Radioactive isotopes are nuclides (isotope-specific atoms) that have unstable nuclei that decay, emitting alpha, beta, and sometimes gamma rays. Such isotopes eventually reach stability in the form of nonradioactive isotopes of other chemical elements, their "radiogenic daughters." Decay of a radionuclide to a stable radiogenic daughter is a function of time measured in units of half-lives.1) alpha (a) decay results from an excess of mass. In this type of decay, alpha particles (consisting of two protons and two neutrons) are emitted from the nucleus.
Both the atomic number and neutron number of the daughter are reduced by two, so the mass number decreases by four. An example is the decay of 238U:2) ß+ - or "positron decay" results from an excess of protons. In this type of decay, a positively charged beta particle and a neutrino are emitted from the nucleus. The atomic number decreases by one and the neutron number is increased by one. An example is the decay of radioactive 18F to stable 18O:where ß+ is the positron, v is the neutrino, and Q is the total energy given off.3) ß- - or "negatron decay" results from an excess of neutrons.
In this type of decay, a negatively charged beta particle and a neutrino are emitted from the nucleus. The atomic number increases by one and the neutron number is reduced by one. An example is the decay of radioactive 14C to stable 14N:where ß- is the beta particle, v is the antineutrino, and Q is the end point energy (0.156 MeV).4) electron capture also results from an excess of protons. In this type of decay, an electron is spontaneously incorporated into the nucleus and a neutrino is emitted from the nucleus. The atomic number decreases by one and the neutron number increases by one. Electron capture may be followed by the emission of a gamma ray.
An example is the decay of 123I to 123Te:Types of radioactive isotopes by origin (return to top)1) Long-lived radioactive nuclidesSome radioactive nuclides that have very long half lives were created during the formation of the solar system (~4.6 billion years ago) and are still present in the earth. These include 40K (t½ = 1.28 billion years), 87Rb (t½ = 48.8 billion years), 238U (t½ = 447 billion years), and 186Os (t½ = 2 x 106 billion years, or 2 million billion years).2) CosmogenicCosmogenic isotopes are a result of cosmic ray activity in the atmosphere. Cosmic rays are atomic particles that are ejected from stars at a rate of speed sufficient to shatter other atoms when they collide.
This process of transformation is called spallation. Some of the resulting fragments produced are unstable atoms having a different atomic structure (and atomic number), and so are isotopes of another element. The resulting atoms are considered to have cosmogenic radioactivity. Cosmogenic isotopes are also produced at the surface of the earth by direct cosmic ray irradiation of atoms in solid geologic materials.Examples of cosmogenic nuclides include 14C, 36Cl, 3H, 32Si, and 10Be. Cosmogenic nuclides, since they are produced in the atmosphere or on the surface of the earth and have relatively short half-lives (10 to 30,000 years), are often used for age dating of waters.3) AnthropogenicAnthropogenic isotopes result from human activities, such as the processing of nuclear fuels, reactor accidents, and nuclear weapons testing.
Such testing in the 1950s and 1960s greatly increased the amounts of tritium (3H) and 14C in the atmosphere; tracking these isotopes in the deep ocean, for instance, allows oceanographers to study ocean flow, currents, and rates of sedimentation. Likewise, in hydrology it allows for the tracking of recent groundwater recharge and flow rates in the vadose zone. Examples of hydrologically useful anthropogenic isotopes include many of the cosmogenic isotopes mentioned above: 3H, 14C, 36Cl, and 85Kr.4) RadiogenicRadiogenic isotopes are typically stable daughter isotopes produced from radioactive decay. In the geosciences, radiogenic isotopes help to determine the nature and timing of geological events and processes.
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Isotopic systems useful in this research are primarily K-Ar, Rb-Sr, Re-Os, Sm-Nd, U-Th-Pb, and the noble gases (4H, 3H-3He, 40Ar).Because of their stable evolution in groundwater, such naturally occurring isotopes are useful hydrologic tracers, allowing evaluation of large geographic areas to determine flowpaths and flow rates. Consequently, they are helpful in building models that predict fracturing, aquifer thickness, and other subterranean features.