Radioelements as old as the Earth and their descendants
Editorial revision 2026
The Crab Nebula
The Crab Nebula is the remnant of an explosion that occurred 6,000 years ago and was observed on Earth in 1054 by Chinese astronomers and Navajo Indians. It is during the explosion of certain supernovae that heavy elements such as platinum, gold and uranium are produced through a rapid succession of neutron captures. These elements are dispersed through the galactic space by the explosion.
© IN2P3/NEPAL
The main source of natural radioactivity is due to the presence in the Earth’s crust of three primordial radioactive nuclei: thorium-232, uranium-235 and uranium-238. Because of their very long lifetimes (of the order of a billion or several billion years), they have not yet disappeared and for this reason are an integral part of our environment.
Primordial means that these elements have been present since the formation of the Solar System and the Earth. In addition to these three primordial radionuclides, a handful of radionuclides have half-lives exceeding 10 billion years and have no radioactive descendants. The most important is an isotope of potassium, potassium-40, which is assimilated by living organisms, followed by rubidium-87.
These three primordial radioactive nuclei are the heaviest observed in nature. They were already present in clouds of interstellar dust whose aggregation led to the formation of stars and planets, as happened for the Sun and the Earth 4.5 billion years ago. These clouds are produced during supernova explosions, spectacular events that mark the end of the lives of very massive stars and make it possible to produce nuclei heavier than iron.
The thorium decay chain
Natural thorium (thorium-232), like uranium-238, is at the origin of a succession of radioactive elements that ends with a stable isotope of lead, lead-208. Like the uranium decay chain, the thorium decay chain includes a gaseous element, radon-220, historically called thoron. This radioactive emanation is less dangerous than the radon from uranium. Since the half-life of thoron is only 55 seconds, the gas has practically no time to escape from the rock.
© IN2P3
« Stardust », thorium-232, uranium-235 and uranium-238 are the ancestors of three radioactive families and generate descendants of other radioactive nuclei with shorter lifetimes. Since uranium-235 has largely disappeared since the formation of the Earth, the two main families are those of thorium-232 and uranium-238.
These two radionuclides, whose lifetimes are measured in billions of years, are the ancestors, progenitors and sources of these two radioactive families, whose descendants coexist with them in rocks and the natural environment. Thus, in granite, traces of radium can be found alongside uranium. This radioactive element has a lifetime of only 1,600 years. It would have disappeared long ago if it were not continuously regenerated as a member of the uranium-238 decay chain.
The « terrestrial » radiation, the radiation from rocks, caused by the presence of uranium, thorium and their descendants, leads to external exposure, of which gamma rays are the main source. The intensity of the radiation escaping from rocks is measured one metre above the ground. The effective dose resulting from exposure over one year is then assessed according to the energy and nature of the radiation. The result is generally expressed in milligray (one thousandth of a Gray or mGy) per year. The worldwide average is estimated at 0.4 mGy per year.
Uranium-238 decay chain
The radioactive decay chains of uranium and thorium are partly responsible for natural radioactivity. Their descendants are found in trace amounts in uranium and thorium ores, for example, 0.34 grams of radium and 0.0000012 milligrams of polonium per tonne of uranium. The uranium-238 decay chain ends with stable lead-206, and the activities of uranium-238 and its 13 descendants are equal. It was by observing that two uranium minerals, pitchblende and chalcolite, were more active than uranium itself that Pierre and Marie Curie discovered polonium and radium, despite their extremely low concentrations.
© IN2P3
In France, the intensity averages 0.7 mGy per year, but can reach 2.5 mGy per year in granitic regions such as certain parts of Brittany, where granite is rich in radioactive elements. These figures remain well below the levels of radiation observed on some black sand beaches in Brazil or southern India, which exceed 50 mGy per year because of their richness in monazite, a mineral containing nearly 10% radioactive thorium.
Terrestrial radiation is supplemented by the contribution of radon and its radioactive descendants. Radon is the only descendant of uranium and thorium that is gaseous and able to leave rocks and enter the atmosphere. The 3.8-day radioactive half-life of uranium radon (radon-222) gives it enough time to do so. The difference between this radon-222 and thorium radon (the thoron or radon-220) is that the latter exists for only 56 seconds. This very short radioactive half-life gives thoron very little time to escape from rocks.
Radon is the main natural source of exposure to radioactivity: approximately 1 millisievert per year on average in France. Radon is produced during the decay of radium. As a gas, it emanates from rocks and disperses into the atmosphere. As a noble gas, it cannot bind to the body. However, its radioactive descendants can. Exposure to radon descendants also varies greatly from one place to another. Current regulations aim to protect against it, mainly through good ventilation.
Other articles on the subject « Natural Radioactivity »
Natural Origins
From Remnants of the Earth’s Formation to Cosmic Rays 2026 Editorial Revision Humans have a[...]
Cosmogenic Radioelements
The Formation of Radioelements from Cosmic Rays 2026 Editorial Revision The Earth is constantly b[...]
Natural Exposure
A chronic but benign exposure All exposure to radioactivity, whether natural or artificial, is me[...]
Ground Radioactivity
Terrestrial Exposure or Radioactivity Emanating from the Ground 2026 Editorial Revision The Earth[...]
Human Internal Exposure
Our bodies are also… slightly radioactive Editorial revision 2026 More than half of interna[...]
Radioactivity in food
In our glasses and on our plates … Editorial revision 2026 The very acts of breathing and w[...]