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Cosmogenic nuclide

Cosmogenic nuclides (or cosmogenic isotopes) are rare isotopes created when a high-energy cosmic ray interacts with the nucleus of an in situ solar system atom, causing cosmic ray spallation. These isotopes are produced within earth materials such as rocks or soil, in Earth's atmosphere, and in extraterrestrial items such as meteorites. By measuring cosmogenic isotopes, scientists are able to gain insight into a range of geological and astronomical processes. There are both radioactive and stable cosmogenic isotopes. Some of these radioisotopes are tritium, carbon-14 and phosphorus-32.

Certain light (low atomic number) primordial nuclides (some isotopes of lithium, beryllium and boron) are thought to have arisen not only during the Big Bang, and also (and perhaps primarily) to have been made after the Big Bang, but before the condensation of the solar system, by the process of cosmic ray spallation on interstellar gas and dust. This explains their higher abundance in cosmic rays as compared with their ratios and abundances of certain other nuclides on Earth. However, the arbitrary defining qualification for cosmogenic nuclides of being formed "in situ in the solar system" (meaning inside an already-aggregated piece of the solar system) prevents primordial nuclides formed by cosmic ray spallation before the formation of the solar system, from being termed "cosmogenic nuclides"— even though the mechanism for their formation is exactly the same. These same nuclides still arrive on Earth in small amounts in cosmic rays, and are formed in meteoroids, in the atmosphere, on Earth, "cosmogenically." However, beryllium (all of it stable beryllium-9) is present primordially in the solar system in much larger amounts, having existed prior to the condensation of the solar system, and thus present in the materials from which the solar system formed.

To make the distinction in another fashion, the timing of their formation determines which subset of cosmic ray spallation-produced nuclides are termed primordial or cosmogenic (a nuclide cannot belong to both classes). By convention, certain stable nuclides of lithium, beryllium, and boron are thought to have been produced by cosmic ray spallation in the period of time between the Big Bang and the solar system's formation (thus making these primordial nuclides, by definition) are not termed "cosmogenic," even though they were formed by the same process as the cosmogenic nuclides (although at an earlier time). The primordial nuclide beryllium-9, the only stable beryllium isotope, is an example of this type of nuclide.

In contrast, even though the radioactive isotopes beryllium-7 and beryllium-10 fall into this series of three light elements (lithium, beryllium, boron) formed mostly by cosmic ray spallation nucleosynthesis, both of these nuclides have half lives too short for them to have been formed before the formation of the solar system, and thus they cannot be primordial nuclides. Since the cosmic ray spallation route is the only possible source of beryllium-7 and beryllium-10 occurrence naturally in the environment, they are therefore cosmogenic.

Production modes

Here is a list of radioisotopes formed by the action of cosmic rays on the atmosphere; the list also contains the production mode of the isotope.[1]

Isotopes formed by the action of cosmic rays on the air
Isotope Mode of formation
³H (tritium) 14N (n, 12C)³H
7Be Spallation (N and O)
10Be Spallation (N and O)
11C Spallation (N and O)
14C 14N (n, p) 14C
18F 18O (p, n)18F and Spallation (Ar)
22Na Spallation (Ar)
24Na Spallation (Ar)
28Mg Spallation (Ar)
31Si Spallation (Ar)
32Si Spallation (Ar)
32P Spallation (Ar)
34mCl Spallation (Ar)
35S Spallation (Ar)
36Cl 35Cl (n, γ)36Cl
37Ar 37Cl (p, n)37Ar
38Cl Spallation (Ar)
39Ar 38Ar (n, γ)39Ar
39Cl 40Ar (n, np)39Cl & spallation (Ar)
41Ar 40Ar (n, γ)41Ar
81Kr 80Kr (n, γ) 81Kr

Some cosmogenic nuclides are formed in situ in soil and rock exposed to cosmic rays. Additional nuclides not listed above include:

Applications in geology listed by isotope

Commonly measured long lived cosmogenic isotopes
element mass half-life (years) typical application
aluminium 26 720,000 exposure dating of rocks, sediment
chlorine 36 308,000 exposure dating of rocks, groundwater tracer
calcium 41 103,000 exposure dating of carbonate rocks
iodine 129 15.7 million groundwater tracer

References

  1. ^ SCOPE 50 - Radioecology after Chernobyl, the Scientific Committee on Problems of the Environment (SCOPE), 1993. See table 1.9 in Section 1.4.5.2.
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