Nuclide
Origins of naturally occurring radionuclides

Natural radionuclides may be conveniently subdivided into three types. First, those whose half-lives t are at least 2% as long as the age of the Earth (for practical purposes, these are difficult to detect with half-lives less than 10% of the age of the Earth) (4.6 × 10 years). These are remnants of nucleosynthesis that occurred in stars before the formation of the Solar System. For example, the isotope U (t = 4.5 × 10 years) of uranium is still fairly abundant in nature, but the shorter-lived isotope U (t = 0.7 × 10 years) is 138 times rarer. About 34 of these nuclides have been discovered (see List of nuclides and Primordial nuclide for details).

The second group of radionuclides that exist naturally consists of radiogenic nuclides such as Ra (t = 1602 years), an isotope of radium, which are formed by radioactive decay. They occur in the decay chains of primordial isotopes of uranium or thorium. Some of these nuclides are very short-lived, such as isotopes of francium. There exist about 51 of these daughter nuclides that have half-lives too short to be primordial, and which exist in nature solely due to decay from longer lived radioactive primordial nuclides.

The third group consists of nuclides that are continuously being made in another fashion that is not simple spontaneous radioactive decay (i.e., only one atom involved with no incoming particle) but instead involves a natural nuclear reaction. These occur when atoms react with natural neutrons (from cosmic rays, spontaneous fission, or other sources), or are bombarded directly with cosmic rays. The latter, if non-primordial, are called cosmogenic nuclides. Other types of natural nuclear reactions produce nuclides that are said to be nucleogenic nuclides.

An example of nuclides made by nuclear reactions, are cosmogenic C (radiocarbon) that is made by cosmic ray bombardment of other elements, and nucleogenic Pu which is still being created by neutron bombardment of natural U as a result of natural fission in uranium ores. Cosmogenic nuclides may be either stable or radioactive. If they are stable, their existence must be deduced against a background of stable nuclides, since every known stable nuclide is present on Earth primordially.
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