How is carbon-14 used to determine the age of an object?

Answer 1

Carbon-14 is an isotope of carbon, with carbon-12 being the primary isotope. Both exist in nature, used by plants, in a relatively stable ratio. Once the plant matter containing carbon-12 and carbon-14 dies, the equilibrium is no longer maintained. Carbon-12 is stable, but carbon-14 is radioactive, decaying with a half-life of 5730 years. By measuring the ratio of carbon-14 to carbon-12, we can estimate how long it has been since the object died.

Unfortunately, its not straightforward, as the natural ratio has varied over the centuries due to various effects, including cosmic ray levels, magnetic field level, and climate. When scientists use radiocarbon dating, they compensate for this based on other observations that tell us what the ratio was over time.

Carbon-14 is produced naturally in the upper atmosphere by the absorption of a thermal neutron (created by cosmic rays) by an ordinary Nitrogen-14 atom, producing Carbon-14 and a stray proton.

In the atmosphere, it disperses sufficiently rapidly (a few decades perhaps), that there is not much delay between its production and the absorption by a plant. In the ocean, the picture is quite different, with dispersion times measured perhaps in centuries before its absorption by a shellfish say. So these biota are much more difficult to date by Carbon-14 methods.

[In a laboratory we used Carbon-11 with a half-life of 20min for plant studies. This made for snappy transit between manufacture and experiment!]

Answer 2

Carbon-14 dating is used to determine the age of organic materials by measuring the decay of carbon-14 in the sample. Carbon-14 is a radioactive isotope with a known half-life, so by measuring the amount of remaining carbon-14 in a sample, scientists can calculate how long it has been since the organism died and stopped absorbing carbon.