The planet we live on has always had its own radioactivity, inherited from the explosion of the stars that forged the heavy elements before the formation of the solar system. Very long-lived radioactive isotopes — chiefly uranium and thorium — still survive today in rocks, billions of years later, and continue to decay.
These heavy nuclei do not reach stability through a single decay, but through long radioactive chains: a succession of and decays that, step by step, transform them into progressively lighter nuclei. The three main chains are the uranium-radium chain, the thorium chain and the actinium chain; all three end in stable isotopes of lead, the common terminus where the decays stop.
It is to these chains that much of the natural radioactive background is due: radon, for example, is a radioactive gas that forms as an intermediate link in the uranium chain present in the subsoil, and can accumulate in buildings. The natural background is not an anomaly, but the ordinary condition under which life on Earth evolved.
Collegamenti
Argomenti: Fisica nucleare Concetti: Decadimento radioattivo
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