Cosmogenic nuclide


Cosmogenic nuclides are rare nuclides created when a high-energy cosmic ray interacts with the nucleus of an in situ Solar System atom, causing nucleons to be expelled from the atom. 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 primordial nuclides are thought to have been created not only during the Big Bang, and also 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. However, the arbitrary defining qualification for cosmogenic nuclides of being formed "in situ in 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 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. 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 are not termed "cosmogenic," even though they were formed by the same process as the cosmogenic nuclides. 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 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. 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.
IsotopeMode of formationhalf life
3H 14NT12.3 y
7BeSpallation 53.2 d
10BeSpallation 1,387,000 y
12BSpallation
11CSpallation 20.3 min
14C14N14C5,730 y
18F18O18F and Spallation 110 min
22NaSpallation 2.6 y
24NaSpallation 15 h
27MgSpallation
28MgSpallation 20.9 h
26AlSpallation 717,000 y
31SiSpallation 157 min
32SiSpallation 153 y
32PSpallation 14.3 d
34mClSpallation 34 min
35SSpallation 87.5 d
36Cl35Cl 36Cl301,000 y
37Ar37Cl 37Ar35 d
38ClSpallation 37 min
39Ar40Ar 39Ar269 y
39Cl40Ar 39Cl & spallation 56 min
41Ar40Ar 41Ar110 min
41Ca40Ca 41Ca102,000 y
45CaSpallation
47CaSpallation
44ScSpallation
46ScSpallation
47ScSpallation
48ScSpallation
44TiSpallation
45TiSpallation
81Kr80Kr 81Kr229,000 y
95Tc95Mo 95Tc
96Tc96Mo 96Tc
97Tc97Mo 97Tc
97mTc97Mo 97mTc
98Tc98Mo 98Tc
99TcSpallation
129ISpallation 15,700,000 y
182YbSpallation
182LuSpallation
183LuSpallation
182HfSpallation
183HfSpallation
184HfSpallation
185HfSpallation
186HfSpallation
185WSpallation
187WSpallation
188WSpallation
189WSpallation
190WSpallation
188ReSpallation
189ReSpallation
190ReSpallation
191ReSpallation
192ReSpallation
191OsSpallation
193OsSpallation
194OsSpallation
195OsSpallation
196OsSpallation
192IrSpallation
194IrSpallation
195IrSpallation
196IrSpallation

Applications in geology listed by isotope

elementmasshalf-life typical application
beryllium101,387,000exposure dating of rocks, soils, ice cores
aluminium26720,000exposure dating of rocks, sediment
chlorine36308,000exposure dating of rocks, groundwater tracer
calcium41103,000exposure dating of carbonate rocks
iodine12915,700,000groundwater tracer
carbon145730radiocarbon dating
sulfur350.24water residence times
sodium222.6water residence times
tritium312.32water residence times
argon39269groundwater tracer
krypton81229,000groundwater tracer