LN2C - Cosmogenic Nuclides Basics

Cosmic Rays

Our planet is continuously impacted by a flux of very energetic particles coming from space constituting cosmic rays. The existence of these radiations was discovered in the early 20^th century, especially by the experiments in manned balloons of Victor Hess (Nobel Prize in Physics 1936). There are two types of cosmic rays: 

• Primary cosmic rays consisting mainly of protons (85%), alpha particles (helium nuclei, 12%). The remaining part (3%) is constituted of heavy nuclei and electrons. The most commonly accepted origin for cosmic rays is the acceleration by shock waves, such as those generated by the explosion of a supernova, of interstellar plasmas. If the most energetic particles of cosmic rays therefore come from outside the Solar System, lower energy particles are also produced by the Sun. The solar and galactic components differ in their compositions, energy distributions and intensities over time.
• Secondary cosmic rays resulting from the primary cosmic rays particles impacts on the components of the atmosphere which results in the formation of cosmic showers (neutrons and muons). When the particles of primary cosmic rays enter the Earth's atmosphere, a cascade of nuclear reactions will occur with the atoms constituting the atmosphere. Their energy is thus dissipated during these nuclear reactions. The flux and energy of primary particles and secondary particles (mainly neutrons) produced in nuclear cascades decrease rapidly according to an exponential law depending on the thickness of the atmosphere crossed.

Cosmogenic nuclides

The very high energy of cosmic rays allows them to trigger nuclear reactions when they impact the nuclei of the elements constituting the atmosphere and the Earth's sub-surface. Most of the reactions take place in the upper atmosphere. At sea level, only 0.00003% of primary protons remain. Likewise, virtually all particles of secondary radiation dissipate their energy in the atmosphere. Only about 0.1% of secondary particles reach the Earth surface with enough energy to induce nuclear reactions in minerals from rocks exposed to the surface.

The nuclei which are produced by nuclear reactions between target atoms constituting the Earth's atmosphere (atmospheric production ) or the minerals of rocks constituting the earth's crust (in situ production, up to a few meters below the surface) and cosmic radiation (primary or secondary) are commonly referred to as cosmogenic nuclides.

These nuclear reactions are mainly spallation reactions, which are reactions where the impacting particle (mainly a neutron) has enough energy to sputter off constituent particles (neutrons and protons) from the target atomic nucleus without being captured, and therefore leave as residue, a nucleus of a different chemical species since both the atomic number (number of protons) and the mass number (number of protons + number of neutrons) are lower than that of the original target nucleus.

In the atmosphere, the production rate of cosmogenic nuclides decreases when the altitude decreases, due to the decreasing flow of particles that travel though the atmosphere. In the Earth's crust, the production rate of cosmogenic nuclides decreases as a function of the depth according to an exponential law.

The production of in situ cosmogenic nuclides is of the order of a few to tens of atoms per gram of rock and per year at the Earth surface, resulting in measured concentrations of the order of a few thousand (10³) to millions (10⁶) of atoms per gram of rock. Comparatively, the total number of atoms in a gram of rock is of the order of several trillions of billions (10²²). The very low abundances of cosmogenic nuclides within a sample requires the use of specific preparation and measurement techniques.

Geochronology applications

Cosmogenic nuclides constitute an extremely versatile tool, allowing numerous geochronological applications.

The accumulation over time of in situ produced cosmonuclides in surface and subsurface rocks depends on the exposure duration, the denudation rate and on the radioactive decay of the considered nuclide. Thus, when a sample is exposed at surface, its concentration will increase until it reaches an equilibrium state where the gains by production will compensate the losses (losses by denudation or radioactive decay). Therefore, the measured concentrations (in samples such as glacial moraines, polished rocks, alluvial terraces or sediments) can be interpreted in terms of exposure age or in term of denudation rates (or both when considering depth profiles).

Cosmonuclides produced in the atmosphere (as the atmospheric ¹⁰Be in sediments and ice) are often interpreted in terms of earth's magnetic field variation. The ¹⁰Be/⁹Be ratio can, in some situations, be used as a proxy for continental weathering.