Cosmogenic nuclides (or cosmogenic isotopes) are rare nuclides (isotopes) created when a high-energy cosmic ray interacts with the nucleus of an in situ Solar System atom, causing nucleons (protons and neutrons) to be expelled from the atom (see cosmic ray spallation). These nuclides are produced within Earth materials such as rocks or soil, in Earth's atmosphere, and in extraterrestrial items such as meteorites. By measuring cosmogenic nuclides, scientists are able to gain insight into a range of geological and astronomical processes. There are both radioactive and stable cosmogenic nuclides. Some of these radionuclides 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 been created 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. This also explains the overabundance of the early transition metals just before iron in the periodic table; the cosmic-ray spallation of iron thus produces scandium through chromium on one hand and helium through boron on the other.[1] 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[citation needed] 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[1] 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[citation needed] 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[citation needed] 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[citation needed] of beryllium-7 and beryllium-10 occurrence naturally in the environment, they are therefore cosmogenic.

Cosmogenic nuclides

Here is a list of radioisotopes formed by the action of cosmic rays; the list also contains the production mode of the isotope.[2] Most cosmogenic nuclides are formed in the atmosphere, but some are formed in situ in soil and rock exposed to cosmic rays, notably calcium-41 in the table below.

Isotopes formed by the action of cosmic rays
Isotope Mode of formation half life
3H (tritium) 14N(n,12C)T 12.3 y
7Be Spallation (N and O) 53.2 d
10Be Spallation (N and O) 1,387,000 y
12B Spallation (N and O)
11C Spallation (N and O) 20.3 min
14C 14N(n,p)14C 5,730 y
18F 18O(p,n)18F and Spallation (Ar) 110 min
22Na Spallation (Ar) 2.6 y
24Na Spallation (Ar) 15 h
27Mg Spallation (Ar)
28Mg Spallation (Ar) 20.9 h
26Al Spallation (Ar) 717,000 y
31Si Spallation (Ar) 157 min
32Si Spallation (Ar) 153 y
32P Spallation (Ar) 14.3 d
34mCl Spallation (Ar) 34 min
35S Spallation (Ar) 87.5 d
36Cl 35Cl (n,γ)36Cl 301,000 y
37Ar 37Cl (p,n)37Ar 35 d
38Cl Spallation (Ar) 37 min
39Ar 40Ar (n,2n)39Ar 269 y
39Cl 40Ar (n,np)39Cl & spallation (Ar) 56 min
41Ar 40Ar (n,γ)41Ar 110 min
41Ca 40Ca (n,γ)41Ca 102,000 y
45Ca Spallation (Fe)
47Ca Spallation (Fe)
44Sc Spallation (Fe)
46Sc Spallation (Fe)
47Sc Spallation (Fe)
48Sc Spallation (Fe)
44Ti Spallation (Fe)
45Ti Spallation (Fe)
81Kr 80Kr (n,γ) 81Kr 229,000 y
95Tc 95Mo (p,n) 95Tc
96Tc 96Mo (p,n) 96Tc
97Tc 97Mo (p,n) 97Tc
97mTc 97Mo (p,n) 97mTc
98Tc 98Mo (p,n) 98Tc
99Tc Spallation (Xe)
129I Spallation (Xe) 15,700,000 y
182Yb Spallation (Pb)
182Lu Spallation (Pb)
183Lu Spallation (Pb)
182Hf Spallation (Pb)
183Hf Spallation (Pb)
184Hf Spallation (Pb)
185Hf Spallation (Pb)
186Hf Spallation (Pb)
185W Spallation (Pb)
187W Spallation (Pb)
188W Spallation (Pb)
189W Spallation (Pb)
190W Spallation (Pb)
188Re Spallation (Pb)
189Re Spallation (Pb)
190Re Spallation (Pb)
191Re Spallation (Pb)
192Re Spallation (Pb)
191Os Spallation (Pb)
193Os Spallation (Pb)
194Os Spallation (Pb)
195Os Spallation (Pb)
196Os Spallation (Pb)
192Ir Spallation (Pb)
194Ir Spallation (Pb)
195Ir Spallation (Pb)
196Ir Spallation (Pb)

Applications in geology listed by isotope

Commonly measured long lived cosmogenic isotopes
element mass half-life (years) typical application
beryllium 10 1,387,000 exposure dating of rocks, soils, ice cores
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 15,700,000 groundwater tracer
carbon 14 5730 radiocarbon dating
sulfur 35 0.24 water residence times
sodium 22 2.6 water residence times
tritium 3 12.32 water residence times
argon 39 269 groundwater tracer
krypton 81 229,000 groundwater tracer

References

  1. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 13–15. ISBN 978-0-08-037941-8.
  2. ^ SCOPE 50 - Radioecology after Chernobyl Archived 2014-05-13 at the Wayback Machine, the Scientific Committee on Problems of the Environment (SCOPE), 1993. See table 1.9 in Section 1.4.5.2.