Primordial nuclide

Short description: Nuclides predating the Earth's formation (found on Earth)

Nuclear physics

Nucleus * Nucleons (p, n) * Nuclear matter * Nuclear force * Nuclear structure * Nuclear reaction

Models of the nucleus

Liquid drop * Nuclear shell model * Interacting boson model * Ab initio

Nuclides' classification

Stable * Magic * Even/odd * Halo&nbsp;(Borromean)

Nuclear stability

Binding energy * p–n ratio * Drip line * Island of stability * Valley of stability * Stable nuclide

Radioactive decay

Alpha α * Beta β (2β, β+) * K/L capture * Isomeric (Gamma γ * Internal conversion) * Spontaneous fission * Cluster decay * Neutron emission * Proton emission

Nuclear fission

Spontaneous * Products (pair breaking) * Photofission

Capturing processes

electron (2×) * neutron (s * r) * proton (p * rp)

High-energy processes

Spallation (by cosmic ray) * Photodisintegration

Nucleosynthesis and
nuclear astrophysics

Nuclear fusion
Processes: Stellar * Big Bang * Supernova
Nuclides: Primordial * Cosmogenic * Artificial

High-energy nuclear physics

Quark–gluon plasma * RHIC * LHC

* v
* t
* e

Relative abundance of the chemical elements in the Earth's upper continental crust, on a per-atom basis

In geochemistry, geophysics and nuclear physics, primordial nuclides, or 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 the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation, or from other processes throughout the history of the universe. They are the stable nuclides plus the fraction of the long-lived radionuclides surviving from the primordial solar nebula through planet accretion until the present; 286 such nuclides are known.

## Contents

* 1 Stability
* 2 Naturally occurring nuclides that are not primordial
* 3 Primordial elements
* 4 Naturally occurring stable nuclides
* 5 Radioactive primordial nuclides
* 6 See also
* 7 References

## Stability

All of the known 251 stable nuclides, plus another 35 nuclides that have half-lives long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 35 primordial radionuclides represent isotopes of 28 separate elements. Cadmium, tellurium, xenon, neodymium, samarium, osmium, and uranium each have two primordial radioisotopes (The element Chemistry:Cadmium does not exist., The element Chemistry:Cadmium does not exist.; The element Chemistry:Tellurium does not exist., The element Chemistry:Tellurium does not exist.; The element Chemistry:Xenon does not exist., The element Chemistry:Xenon does not exist.; The element Chemistry:Neodymium does not exist., The element Chemistry:Neodymium does not exist.; The element Chemistry:Samarium does not exist., The element Chemistry:Samarium does not exist.; The element Chemistry:Osmium does not exist., The element Chemistry:Osmium does not exist.; and The element Chemistry:Uranium does not exist., The element Chemistry:Uranium does not exist.).

Because the age of the Earth is 4.58×109 years (4.58 billion years), the half-life of the given nuclides must be greater than about 108 years (100 million years) for practical detectability. 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 seven shortest-lived primordial nuclides (i.e., the nuclides with the shortest half-lives) to be detected as primordial are The element Chemistry:Rubidium does not exist. (4.92×1010 years), The element Chemistry:Rhenium does not exist. (4.12×1010 years), The element Chemistry:Lutetium does not exist. (3.70×1010 years), The element Chemistry:Thorium does not exist. (1.40×1010 years), The element Chemistry:Uranium does not exist. (4.46×109 years), The element Chemistry:Potassium does not exist. (1.25×109 years), and The element Chemistry:Uranium does not exist. (7.04×108 years).

These are the seven nuclides with half-lives comparable to, or somewhat less than, the estimated age of the universe. (87Rb, 187Re, 176Lu, and 232Th have half-lives somewhat longer than the age of the universe.) For a complete list of the 35 known primordial radionuclides, including the next 28 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. 87Rb, 187Re, 176Lu, 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 remain present in significant amount on Earth.

The longest-lived isotope not proven to be primordial[1] is The element Chemistry:Samarium does not exist., which has a half-life of 9.20×107 years, followed by The element Chemistry:Plutonium does not exist. (8.13×107 years) and The element Chemistry:Niobium does not exist. (3.47×107 years). 244Pu was reported to exist in nature as a primordial nuclide in 1971,[2] but this detection could not be confirmed by further studies in 2012 and 2022.[3][4]

Taking into account that all these nuclides must exist for at least 4.58×109 years, 146Sm must survive 50 half-lives (and hence be reduced by 250 ≈ 1×1015), 244Pu must survive 57 (and be reduced by a factor of 257 ≈ 1×1017), and 92Nb must survive 130 (and be reduced by 2130 ≈ 1×1039). Mathematically, considering the likely initial abundances of these nuclides, primordial 146Sm and 244Pu should persist somewhere within the Earth today, even if they are not identifiable in the relatively minor portion of the Earth's crust available to human assays, while 92Nb and all shorter-lived nuclides should not. 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.[5] As for 244Pu, calculations suggest that as of 2022, sensitivity limits were about one order of magnitude away from detecting it as a primordial nuclide.[4]

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).

## Naturally occurring nuclides that are not primordial

Some unstable isotopes which occur naturally (such as The element Chemistry:Carbon does not exist., The element Chemistry:Hydrogen does not exist., and The element Chemistry:Plutonium does not exist.) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as The element Chemistry:Carbon does not exist. and The element Chemistry:Hydrogen does not exist.), or (rarely) by such processes as geonuclear transmutation (neutron capture by uranium in the case of The element Chemistry:Neptunium does not exist. and The element Chemistry:Plutonium does not exist.). Other examples of common naturally occurring but non-primordial nuclides are isotopes of radon, polonium, and radium, which are all radiogenic daughters of uranium decay and are found in uranium ores. The stable argon isotope 40Ar is actually more common as a radiogenic nuclide than as a primordial nuclide, forming almost 1% of the Earth's atmosphere, which is generated by the electron capture decay of the extremely long-lived radioactive primordial isotope 40K, whose half-life is on the order of a billion years and thus has been generating argon since early in the Earth's existence. (Primordial argon was dominated by the alpha process nuclide 36Ar, which is significantly rarer than 40Ar on Earth.) And the classical decay chains of radiogenic elements derive from the long-lived radioactive primordial nuclides 232Th, 235U, and 238U.

These nuclides are described as geogenic, meaning that they are decay or fission products of uranium or other actinides in subsurface rocks.[6] All such nuclides have shorter half-lives than their parent radioactive primordial nuclides. Some other geogenic nuclides 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.[7] Another, 99Tc, has also been detected,[8] and there are five other long-lived fission products known.

## Primordial elements

"Primordial element" redirects here. For a concept in algebra, see Primordial element (algebra).

A primordial element is a chemical element with at least one primordial nuclide. There are 251 stable primordial nuclides and 35 radioactive primordial nuclides, but only 80 primordial stable elements—hydrogen through lead, atomic numbers 1 to 82, except for technetium (43) and promethium (61)—and three radioactive primordial elements—bismuth (83), thorium (90), and uranium (92). If plutonium (94) turns out to be primordial (specifically, the long-lived isotope 244Pu), then it would be a fourth radioactive primordial, though practically speaking it would still be more convenient to produce synthetically. Bismuth's half-life is so long that it is often classed with the 80 stable elements instead, since its radioactivity is not a cause for concern. The number of elements is smaller than the number of nuclides, because many of the primordial elements are represented by multiple isotopes. See chemical element for more information.

## Naturally occurring stable nuclides

As noted, this number is about 251. For a list, see the article list of elements by stability of isotopes. For a complete list noting which of the "stable" 251 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 also primordial.

## Radioactive primordial nuclides

Although it is estimated that about 35 primordial nuclides are radioactive (see list of nuclides), it is very hard to determine the exact number of radioactive primordials, as there are many extremely long-lived nuclides whose half-lives are still unknown; in fact, all nuclides heavier than dysprosium-164 are theoretically radioactive.[9] For example, it is predicted theoretically that all isotopes of tungsten, including those indicated by even the most modern empirical methods to be stable, must be radioactive to alpha decay, but this can be detected experimentally only for 180W.[10] Similarly, all four primordial isotopes of lead are expected to decay to mercury, but the predicted half-lives are so long (some exceeding 10100 years) that such decays could hardly be observed in the near future. 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, it merely moves from the stable to the unstable list of primordials, and the total number of primordial nuclides remains unchanged. For practical purposes, such nuclides, whose radioactivity is not detectable by ordinary means, may be considered stable for all purposes outside specialized research.[citation needed]

These 35 primordial radionuclides are isotopes of 28 elements (cadmium, neodymium, osmium, samarium, tellurium, uranium, and xenon each have two primordial radioisotopes). These nuclides are listed in order of decreasing stability. Many of them are so nearly stable that they compete for abundance with stable isotopes of their respective elements; in fact, for three elements (indium, tellurium, and rhenium) a very long-lived radioactive primordial nuclide is more abundant than a stable nuclide.

The longest-lived radionuclide known, 128Te, has a half-life of 2.25×1024 years: 1.6 × 1014 times the age of the Universe. Only four of these 35 nuclides have half-lives shorter than, or nearly equal to, the age of the universe. Most of the other 30 have half-lives much longer. The shortest-lived primordial, 235U, has a half-life of 704 million years, about 15% of the age of the Earth and Solar System. Many of these nuclides decay by double beta decay, although some like 209Bi decay by other means like alpha decay.

## See also

* Alpha nuclide
* List of nuclides
* Table of nuclides
* Isotope geochemistry
* Radionuclide
* Mononuclidic element
* Monoisotopic element
* Stable isotope
* List of nuclides
* List of elements by stability of isotopes
* Big Bang nucleosynthesis

## References

1. ↑ Samir Maji et al. (2006). "Separation of samarium and neodymium: a prerequisite for getting signals from nuclear synthesis". Analyst 131 (12): 1332–1334. doi:10.1039/b608157f. PMID 17124541. Bibcode: 2006Ana...131.1332M.
2. ↑ Hoffman, D. C.; Lawrence, F. O.; Mewherter, J. L.; Rourke, F. M. (1971). "Detection of Plutonium-244 in Nature". Nature 234 (5325): 132–134. doi:10.1038/234132a0. Bibcode: 1971Natur.234..132H.
3. ↑ Lachner, J. (2012). "Attempt to detect primordial 244Pu on Earth". Physical Review C 85 (1). doi:10.1103/PhysRevC.85.015801. Bibcode: 2012PhRvC..85a5801L.
4. ↑ 4.0 4.1 Wu, Yang; Dai, Xiongxin; Xing, Shan; Luo, Maoyi; Christl, Marcus; Synal, Hans-Arno; Hou, Shaochun (2022). "Direct search for primordial 244Pu in Bayan Obo bastnaesite". Chinese Chemical Letters 33 (7): 3522–3526. doi:10.1016/j.cclet.2022.03.036. http://www.ccspublishing.org.cn/article/doi/10.1016/j.cclet.2022.03.036?pageType=en. Retrieved 29 January 2024.
5. ↑ P. K. Kuroda (1979). "Origin of the elements: pre-Fermi reactor and plutonium-244 in nature". Accounts of Chemical Research 12 (2): 73–78. doi:10.1021/ar50134a005.
6. ↑ Clark, Ian (2015). Groundwater geochemistry and isotopes. CRC Press. p. 118. ISBN 978-1-4665-9174-5. https://books.google.com/books?id=LnN3CAAAQBAJ&q=nuclides+produced+in+uranium+spontaneous+fission&pg=PA118. Retrieved 13 July 2020.
7. ↑ H.-T. Shen. "Research on measurement of 126Sn by AMS". https://accelconf.web.cern.ch/accelconf/HIAT2009/papers/g-07.pdf.
8. ↑ David Curtis, June Fabryka-Martin, Paul Dixon, Jan Cramer (1999), "Nature's uncommon elements: plutonium and technetium", Geochimica et Cosmochimica Acta 63 (2): 275–285, doi:10.1016/S0016-7037(98)00282-8, Bibcode: 1999GeCoA..63..275C
9. ↑ arXiv:1908.11458
10. ↑ National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/.

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