Primordial nuclide

In geochemistry, geophysics and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the interstellar medium from which the solar system was formed, and were formed in, or after, the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation, and potentially from other processes. 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 286 such nuclides are known. In geochemistry, geophysics and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the interstellar medium from which the solar system was formed, and were formed in, or after, the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation, and potentially from other processes. 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 286 such nuclides are known. All of the known 252 stable nuclides, plus another 34 nuclides that have half-lives long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 34 primordial radionuclides represent isotopes of 28 separate elements.Cadmium, tellurium, xenon, neodymium, samarium and uranium each have two primordial radioisotopes (113Cd, 116Cd; 128Te, 130Te; 124Xe, 136Xe; 144Nd, 150Nd; 147Sm, 148Sm; and 235U, 238U). Because the age of the Earth is 4.58×109 years (4.6 billion years), the half-life of the given nuclides must be greater than about 1×108 years (100 million years) for practical considerations. For example, for a nuclide with half-life 6×107 years (60 million years), this means 77 half-lives have elapsed, meaning that for each mole (6.02×1023 atoms) of that nuclide being present at the formation of Earth, only 4 atoms remain today. The four shortest-lived primordial nuclides (i.e. nuclides with shortest half-lives) are 232Th, 238U, 40K, and 235U.These are the 4 nuclides with half-lives comparable to, or less than, the estimated age of the universe. (In the case of 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 30 with half-lives much longer than the age of the universe, see the complete list below. For practical purposes, nuclides with half-lives much longer than the age of the universe may be treated as if they were stable. 232Th and 238U have half-lives long enough that their decay is limited over geological time scales; 40K and 235U have shorter half-lives and are hence severely depleted, but are still long-lived enough to persist significantly in nature. The next longest-living nuclide after the end of the list given in the table is 244Pu, with a half-life of 8.08×107 years. It has been reported to exist in nature as a primordial nuclide, although later studies could not detect it. Likewise, the second-longest-lived non-primordial 146Sm has a half-life of 6.8×107 years, about double that of the third-longest-lived non-primordial 92Nb (3.5×107 years). Taking into account that all these nuclides must exist for at least 4.6×109 years, 244Pu must survive 57 half-lives (and hence be reduced by a factor of 257 ≈ 1.4 × 1017), 146Sm must survive 67 (and be reduced by 267 ≈ 1.5 × 1020), and 92Nb must survive 130 (and be reduced by 2130 ≈ 1.4 × 1039). Considering the likely initial abundances of these nuclides, possibly measurable quantities of 244Pu and 146Sm should persist today, while they should not for 92Nb and all shorter-lived nuclides. Nuclides such as 92Nb that were present in the primordial solar nebula but have long since decayed away completely are termed extinct radionuclides if they have no other means of being regenerated. Because primordial chemical elements often consist of more than one primordial isotope, there are only 83 distinct primordial chemical elements. Of these, 80 have at least one observationally stable isotope and three additional primordial elements have only radioactive isotopes (bismuth, thorium, and uranium). Some unstable isotopes which occur naturally (such as 14C, 3H, and 239Pu) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as 14C and 3H), or (rarely) by such processes as geonuclear transmutation (neutron capture of uranium in the case of 237Np and 239Pu). Other examples of common naturally occurring but non-primordial nuclides are isotopes of 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 232Th. All of such nuclides have shorter half-lives than their parent radioactive primordial nuclides. Some other geogenic nuclides do not occur in the decay chains of 232Th, 235U, or 238U but can still fleetingly occur naturally as products of the spontaneous fission of one of these three long-lived nuclides, such as 126Sn, which makes up about 10−14 of all natural tin. There are 252 stable primordial nuclides and 34 radioactive primordial nuclides, but only 80 primordial stable elements (1 through 82, i.e. hydrogen through lead, exclusive of 43 and 61, technetium and promethium respectively) and three radioactive primordial elements (bismuth, thorium, and uranium). Bismuth's half-life is so long that it is often classed with the 80 primordial stable elements instead, since its radioactivity is not a cause for serious concern. The number of elements is fewer than the number of nuclides, because many of the primordial elements are represented by multiple isotopes. See chemical element for more information. As noted, these number about 252. For a list, see the article list of elements by stability of isotopes. For a complete list noting which of the 'stable' 252 nuclides may be in some respect unstable, see list of nuclides and stable nuclide. These questions do not impact the question of whether a nuclide is primordial, since all 'nearly stable' nuclides, with half-lives longer than the age of the universe, are primordial also.