From Remnants of the Earth’s Formation to Cosmic Rays
2026 Editorial Revision
DISTANT SUPERNOVA EXPLOSION:
The explosion of a supernova, one of the major cataclysms of the universe, is a rare event on the scale of a galaxy. It seeds intergalactic space with elements heavier than iron, which later become incorporated into newly forming stars and planets. Some of these heavy elements, such as radioactive thorium and uranium, have survived because of their extremely long lifetimes. This remarkably sharp image was taken in 1996 by the ISO satellite.
© CEA/ESA/ISO
Humans have always been exposed to natural radioactivity, although its existence remained unsuspected until the end of the 19th century. It was only in 1896, with Henri Becquerel’s discovery of uranium rays, that humanity became aware of this radiation and began to investigate its many origins.
Natural radioactivity comes mainly from radioactive elements produced in stars billions of years ago. Traces of these radioactive elements and their decay products can still be found in our environment.
Four elements dating back to the birth and death of stars have survived because their lifetimes are measured in billions of years: two isotopes of uranium, uranium-238 and uranium-235; thorium-232; and finally potassium-40, which accounts for approximately one ten-thousandth of natural potassium.
As uranium and thorium nuclei decay, they are transformed through radioactive “decay chains” into a succession of radioactive elements. These decay products are found in uranium and thorium ores in radioactive equilibrium (meaning that, for each of these decay products, as many nuclei are produced as decay). The best known are radium and polonium, identified by Pierre and Marie Curie, and radon, because of its significant contribution to natural radioactivity. Eventually, after billions of years and after passing through the entire series of their decay products, uranium and thorium nuclei become stable lead nuclei.
Natural radioactivity also results from the bombardment of the Earth by high-energy particles originating from space: cosmic rays. The atmosphere and the Earth’s magnetic field act as shields, reducing their impact. The bombardment of atoms in the atmosphere also generates radioactive elements, the best known of which are carbon-14 and tritium.
All these sources of natural radioactivity are modest and remain relatively constant on human timescales. Overall, the effects of exposure are low or minor, as illustrated by the abundance and diversity of living species. When life first appeared on Earth, natural radioactivity from uranium was twice as high as it is today.
Over the past few decades, new sources associated with human activity have been added to these natural sources. The most significant comes from medical procedures and treatments.
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