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High-energy processes |
Ingeochemistry,geophysics andnuclear physics,primordial nuclides, orprimordial isotopes, arenuclides found onEarth that have existed in their current form since beforeEarth was formed. Primordial nuclides were present in the interstellar medium from which theSolar System was formed, and were formed in theBig Bang, bynucleosynthesis in stars and supernovae followed by mass ejection, bycosmic ray spallation, or from other processes throughout the history of the universe. They are the stable nuclides plus the fraction of the long-livedradionuclides surviving from the primordial solar nebula through planetaccretion until the present; 286 such nuclides are known.
All of the known 251stable nuclides, plus another 35 nuclides that havehalf-lives long enough to have survived from the formation of the Earth, occur as primordial nuclides. These 35 primordial radionuclides representisotopes of 28 separateelements.Cadmium,tellurium,xenon,neodymium,samarium,osmium, anduranium each have two primordial radioisotopes (113
Cd,116
Cd;128
Te,130
Te;124
Xe,136
Xe;144
Nd,150
Nd;147
Sm,148
Sm;184
Os,186
Os; and235
U,238
U).
Because theage of the Earth is4.58×109 years (4.58 billion years), thehalf-life of the given nuclides must be greater than about108 years (100 million years) for practical detectability. For example, for a nuclide with half-life6×107 years (60 million years), this means 77 half-lives have elapsed, meaning that for eachmole (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 are87
Rb (4.92×1010 years),187
Re (4.12×1010 years),176
Lu (3.70×1010 years),232
Th (1.40×1010 years),238
U (4.46×109 years),40
K (1.25×109 years), and235
U (7.04×108 years).
These are the seven nuclides with half-lives comparable to, or somewhat less than, the estimatedage of the universe. (87Rb,187Re,176Lu, and232Th 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, and238U have half-lives long enough that their decay is limited over geological time scales;40K and235U 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] is146
Sm, which has a half-life of9.20×107 years, followed by244
Pu (8.13×107 years) and92
Nb (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 least4.58×109 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), and92Nb must survive 130 (and be reduced by 2130 ≈ 1×1039). Mathematically, considering the likely initial abundances of these nuclides, primordial146Sm and244Pu 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, while92Nb and all shorter-lived nuclides should not. Nuclides such as92Nb that were present in the primordial solar nebula but have long since decayed away completely are termedextinct radionuclides if they have no other means of being regenerated.[5] As for244Pu, calculations suggest that as of 2022, sensitivity limits were about one order of magnitude away from detecting it as a primordial nuclide.[4]
Becauseprimordial chemical elements often consist of more than one primordial isotope, there are only 83 distinct primordialchemical elements. Of these, 80 have at least oneobservationally stable isotope and three additional primordial elements have only radioactive isotopes (bismuth,thorium, and uranium).
Some unstable isotopes which occur naturally (such as14
C,3
H, and239
Pu) are not primordial, as they must be constantly regenerated. This occurs bycosmic radiation (in the case ofcosmogenic nuclides such as14
C and3
H), or (rarely) by such processes as geonuclear transmutation (neutron capture by uranium in the case of237
Np and239
Pu). Other examples of common naturally occurring but non-primordial nuclides are isotopes ofradon,polonium, andradium, which are allradiogenic daughters of uranium decay and are found in uranium ores. The stableargon isotope40Ar is actually more common as a radiogenic nuclide than as a primordial nuclide, forming almost 1% of the Earth'satmosphere, which is generated by theelectron capture decay of the extremely long-lived radioactive primordial isotope40K, 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 thealpha process nuclide36Ar, which is significantly rarer than40Ar on Earth.) And the classicaldecay chains of radiogenic elements derive from the long-lived radioactive primordial nuclides232Th,235U, and238U.
These nuclides are described asgeogenic, 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 thespontaneous fission of one of these three long-lived nuclides, such as126Sn, which makes up about 10−14 of all naturaltin.[7] Another,99Tc, has also been detected,[8] and there are five otherlong-lived fission products known.
A primordial element is achemical element with at least one primordial nuclide. There are 251 stable primordial nuclides and 35 radioactive primordial nuclides, but only 80 primordial stableelements—hydrogen through lead, atomic numbers 1 to 82, except fortechnetium (43) andpromethium (61)—and three radioactive primordialelements—bismuth (83), thorium (90), and uranium (92). If plutonium (94) turns out to be primordial (specifically, the long-lived isotope244Pu), 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 multipleisotopes. Seechemical element for more information.
As noted, this number is about 251. For a list, see the articlelist of elements by stability of isotopes. For a complete list noting which of the "stable" 251 nuclides may be in some respect unstable, seelist of nuclides andstable 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.
Although it is estimated that about 35 primordial nuclides areradioactive (seelist 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 thandysprosium-164 are theoretically radioactive.[9] For example, it is predicted theoretically that allisotopes of tungsten, including those indicated by even the most modern empirical methods to be stable, must be radioactive toalpha decay, but this can be detected experimentally only for180W.[10] Similarly, all four primordialisotopes of lead are expected to decay tomercury, 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 bestable nuclides—is limited. Even when a "stable" nuclide is found to be radioactive, it merely moves from thestable to theunstable 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, andrhenium) a very long-lived radioactive primordial nuclide is more abundant than a stable nuclide.
The longest-lived radionuclide known,128Te, has a half-life of2.25×1024 years: 1.6 × 1014 times theage 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 andSolar System. Many of these nuclides decay bydouble beta decay, although some like209Bi decay by other means likealpha decay.
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