Natural radionuclides may be conveniently subdivided into three types.[citation needed] First, those whose half-lives T1/2 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) . These are remnants of nucleosynthesis that occurred in stars before the formation of the solar system. For example, the isotope 238
U of uranium is still fairly abundant in nature, but the shorter-lived isotope 235
U is 138 times rarer. About 33 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 226
Ra , 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 14
C (radiocarbon) that is made by cosmic-ray bombardment of other elements, and nucleogenic 239
Pu which is still being created by neutron bombardment of natural 238
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.Nuclides produced by radioactive decay are called radiogenic nuclides, whether they themselves are stable or not. There exist stable radiogenic nuclides that were formed from short-lived extinct radionuclides in the early solar system[citation needed]. The extra presence of stable radiogenic nuclides against the background of primordial stable nuclides can be inferred by various means. Presently-radioactive nuclides are from three sources: many naturally-occurring radionuclides are short-lived radiogenic nuclides that are the daughters of ongoing radioactive primordial nuclides (types of radioactive atoms that have been present since the beginning of the Earth and solar system). Other naturally-occurring radioactive nuclides are cosmogenic nuclides, formed by cosmic ray bombardment of material in the Earth's atmosphere or crust. Finally, some primordial nuclides are radioactive, but are so long-lived that they remain present from the primordial solar nebula. For a summary table showing the number of stable nuclides and of radioactive nuclides in each category, see radionuclide.
passage of time
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