A rare nuclide produced by nuclear reactions between high-energy cosmic radiation and terrestrial or extraterrestrial material. Cosmogenic nuclides may be used to examine the history of exposure to cosmic rays, and have numerous applications in earth science and archeology.
Primary cosmic radiation is present in space and consists of nuclear particles (mostly protons and alpha particles) with energies that are greater than typical nuclear binding energies. Collisions between cosmic-ray particles and atomic nuclei produce fragments, including cosmogenic nuclides, and also alter the characteristics of the cosmic radiation. As cosmic radiation enters the atmosphere, its flux decreases and its composition becomes dominated by neutrons rather than protons and alpha particles (which react with atmospheric gases). This reduction in flux continues through the atmosphere; at ground level, small but measurable quantities of cosmogenic nuclides are produced in solid material within about 1 m (3.3 ft) of the Earth's surface. The cosmogenic nuclides produced in these two reservoirs (atmosphere and lithosphere) may be employed for examination of different geological processes. Because of their wide range of half-lives and chemical properties, cosmogenic nuclides have a wide range of applications in geological, geomorphological, and biogeochemical studies. See also Alpha particles; Cosmic rays; Cosmochemistry; Proton.
Numerous nuclides are produced in measurable quantities in the atmosphere. Their half-lives and their chemical reactivities determine their applications in earth science.
Short-lived nuclides, including beryllium-7 (7Be), sodium-22 (22Na), phosphorus-32 (32P), and phosphorus-33 (33P), have been employed in studies of atmospheric circulation, particularly stratospheric-tropospheric exchange. Because of its highly successful applications in dating, carbon-14 (14C; radiocarbon) is the best-known cosmogenic nuclide. Essentially all atmospheric 14C is in the form of gaseous carbon dioxide (14CO2); it thus has an atmospheric residence time long enough for the ratio of 14C to 12C (the stable isotope) to become homogeneous throughout the atmosphere. Living organisms and inorganic carbonates incorporate carbon with an isotope ratio reflecting that of the atmosphere. Upon removal from contact with the atmosphere, the ratio 14C/12C decreases through radioactive decay of 14C. With knowledge of the initial ratio and the half-life of 14C, measurement of 14C/12C allows calculation of the age of a sample. This technique has been extensively used in archeology and geology for samples as old as 4 × 104 years. High-precision 14C dates of inorganic carbon dissolved in seawater are also used to deduce global oceanic circulation rates and patterns by determining the time since a particular water mass had contact with the atmosphere. See also Ocean circulation; Radiocarbon dating; Seawater.
A small but significant flux of cosmic rays is present at ground level. This radiation produces cosmogenic nuclides within mineral lattices of exposed rock. In-situ-produced cosmogenic nuclides are used to examine geological problems. In-situ-produced cosmogenic nuclides are useful for dating periods of surface exposure. Their production rates decrease by a factor of 2 for every ≫40 cm (16 in.) depth below an exposed rock surface, so accumulation of cosmogenic nuclides can date geological events that bring material to the Earth's surface. For example, the theoretical evolution of the concentration of 10Be as a function of time for various erosion rates can be calculated. The method has been used to date exposure of rocks brought to the surface by processes including glaciation, volcanic activity, and meteor impact. It has also been used to date the formation of geomorphological features (such as alluvial fans or glacial moraines) deposited over active faults and subsequently offset by fault movement. This provides a quantitative approach to determine slip rates on faults and earthquake recurrence intervals. Such results could not have been obtained by conventional methods. See also Geomorphology.
Because cosmic rays penetrate just a few meters of solid matter, cosmogenic nuclides cannot form deep in the interiors of large bodies such as asteroids. The accumulation of cosmogenic nuclides begins when a collision lifts deep-seated material to the surface or ejects it into space as part of a small body. When a meteorite hits the Earth, the accumulation of cosmogenic nuclides effectively stops. The reason is that the Earth's atmosphere and magnetic field screen out most cosmic rays. The total amounts of the cosmogenic nuclides in a meteorite are related to its exposure age. They also indicate the size of the meteoroid. To get this information, it is necessary to be able to identify cosmogenic nuclides as such and measure their concentrations.
