Nucleosynthesis - Minor Mechanisms and Processes

Minor Mechanisms and Processes

Amounts of certain nuclides are produced on Earth by artificial means, and this is their major source (for example, technetium). However, some nuclides are also produced by a number of natural means that have continued after primordial production of elements, discussed above, ceased. Often these act to produce new elements in ways that can be used to date rocks or check on the timing or source of geological processes. Although these processes are usually not major sources of nuclides, in the cases of the short-lived naturally-occurring nuclides that exhibit half-lives too short to be primordial (see list of nuclides), these processes are the entire source of the existing natural supply of the nuclide.

These mechanisms include:

• Radioactive decay leading to specific radiogenic daughter nuclides. The nuclear decay of many long-lived primordial isotopes, especially uranium-235, uranium-238, and thorium-232 produce many intermediate daughter nuclides, some of them quite short-lived, before finally decaying to isotopes of lead. The Earth's natural supply of elements like radon and polonium is via this mechanism. The atmosphere's supply of argon-40 is due mostly to the radioactive decay of potassium-40 in the time since the formation of the Earth, so most of this atmospheric argon is not primordial. In the case of alpha-decay, helium-4 is produced directly by alpha-decay, and so the helium trapped in Earth's crust is also mostly non-primordial. In other types of radioactive decay, such as cluster decay, other types of nuclei are ejected (for example, neon-20), and these eventually become newly-formed neutral atoms.

• Radioactive decay leading to spontaneous fission. This is not cluster decay, for the fission products may be split among nearly any type of atom. Uranium-235 and uranium-238 are both primordial isotopes that undergo spontaneous fission. Natural technetium and promethium are produced in this way.

• Nuclear reactions. Naturally-occurring nuclear reactions powered by radioactive decay give rise to so-called nucleogenic nuclides. This process happens when an energetic particle from a radioactive decay, often an alpha particle, reacts with a nucleus of another atom to change the nucleus into another nuclide. This process may also cause production of further subatomic particles, such as neutrons. Neutrons can also be produced in spontaneous fission and by neutron emission (a type of radioactive decay). These neutrons can then go on to produce other nuclides via neutron-induced fission, or by neutron capture. For example, some stable isotopes like neon-21 and neon-22 are produced in several routes of nucleogenic synthesis, and thus only part of their abundance is primordial.

• Nuclear reactions due to cosmic rays. By convention, these reaction-products are not termed "nucleogenic" nuclides, but rather cosmogenic nuclides. Cosmic rays continue to produce new elements on Earth by the same cosmogenic processes discussed above that produced primordial beryllium and boron. An important example is carbon-14, produced from nitrogen-14 in the atmosphere by cosmic rays. See also iodine-129 for another example.