Go back to main page  

IGERT program in Archeological Science

Principles of AMS

Radiocarbon dating





Age Calculation


Cosmogenic Radioisotopes



production of C-14 diagram

Diagram of production of carbon-14 in the upper atmosphere
by reaction of neutrons with nitrogen, and of the subsequent
incorporation of carbon-14 into the biosphere.

    Carbon-14 is produced in the upper atmosphere by secondary neutrons, which are the result of the reaction of cosmic radiation with atmospheric nitrogen. The "hot" carbon-14 atom rapidly reacts with oxygen to produce CO, which is also rapidly oxidized to CO2 [Pandow et al. 1960]. Nearly all the carbon in the atmosphere is present as carbon dioxide. We know that CO2 in the atmosphere maintains an equilibrium with the biosphere and the oceans. As plants absorb carbon from the atmosphere during photosynthesis, and as animals eat plants, they form what is known as the "carbon cycle" and all terrestrial plants and animals will also contain about the same level of 14C. We also know that plants and animals in the ocean maintain a different level of 14C from those on land, since the carbon cycle in the oceans turns over more slowly.

    The principle behind radiocarbon dating is when a living plant or animal dies, it ceases to take up 14C, and thus no longer maintains an equilibrium level of 14C with this carbon cycle. The amount of 14C in the carbon from this material will then decay from the equilibrium level. Radioactive decay is a statistical process, so the number of 14C atoms decaying in a given time is proportional only to the number of 14C atoms present.

arrow back Back