Carbon-14

A by-product of cosmic rays

Carbon-14 (C-14) formation in the atmosphere

The nucleus of carbon-14 contains 6 protons and 8 neutrons, instead of the 6 and 6 found in ordinary carbon-12.
The imbalance makes radiocarbon a radioisotope with a half-life of 5,700 years, and an emitter of beta particles. This radioactive isotope of carbon is called radiocarbon.

The C-14 found in nature is constantly being regenerated by cosmic rays hitting the atmosphere. The regeneration rate has been virtually unchanged for centuries; it depends on the flux of particles bombarding the Earth and the strength of the magnetic field that diverts them. This magnetic shield, and consequently the particle flux, has slowly changed over time, so the amount of radiocarbon produced on Earth also varies.

Incoming cosmic rays create atoms of radiocarbon by colliding with nuclei in the upper atmosphere, liberating neutrons.
These neutrons in turn interact with atmospheric nitrogen, replacing one of its 7 protons with an extra neutron.
The resulting atom, now containing 6 protons and 8 neutrons, is one of C-14.

Carbon gases formed with ^14C are chemically indistinguishable from gases with the ordinary carbon, carbon-12.
The radioactive atom is absorbed by plants and living matter in the same way as its non-radioactive isotope; in every thousand billion (10¹²) atoms of carbon-12, there will be on average one atom of C-14.

This tiny ratio exists in all molecules involving carbon atoms, including all living matter. This is why radiocarbon, along with potassium-40, accounts for almost all the natural radioactivity of our body.

When a living organism dies, radioactive carbon is no longer absorbed, and the ratio of ^14C present begins to decrease.
The amount still present in a sample of what was once a living creature can thus be used to determine its age.

Radiocarbon can also be used as a radioactive marker.