Primordial nuclide

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

In geochemistry 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 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 253 stable nuclides occur as primordial nuclides, plus another 33 nuclides that have half-lives long enough to have survived from the formation of the Earth. These 33 primordial radionuclides represent isotopes of 28 separate elements. Cadmium, tellurium, neodymium, samarium and uranium each have two primordial radioisotopes (113
Cd
, 116
Cd
; 128
Te
, 130
Te
; 144
Nd
, 150
Nd
; 147
Sm
, 148
Sm
; and 235
U
, 238
U
).

Because the age of the Earth is 4.58×109 years (4.6 billion years), this means that 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 shortest-lived primordial nuclides (i.e. nuclides with shortest half-lives) are:

..., 232
Th
, 238
U
, 40
K
, and 235
U
.

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 33 known primordial radionuclides, including the next 29 with half-lives much longer than the age of the universe, see the complete list in the section below. For practical purposes, nuclides with half-lives much longer than the age of the universe may be treated as if they really were stable. 232Th and 238U have half-lives long enough that their decay is negligible 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 244
Pu
, 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.[1] Likewise, the second-longest-lived non-primordial 146
Sm
, was once reported as primordial, but this could not be replicated.[2] Taking into account that all these nuclides must exist since at least 4.6×109 years, meaning survive 57 half-lives, their original number is now reduced by a factor of 257 which equals more than 1017: thus any remaining primordial 244Pu and 146Sm would only exist on Earth in negligibly small quantities, if they still exist at all.[3]

Although it is estimated that about 33 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 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
.[4] 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 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 239
Pu
). 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 232Th. 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).

Primordial elements

There are 253 stable primordial nuclides and 33 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 numbers of elements are smaller, because many primordial elements are represented by more than one primordial nuclide. See chemical element for more information.

Naturally occurring stable nuclides

As noted, these number about 253. For a list, see the article list of stable isotopes. For a complete list noting which of the "stable" 253 nuclides may be in some respect unstable, see list of nuclides and stable isotope. 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.

List of 33 radioactive primordial nuclides and measured half-lives

These 33 primordial nuclides represent radioisotopes of 29 distinct chemical elements (cadmium, neodymium, samarium, tellurium, and uranium 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, a very long lived radioactive primordial nuclide is found to be the most abundant nuclide for an element that also has a stable nuclide. These unusual elements are tellurium, indium, and rhenium.

The longest has a half-life of 2.2×1024 years, which is 160 trillion times the age of the Universe (the latter is about 4.32×1017 s). Only four of these 33 nuclides have half-lives shorter than, or equal to, the age of the universe. Most of the remaining 29 have half-lives much longer. The shortest-lived primordial isotope, 235U, has a half-life of 704 million years, about one sixth of the age of the Earth and Solar System.

nonuclideenergyhalf-life (years)decay modedecay energy (MeV)approx ratio half-life to age of universe
254128Te8.7432612.2×10242 β 2.530160 trillion
25578Kr9.0223499.2×1021 KK 2.846670 billion
256136Xe8.7068052.165×10212 β 2.462150 billion
25776Ge9.0346561.8×10212 β 2.039130 billion
258130Ba8.7425741.2×1021 KK 2.62090 billion
25982Se9.0175961.1×10202 β 2.9958 billion
260116Cd8.8361463.102×10192 β 2.8092 billion
26148Ca8.9924522.301×10192 β 4.274, .00582 billion
26296Zr8.9613592.0×10192 β 3.41 billion
263209Bi8.1586891.9×1019α 3.1371 billion
264130Te8.7665788.806×10182 β .868600 million
265150Nd8.5625947.905×10182 β 3.367600 million
266100Mo8.9331677.804×10182 β 3.035600 million
267151Eu8.5657595.004×1018α 1.9644300 million
268180W8.3471271.801×1018α 2.509100 million
26950V9.0557591.4×1017β+ or β 2.205, 1.03810 million
270113Cd8.8593727.7×1015β .321600,000
271148Sm8.6074237.005×1015α 1.986500,000
272144Nd8.6529472.292×1015α 1.905200,000
273186Os8.3025082.002×1015α 2.823100,000
274174Hf8.3922872.002×1015α 2.497100,000
275115In8.8499104.4×1014β .49930,000
276152Gd8.5628681.1×1014α 2.2038000
277190Pt8.2677646.5×1011α 3.25260
278147Sm8.6105931.061×1011α 2.3108
279138La8.6983201.021×1011K or β 1.737, 1.0447
28087Rb9.0437184.972×1010β .2834
281187Re8.2917324.122×1010β or α .0026, 1.6533
282176Lu8.3746653.764×1010β 1.1933
283232Th7.9185331.406×1010α or SF 4.0831
284238U7.8725514.471×109α or SF or 2 β 4.2700.3
28540K8.9097071.25×109β or K or β+ 1.311, 1.505, 1.5050.09
286235U7.8971987.04×108α or SF 4.6790.05

List legends

no (number)

A running positive integer for reference. These numbers may change slightly in the future since there are 163 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 254, to follow the 253 nuclides (or stable isotopes) not yet found to be radioactive.

nuclide column

Nuclide identifiers are given by their mass number A and the symbol for the corresponding chemical element (implies a unique proton number).

energy column

The column labeled "energy" denotes the 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: mnmnuclide / A.

half-life column

All times are given in years.

decay mode column
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 column

Multiple values for (maximal) decay energy in MeV are mapped to decay modes in their order.

See also

References

  1. D.C. Hoffman; F.O. Lawrence; J.L. Mewherter; F.M. Rourke (1971). "Detection of Plutonium-244 in Nature". Nature. 234 (5325): 132–134. Bibcode:1971Natur.234..132H. doi:10.1038/234132a0.
  2. S. Maji; S. Lahiri; B. Wierczinski; G. Korschinek (2006). "Separation of samarium and neodymium: a prerequisite for getting signals from nuclear synthesis". Analyst. 131 (12): 1332–1334. Bibcode:2006Ana...131.1332M. PMID 17124541. doi:10.1039/b608157f.
  3. 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.
  4. "Interactive Chart of Nuclides (Nudat2.5)". National Nuclear Data Center. Retrieved 2009-06-22.
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