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Concept
Primordial nuclide
Summary
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.
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 (, ; , ; , ; , ; , ; , ; and , ).
Because the age of the Earth is 4.58e9years (4.6 billion years), the half-life of the given nuclides must be greater than about e8years (100 million years) for practical considerations. For example, for a nuclide with half-life 6e7years (60 million years), this means 77 half-lives have elapsed, meaning that for each mole (6.02e23atoms) of that nuclide being present at the formation of Earth, only 4 atoms remain today.
The four shortest-lived primordial nuclides (i.e., the nuclides with the shortest half-lives) to have been experimentally verified are (1.4e10years), (4.5e9years), (1.25e9years), and (7.0e8years).
These are the four nuclides with half-lives comparable to, or somewhat less than, the estimated age of the universe. (232Th has a half life slightly longer than the age of the universe.) For a complete list of the 35 known primordial radionuclides, including the next 30 with half-lives much longer than the age of the universe, see the complete list below.
Official source
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