle-bound radionuclides are very useful in determining sedimentation and mixing rates, as well as the overall fate of important elements in estuarine and coastal biogeochemical cycles. Radioactivity is defined as the spontaneous adjustment of nuclei of unstable nuclides to a more stable state. Radiation (e.g., alpha, beta, and gamma rays) is released in different forms as a direct result of changes in the nuclei of these nuclides. The general composition of an atom can simply be divided into the atomic number, which is the number of protons (Z) in a nucleus. The mass number (A) is the number of neutrons (N) plus protons in a nucleus (A = Z + N). Isotopes are different forms of an element that have the same Z value but a different N. Instability in nuclei is generally caused by having an inappropriate number of neutrons relative to the number of protons. Some of the pathways by which a nucleus can spontaneously transform are as follows: (1) alpha decay, or loss of an alpha particle (nucleus of a 4He atom) from the nucleus, which results in a decrease in the atomic number by two (two protons) and the mass number by four units (two protons and two neutrons); (2) beta (negatron) decay, which occurs when a neutron changes to a proton and a negatron (negatively charged electron) is emitted, thereby increasing the atomic number by one unit; (3) emission of a positron (positively charged electron) which results in a proton becoming a neutron and a decrease in the atomic number by one unit; and (4) electron capture, where a proton is changed to a neutron after combining with the captured extranuclear electron (from the K shell)—the atomic number is decreased by one unit.
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