Primordial nuclide

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Relative abundance of the chemical elements in the Earth's upper continental crust, on a per-atom basis Elemental abundances.svg
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, 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; 286 such nuclides are known.

Contents

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 ( 113
Cd
 , 116
Cd
 ; 128
Te
 , 130
Te
 ; 124
Xe
 , 136
Xe
 ; 144
Nd
 , 150
Nd
 ; 147
Sm
 , 148
Sm
 ; 184
Os
 , 186
Os
 ; and 235
U
 , 238
U
 ).

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 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 seven shortest-lived primordial nuclides (i.e., the nuclides with the shortest half-lives) to have been experimentally verified are 87
Rb
 (5.0×1010 years), 187
Re
 (4.1×1010 years), 176
Lu
 (3.8×1010 years), 232
Th
 (1.4×1010 years), 238
U
 (4.5×109 years), 40
K
 (1.25×109 years), and 235
U
 (7.0×108 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 persist significantly in nature.

The longest-lived isotope not proven to be primordial [1] is 146
Sm
 , which has a half-life of 1.03×108 years, followed by 244
Pu
 (8.08×107 years) and 92
Nb
 (3.5×107 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 4.6×109 years, 146Sm must survive 45 half-lives (and hence be reduced by 245 ≈ 4×1013), 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 14
C
 , 3
H
 , and 239
Pu
 ) are not primordial, as they must be constantly regenerated. This occurs by cosmic radiation (in the case of cosmogenic nuclides such as 14
C
 and 3
H
), or (rarely) by such processes as geonuclear transmutation (neutron capture of uranium in the case of 237
Np
 and 239
Pu
). 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. 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 regenerated by the beta 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.)

A similar radiogenic series is derived from the long-lived radioactive primordial nuclide 232Th. 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 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. [7] Another, 99Tc, has also been detected. [8] There are five other long-lived fission products known.

Primordial elements

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, with the exceptions of 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 primordial stable elements instead, since its radioactivity is not a cause for serious 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, these number 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 (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, in fact, all nuclides heavier than dysprosium-164 are theoretically radioactive. 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 and can decay by alpha emission, but as of 2013 this could only be measured experimentally for 180
W
 . [9] 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 primordial nuclides, and the total number of primordial nuclides remains unchanged. For practical purposes, these nuclides may be considered stable for all purposes outside specialized research.[ citation needed ]

List of 35 radioactive primordial nuclides and measured half-lives

These 35 primordial nuclides represent radioisotopes of 28 distinct chemical elements (cadmium, neodymium, osmium, samarium, tellurium, uranium, and xenon each have two primordial radioisotopes). The radionuclides are listed in order of stability, with the longest half-life beginning the list. These radionuclides in many cases are so nearly stable that they compete for abundance with stable isotopes of their respective elements. For three chemical elements, indium, tellurium, and rhenium, a very long-lived radioactive primordial nuclide is found in greater abundance than a stable nuclide.

The longest-lived radionuclide known, 128Te, has a half-life of 2.2×1024 years, which is 160 trillion times the age of the Universe. Only four of these 35 nuclides have half-lives shorter than, or equal to, the age of the universe. Most of the remaining 30 have half-lives much longer. The shortest-lived primordial isotope, 235U, has a half-life of 703.8 million years, about one sixth of the age of the Earth and the Solar System. Many of these nuclides decay by double beta decay, although some like 209Bi decay by other methods such as alpha decay.

At the end of the list, two more nuclides have been added: 146Sm and 244Pu. They have not been confirmed as primordial, but their half-lives are long enough that minute quantities should persist today.

No.NuclideEnergyHalf-
life
(years)		Decay
mode		 Decay energy
(MeV)		Approx. ratio
half-life to
age of universe
252128Te8.7432612.2×10242 β−2.530160 trillion
253124Xe8.7782641.8×1022KK2.8641.3 trillion
25478Kr9.0223499.2×1021KK2.846670 billion
255136Xe8.7068052.165×10212 β−2.462160 billion
25676Ge9.0346561.8×10212 β−2.039130 billion
257130Ba8.7425741.2×1021KK2.62087 billion
25882Se9.0175961.1×10202 β−2.9958.0 billion
259116Cd8.8361463.102×10192 β−2.8092.3 billion
26048Ca8.9924522.301×10192 β−4.274, .00581.7 billion
261209Bi8.1586892.01×1019α3.1371.5 billion
26296Zr8.9613592.0×10192 β−3.41.5 billion
263130Te8.7665788.806×10182 β−.868640 million
264150Nd8.5625949.3×1018 [10] 2 β−3.367671 million
265100Mo8.9331677.07×1018 [10] 2 β−3.035510 million
266151Eu8.5657594.62×1018α1.9644333 million
267180W8.3471271.801×1018α2.509130 million
26850V9.0557591.4×1017β+ or β−2.205, 1.03810 million
269174Hf8.3922877.0×1016α2.4975 million
270113Cd8.8593727.7×1015β−.321560,000
271148Sm8.6074237.005×1015α1.986510,000
272144Nd8.6529472.292×1015α1.905170,000
273186Os8.3025082.002×1015α2.823150,000
274115In8.8499104.4×1014β−.49932,000
275152Gd8.5628681.1×1014α2.2038000
276184Os8.3118501.12×1013α2.963810
277190Pt8.2677644.83×1011 [10] α3.25235
278147Sm8.6105931.061×1011α2.3107.7
279138La8.6983201.021×1011β− or K or β+1.044, 1.737, 1.7377.4
28087Rb9.0437184.972×1010β−.2833.6
281187Re8.2917324.122×1010β−.00263.0
282176Lu8.3746653.764×1010β−1.1932.7
283232Th7.9185331.405×1010α or SF4.0831.0
284238U7.8725514.468×109α or SF or 2 β−4.2700.3
28540K8.9097071.251×109β− or K or β+1.311, 1.505, 1.5050.09
286235U7.8971987.038×108α or SF4.6790.05
287146Sm8.6261361.03×108α2.5290.008
288244Pu7.8262218.0×107α or SF4.6660.006

List legends

No. (number)
A running positive integer for reference. These numbers may change slightly in the future since there are 251 nuclides now classified as stable, but which are theoretically predicted to be unstable (see Stable nuclide § Still-unobserved decay), so that future experiments may show that some are in fact unstable. The number starts at 252, to follow the 251 (observationally) stable nuclides.
Nuclide
Nuclide identifiers are given by their mass number A and the symbol for the corresponding chemical element (implies a unique proton number).
Energy
Mass of the average nucleon of this nuclide relative to the mass of a neutron (so all nuclides get a positive value) in MeV/c2, formally: mn − mnuclide / A.
Half-life
All times are given in years.
Decay mode
α α decay
β− β− decay
K electron capture
KK double electron capture
β+ β+ decay
SF spontaneous fission
2 β− double β− decay
2 β+ double β+ decay
I isomeric transition
p proton emission
n neutron emission
Decay energy
Multiple values for (maximal) decay energy in MeV are mapped to decay modes in their order.

See also

Related Research Articles

The actinide or actinoid series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.

A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle (alpha particle or beta particle) from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay. These emissions are considered ionizing radiation because they are energetic enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single nuclide the decay rate, and thus the half-life (t1/2) for that collection, can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms has no known limits and spans a time range of over 55 orders of magnitude.

<span class="mw-page-title-main">Stable nuclide</span> Nuclide that does not undergo radioactive decay

Stable nuclides are nuclides that are not radioactive and so do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes.

<span class="mw-page-title-main">Nuclide</span> Atomic species

A nuclide is a class of atoms characterized by their number of protons, Z, their number of neutrons, N, and their nuclear energy state.

<span class="mw-page-title-main">Decay chain</span> Series of radioactive decays

In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". The typical radioisotope does not decay directly to a stable state, but rather it decays to another radioisotope. Thus there is usually a series of decays until the atom has become a stable isotope, meaning that the nucleus of the atom has reached a stable state.

Uranium (92U) is a naturally occurring radioactive element (radioelement) with no stable isotopes. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from 214U to 242U. The standard atomic weight of natural uranium is 238.02891(3).

Lead (82Pb) has four observationally stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uranium series, the actinium series, and the thorium series, respectively; a fourth decay chain, the neptunium series, terminates with the thallium isotope 205Tl. The three series terminating in lead represent the decay chain products of long-lived primordial 238U, 235U, and 232Th. Each isotope also occurs, to some extent, as primordial isotopes that were made in supernovae, rather than radiogenically as daughter products. The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium.

Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was 239Np in 1940, produced by bombarding 238
U
 with neutrons to produce 239
U
, which then underwent beta decay to 239
Np
.

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being plutonium-238 in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years; plutonium-242 with a half-life of 373,300 years; and plutonium-239 with a half-life of 24,110 years; and plutonium-240 with a half-life of 6,560 years. This element also has eight meta states; all have half-lives of less than one second.

<span class="mw-page-title-main">Actinides in the environment</span>

The actinide series is a group of chemical elements with atomic numbers ranging from 89 to 102, including notable elements such as uranium and plutonium. The nuclides thorium-232, uranium-235, and uranium-238 occur primordially, while trace quantities of actinium, protactinium, neptunium, and plutonium exist as a result of radioactive decay and neutron capture of uranium. These elements are far more radioactive than the naturally occurring thorium and uranium, and thus have much shorter half-lives. Elements with atomic numbers greater than 94 do not exist naturally on Earth, and must be produced in a nuclear reactor. However, certain isotopes of elements up to californium still have practical applications which take advantage of their radioactive properties.

Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

<span class="mw-page-title-main">Plutonium-244</span> Isotope of plutonium

Plutonium-244 (244Pu) is an isotope of plutonium that has a half-life of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any other actinide isotope except for the three naturally abundant ones: uranium-235, uranium-238, and thorium-232. Given the mathematics of the decay of plutonium-244, an exceedingly small amount should still be present in the Earth's composition, making plutonium a likely although unproven candidate as the shortest lived primordial element.

An extinct radionuclide is a radionuclide that was formed by nucleosynthesis before the formation of the Solar System, about 4.6 billion years ago, but has since decayed to virtually zero abundance and is no longer detectable as a primordial nuclide. Extinct radionuclides were generated by various processes in the early Solar system, and became part of the composition of meteorites and protoplanets. All widely documented extinct radionuclides have half-lives shorter than 100 million years.

<span class="mw-page-title-main">Isotope</span> Different atoms of the same element

Isotopes are distinct nuclear species of the same chemical element. They have the same atomic number and position in the periodic table, but differ in nucleon numbers due to different numbers of neutrons in their nuclei. While all isotopes of a given element have similar chemical properties, they have different atomic masses and physical properties.

<span class="mw-page-title-main">Monoisotopic element</span> Element that has only a single stable isotope

A monoisotopic element is an element which has only a single stable isotope (nuclide). There are 26 such elements, as listed.

<span class="mw-page-title-main">Radiogenic nuclide</span>

A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive or stable.

<span class="mw-page-title-main">Nuclear transmutation</span> Conversion of an atom from one element to another

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.

<span class="mw-page-title-main">Even and odd atomic nuclei</span> Nuclear physics classification method

In nuclear physics, properties of a nucleus depend on evenness or oddness of its atomic number Z, neutron number N and, consequently, of their sum, the mass number A. Most importantly, oddness of both Z and N tends to lower the nuclear binding energy, making odd nuclei generally less stable. This effect is not only experimentally observed, but is included in the semi-empirical mass formula and explained by some other nuclear models, such as the nuclear shell model. This difference of nuclear binding energy between neighbouring nuclei, especially of odd-A isobars, has important consequences for beta decay.

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. Bibcode:2006Ana...131.1332M. doi:10.1039/b608157f. PMID   17124541.
  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. Bibcode:1971Natur.234..132H. doi:10.1038/234132a0. S2CID   4283169.
  3. ↑ Lachner, J.; et al. (2012). "Attempt to detect primordial 244Pu on Earth". Physical Review C. 85 (1): 015801. Bibcode:2012PhRvC..85a5801L. doi:10.1103/PhysRevC.85.015801.
  4. 1 2 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 . 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   9781466591745 . Retrieved 13 July 2020.
  7. ↑ H.-T. Shen; et al. "Research on measurement of 126Sn by AMS" (PDF). accelconf.web.cern.ch. Archived from the original (PDF) on 2017-11-25. Retrieved 2018-02-06.
  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, Bibcode:1999GeCoA..63..275C, doi:10.1016/S0016-7037(98)00282-8 {{citation}}: CS1 maint: multiple names: authors list (link)
  9. ↑ "Interactive Chart of Nuclides (Nudat2.5)". National Nuclear Data Center . Retrieved 2009-06-22.
  10. 1 2 3 Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
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