Cosmogenic nuclide

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.

See also: Environmental radioactivity § Natural

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 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 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.

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. 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.