The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant.
Maybe one in a trillion carbon atoms are carbon-14.
It is not uncommon for a cosmic ray to collide with an atom in the atmosphere, creating a secondary cosmic ray in the form of an energetic neutron, and for these energetic neutrons to collide with nitrogen atoms.
When the neutron collides, a nitrogen-14 (seven protons, seven neutrons) atom turns into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons).
On death this equilibrium is broken and the remaining carbon-14 slowly decays away without anymore replacing it from the environment. Carbon 14 percentages is what is used in radiometric dating (carbon dating) to give some idea how old a specimen might be.
Carbon-14 dating is normally used by archaeologists rather than geologists because it ceases to be accurate at ages over 50,000-60,000 years.
Carbon-14 is radioactive, with a half-life of about 5,700 years.
For more information on cosmic rays and half-life, as well as the process of radioactive decay, see How Nuclear Radiation Works.
Other types of radiometric dating, however, are good for hundreds of thousands or millions of years, and these are very useful for fossils.
In fact they can be used to estimate the ages of various kinds of rocks… Carbon 14 has a half life of 5,730 years and by checking the amount of the carbon 14 is in a fossil the can see how old it is. Radiocarbon dating can be done at a variety of research institutions including Woods Whole and UC Irvine.
Carbon dating is radiometric dating, using the carbon 14 isotope.
Carbon 14 is used for fossils of fairly recent origin, as it becomes less and less accurate beyond 10 half lives (about 50 thousand years). Longer lived isotopes such as uranium/uranium, uranium/thorium, and potassium/argon are used to date inorganic materials of volcanic origin, such as rock or layers of volcanic ash, and can yield results ranging…
Typically, Carbon 14 is used for fossil dating for fossils 100-30,000 years old, but older than that, Potassium 40 or Rubidium 87 are more effective. Carbon-14 has a short half-life and is normally only used by archaeologists or anyone working on sediments less than 50,000 years old. Scientists used many different isotopes for dating rocks in Radiometric Dating, uranium/lead, potassium/argon and others are used.