Primordial nuclide

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A radionuclide is called primordial ( Latin for "first order") if it was already present when the earth was formed and has not yet completely disintegrated. It therefore occurs in nature without being replenished through natural or technical processes. The term “primordial radionuclide” is usually shortened to “primordial nuclide”.

Assuming an earth age of 4.6 billion years, the half-life of a nuclide must be above 50 million years for there to be any chance of detection. This means that a maximum of 288 nuclides are possible. According to the current state of knowledge, these are divided into 253 stable and 35 primordial. The list, sorted by falling half-lives, ends with

The plutonium isotope 244 Pu (half-life 80 million years) could be detected as a radionuclide in 1971 using the method of mass spectrometry . Its half-life has expired more than 57 times in the ages , making it the most ephemeral primordial nuclide. Its original concentration was about 1.5 · 10 17 times as high as it is today. Its mass fraction in some ores is 10 −18 . However, later measurements with more sensitive methods did not detect any traces of 244 Pu in the same samples .

The primordial nuclides are mostly mixed with other, sometimes stable, isotopes of the same element. Other important primordial nuclides besides those already mentioned above are e.g. B. 190 Pt , 204 Pb , 209 Bi and 40 K . The latter - contained in all living organisms - has a half-life of 1.28 billion years.

The distinction between stable and primordial (radio) nuclides is difficult because of the long half-lives. For some theoretically unstable nuclides, the decay has not yet been proven experimentally. One example is the metastable nuclide 180m Ta , whose decay into the ground state 180 Ta has not yet been observed. The longest observed half-lives are in the range of quadrillion years ( 128 Te with 7.2 · 10 24  a).

With some primordial nuclides - in particular 235 U , 238 U and 232 Th  - the decay product (“daughter nuclide”) is not stable but also radioactive. This is the case with the aforementioned nuclides over several generations of daughter nuclides. If, as with the aforementioned nuclides, the daughter nuclides have shorter half-lives than the parent nuclide, a secular equilibrium is established after a longer period of time , in which the activity of the daughter nuclides is equal to the activity of the parent nuclides. In undisturbed rocks that contain uranium or thorium , all daughter nuclides of the uranium-radium and uranium-actinium decay series or the thorium decay series are therefore always included.

In some nuclide maps the primordial radionuclides are specially marked, e.g. B. in the Karlsruhe nuclide map by a black bar at the top of its colored field.

The longest-lived primordial nuclides are those that can only transform through the rare process of double beta decay, while single beta decay is not possible with them. The “record holder” is the mentioned Tellurium-128 with a half-life of 7.2 · 10 24  years (see list of isotopes / atomic number 51 to atomic number 60 ); this is roughly 520 trillion times the age of the universe.

Thorium-232, uranium-238, uranium-235 and potassium-40 are of practical importance - in technical terms or as part of natural exposure to terrestrial radiation .


  • Hanno Krieger: Radiation Physics, Dosimetry and Radiation Protection. Volume 1 Basics . 4th edition, Springer 1998, ISBN 978-3-519-33052-3 .
  • Winfried Koelzer: Lexicon on Nuclear Energy 2017 . KIT Scientific Publishing, ISBN 978-3-7315-0631-7 , p. 167.
  • Hans Volker Klapdor-Kleingrothaus and Andreas Staudt: Particle physics without accelerators. Teubner 1995, ISBN 978-3-519-03088-1 .

Individual evidence

  1. List of isotopes / atomic number 91 to atomic number 100 # 94 plutonium
  2. DC Hoffman, FO Lawrence, JL Mewherter, FM Rourke: Detection of Plutonium-244 in Nature. In: Nature . Vol. 234, 1971, pp. 132-134 ( doi: 10.1038 / 234132a0 ).
  3. J. Lachner: Attempt to detect primordial 244 Pu on Earth . In: Physical Review C . 85, 2012, p. 015801. doi : 10.1103 / PhysRevC.85.015801 .

See also