Cosmic Ray Spallation

Cosmic ray spallation is a form of naturally occurring nuclear fission and nucleosynthesis. It refers to the formation of elements from the impact of cosmic rays on an object. Cosmic rays are highly energetic charged particles from outside of Earth ranging from protons, alpha particles, and nuclei of many heavier elements. About 1% of cosmic rays also consist of free electrons.

Cosmic rays cause spallation when a ray particle (e.g. a proton) impacts with matter, including other cosmic rays. The result of the collision is the expulsion of large numbers of nucleons (protons and neutrons) from the object hit. This process goes on not only in deep space, but in Earth's upper atmosphere and crustal surface (typically the upper ten meters) due to the ongoing impact of cosmic rays.

Cosmic ray spallation after the Big Bang is thought to be responsible for the abundance in the universe of some light elements such as lithium, beryllium, and boron. This process (cosmogenic nucleosynthesis) was discovered somewhat by accident during the 1970s: models of Big Bang nucleosynthesis suggested that the amount of deuterium was too large to be consistent with the expansion rate of the universe and there was therefore great interest in processes that could generate deuterium after the Big Bang. Cosmic ray spallation was investigated as a possible process to generate deuterium. As it turned out, spallation could not generate much deuterium, nor could nucleosynthesis in stars. (The excess deuterium in the universe was finally explained by assuming the existence of non-baryonic dark matter). However, the new studies of spallation showed that this process could generate lithium, beryllium and boron, and indeed these isotopes are over-represented in cosmic ray nuclei, as compared with solar atmospheres (whereas hydrogen and helium are present in about primordial ratios in cosmic rays).

In addition to the above light elements, isotopes of aluminium, carbon (carbon-14), tritium, chlorine, iodine and neon are formed within solar system materials through cosmic ray spallation, and are termed cosmogenic nuclides. Since they remain trapped in the atmosphere or rock in which they formed, some can be very useful in the dating of materials by cosmogenic radionuclide dating, particularly in the geological field. In formation of a cosmogenic nuclide, a cosmic ray interacts with the nucleus of an in situ solar system atom, causing cosmic ray spallation. These isotopes are produced within earth materials such as rocks or soil, in Earth's atmosphere, and in extraterrestrial items such as meteorites. By measuring cosmogenic isotopes, scientists are able to gain insight into a range of geological and astronomical processes. There are both radioactive and stable cosmogenic isotopes. Some of the well-known naturally-occurring radioisotopes are tritium, carbon-14 and phosphorus-32.

The timing of their formation determines which subset of nuclides formed by cosmic ray spallation, are termed primordial or cosmogenic (a nuclide cannot belong to both classes). By convention, certain stable nuclides of lithium, beryllium, and boron 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 are were formed by the same process as the cosmogenic nuclides (although at an earlier time). In contrast, the radioactive nuclide beryllium-7 falls into this light element range, but this nuclide has a half-life too short for it to have been formed before the formation of the solar system, so that it cannot be a primordial nuclide. Since the cosmic ray spallation route is the most likely source of beryllium-7 in the environment, it is therefore cosmogenic.