Carbon dating reference
Carbon-14 is only a small percentage of the total carbon in the environment and originates in the atmosphere when cosmic rays and high-speed particles from space hit nitrogen atoms.Although the physics are more complicated than this, essentially the nitrogen atoms lose a proton and gain a neutron to become carbon-14.
Carbon-14 has a half-life of 5,730 years, which is the amount of time that it takes for half of a given sample of carbon-14 to decay.
It must be 1 carbon 14 half-life (or 5730 years) old.' This is the basic idea behind carbon dating. In the atmosphere, cosmic rays smash into normal carbon 12 atoms (in atmospheric carbon dioxide), and create carbon 14 isotopes.
So in the real world, looking at a sample like say a bone dug up by an archaeologist, how do we know how much carbon 14 we started with? This process is constantly occurring, and has been for a very long time, so there is a fairly constant ratio of carbon 14 atoms to carbon 12 atoms in the atmosphere.
Carbon 14 has a half-life of 5,780 years, and is continuously created in Earth's atmosphere through the interaction of nitrogen and gamma rays from outer space.
Because atmospheric carbon 14 arises at about the same rate that the atom decays, Earth's levels of carbon 14 have remained fairly constant.