Primordial radionuclide
In geochemistry, geophysics 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
).
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