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In geochemistry and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on the Earth that have existed in their current form since before Earth was formed. Primordial nuclides are residues from the Big Bang, from cosmogenic sources, and from ancient supernova explosions which occurred before the formation of the Solar System. They are the stable nuclides plus the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present. Only 288 such nuclides are known. All of the known 254 stable nuclides occur as primordial nuclides, plus another 34 nuclides that have half-lives long enough to have survived from the formation of the Earth. These 34 primordial radionuclides represent isotopes of 28 separate elements. Cadmium, tellurium, neodymium and uranium each have two primordial radioisotopes (, ; , ; , ; and , ), and samarium has three (, , ). Due to the age of the Earth of (4.6 billion years), this means that the half-life of the given nuclides must be greater than about (50 million years) for practical considerations. For example, for a nuclide with half-life (60 million years), this means 77 half-lives have elapsed, meaning that for each mole () of that nuclide being present at the formation of Earth, only 4 atoms remain today. The shortest-lived primordial nuclides (i.e. nuclides with shortest half-lives) are: : ..., , , , , and . These are the 6 nuclides with half-lives comparable to, or less than, the estimated age of the universe. (In the case for 232Th, it has a half life of more than 14 billion years, slightly longer than the age of the universe.) For a complete list of the 34 known primordial radionuclides, including the next 28 with half-lives much ''longer'' than the age of the universe, see the complete list in the section below. The next longest-living nuclide after the end of the list given in the table is niobium-92 with a half-life of . (See list of nuclides for the list of all nuclides with half-lives longer than 60 minutes.) To be detected primordially, 92Nb would have to survive at least 132 half-lives since the Earth's formation, meaning its original concentration will have decreased by a factor of 1040. , it has not been detected. It has been found that the next longer-lived nuclide, , with a half-life of is primordial, although just barely, as its concentration in a few ores is nearly 10−18 weight parts.〔 〕〔 〕 Taking into account that all these nuclides must exist since at least , meaning survive 57 half-lives, their original number is now reduced by a factor of 257 which equals more than 1017.〔 〕 Although it is estimated that about 34 primordial nuclides are radioactive (list below), it becomes very difficult to determine the exact total number of radioactive primordials, because the total number of stable nuclides is uncertain. There exist many extremely long-lived nuclides whose half-lives are still unknown. For example, it is known theoretically that all isotopes of tungsten, including those indicated by even the most modern empirical methods to be stable, must be radioactive and can decay by alpha emission, but this could only be measured experimentally for .〔 (【引用サイトリンク】url=http://www.nndc.bnl.gov/nudat2/ )〕 Nevertheless, the number of nuclides with half-lives so long that they cannot be measured with present instruments—and are considered from this viewpoint to be stable nuclides—is limited. Even when a "stable" nuclide is found to be radioactive, the fact merely moves it from the ''stable'' to the ''unstable'' list of primordial nuclides, and the total number of primordial nuclides remains unchanged. Because primordial chemical elements often consist of more than one primordial isotope, there are only 84 distinct primordial chemical elements. Of these, 80 have at least one observationally stable isotope and four additional primordial elements have only radioactive isotopes. ==Naturally occurring nuclides that are not primordial== Some unstable isotopes which occur naturally (such as , , and ) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as and ), or (rarely) by such processes as geonuclear transmutation (neutron capture of uranium in the case of ). Other examples of common naturally-occurring but non-primordial nuclides are radon, polonium, and radium, which are all radiogenic nuclide daughters of uranium decay and are found in uranium ores. A similar radiogenic series is derived from the long-lived radioactive primordial nuclide thorium-232. All of such nuclides have shorter half-lives than their parent radioactive primordial nuclides. There are about 51 nuclides which are radioactive and exist naturally on Earth but are not primordial (making a total of fewer than 340 total nuclides to be found naturally on Earth). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Primordial nuclide」の詳細全文を読む スポンサード リンク
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