The Earth is constantly being bombarded by 'cosmic rays', highly energetic particles coming from the depths of space. These particles (known as primary) can be either electrically charged - with protons making up 86% of charged particles, alphas making up 13% and the remainder consisting of heavy nuclei - or neutral, in which case they primarily consist of photons and neutrinos.
The energy of these cosmic rays varies enormously, as the variable magnetic field of the stars is capable of accelerating certain charged particles to energies of over 1 billion electronvolts and much more. When these particles then approach the Earth they are deflected by our planet magnetic field, which serves as a giant shield against these extraterrestrial bombardments. This shield is weakest, however, near the magnetic poles, and it is the cosmic rays which manage to penetrate that are the cause of the aurora borealis.
Upon entering the higher layers of the atmosphere the cosmic rays start colliding with nuclei, causing an explosion of pieces of subatomic shrapnel which in turn crash into other atoms. This chain reaction of collisions produces a host of 'secondary' particles, some of which will reach the Earth surface like muons (particles similar to electrons, but 200 times heavier and highly unstable) and neutrinos (*).
Muons interact little and do not produce radioactivity. Neutrinos, which are capable of passing unhindered through an object as wide as a star, very rarely produce any secondary particles.
During the development of the shower of particles that follows the initial collision, many of the atomic nuclei in the atmosphere that serve as a target could be broken, protons and neutrons emitted. These neutrons can penetrate into the nuclei of other atoms, mainly oxygen and nitrogen, to make them radioactive. These radioactive nuclei, byproducts of cosmic radiations, are called cosmogenic.
One of these neutrons entering a nucleus of nitrogen from the air triggers the ejection of a proton or a proton bound to two neutrons (a tritium nucleus). In the first case, the nitrogen is transformed into radioactive carbon-14; in the second case the nitrogen is transformed into stable carbon-12 but is accompanied by radioactive tritium.
Among the “cosmogenic” nuclei created in these atmospheric collisions, some are radioactive: notably carbon 14, tritium, beryllium 7 and beryllium 10. Some of these decay rapidly whereas others, such as carbon 14 (created from nitrogen found in the air) live for longer. Molecules of this carbon 14 are therefore often absorbed by plants alongside ordinary carbon, meaning that all living matter contains traces of this radioisotope.
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