Cosmogenic nuclides (orcosmogenic isotopes) are rarenuclides (isotopes) created when a high-energycosmic ray interacts with thenucleus of anin situ Solar Systematom, causing nucleons (protons and neutrons) to be expelled from the atom (seecosmic ray spallation). These nuclides are produced within Earth materials such asrocks orsoil, inEarth'satmosphere, and in extraterrestrial items such asmeteoroids. By measuring cosmogenic nuclides,scientists are able to gain insight into a range ofgeological andastronomical processes. There are bothradioactive andstable cosmogenic nuclides. Some of these radionuclides aretritium,carbon-14 andphosphorus-32.

Certain light (low atomic number)primordial nuclides (isotopes oflithium,beryllium andboron) are thought to have been created not only during theBig Bang, but 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 dust as compared with their abundances on Earth. This also explains the overabundance of the earlytransition metals just beforeiron in the periodic table – the cosmic-ray spallation of iron producesscandium throughchromium on the one hand andhelium 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 spallationbefore 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^[2] 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, thetiming of their formation determines which subset of cosmic ray spallation-produced nuclides are termedprimordial orcosmogenic (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 timebetween theBig Bang and the Solar System's formation (thus making theseprimordial 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 isotopesberyllium-7 andberyllium-10 fall into this series of three light elements (lithium, beryllium, boron) formed mostly by cosmic ray spallationnucleosynthesis, both of these nuclides have half-lives too short (53 days and c. 1.4 million years, resp.) 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.

Here is a list of radioisotopes formed by the action ofcosmic rays; the list also contains the production mode of the isotope.^[4] 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.

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.^[5] These nuclides all share the common feature of being absent in the host material at the time of formation. These nuclides are chemically distinct and fall into two categories. The nuclides of interest are eithernoble gases which due to their inert behavior are inherently not trapped in a crystallized mineral or has a short enough half-life such that it has decayed sincenucleosynthesis, but a long enough half-life such that it has built up measurable concentrations. The former includes measuring abundances of⁸¹Kr and³⁹Ar whereas the latter includes measuring abundances of¹⁰Be,¹⁴C, and²⁶Al.

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 acollision 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.

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.

• Environmental radioactivity

• Concepts in astrophysics
• Environmental isotopes
• Geochemistry
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• Radiometric dating

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