Cosmogenic nuclide
Cosmogenic nuclides (or cosmogenic isotopes) are rare
.Certain light (low atomic number)
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).[1][3] 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
Cosmogenic nuclides
Here is a list of radioisotopes formed by the action of
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 |
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 |
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 |
33P | Spallation (Ar) | 25.3 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 |
81Kr | 80Kr (n,γ) 81Kr | 229,000 y |
129I | Spallation (Xe) | 15,700,000 y |
Applications in geology listed by isotope
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 |
Use in geochronology
As seen in the table above, there are a wide variety of useful cosmogenic nuclides which can be measured in soil, rocks, groundwater, and the atmosphere.
Three types of cosmic-ray reactions can occur once a cosmic ray strikes matter which in turn produce the measured cosmogenic nuclides.[6]
- cosmic ray spallation, which is the most common reaction on the near-surface (typically 0 to 60 cm below) the Earth and can create secondary particles which can cause additional reaction upon interaction with another nuclei called a collision cascade.
- muon capture, which pervades at depths a few meters below the subsurface because muons are inherently less reactive; in some cases, high-energy muons can reach greater depths[7]
- neutron capture, which due to the neutron's low energy are captured into a nucleus, most commonly by water,[clarification needed] but this process is highly dependent on snow, soil moisture and trace element concentrations.
Corrections for cosmic-ray fluxes
Since the Earth bulges at the equator and mountains and deep oceanic trenches allow for deviations of several kilometers relative to a uniformly smooth spheroid, cosmic rays bombard the Earth's surface unevenly based on the latitude and altitude. Thus, many geographic and geologic considerations must be understood in order for cosmic-ray flux to be accurately determined. Atmospheric pressure, for example, which varies with altitude, can change the production rate of nuclides within minerals by a factor of 30 between sea level and the top of a 5 km high mountain. Even variations in the slope of the ground can affect how far high-energy muons can penetrate the subsurface.[8] Geomagnetic field strength which varies over time affects the production rate of cosmogenic nuclides though some models assume variations of the field strength are averaged out over geologic time and are not always considered.
References
- ^ ISBN 978-0-08-037941-8.
- ^ "Beryllium | Properties, Uses, & Facts | Britannica". www.britannica.com. 2023-09-17. Retrieved 2023-10-19.
- ^ Sapphire Lally (Jul 24, 2021). "How is gold made? The mysterious cosmic origins of heavy elements". New Scientist.
- ^ 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.
- S2CID 247396585.
- ISBN 978-3-642-46081-4.
- .
- .