Carbon dating, or radiocarbon dating, is a technique used to determine the age of organic materials (wood, animal fossils) that are up to 60,000 years old. The key to carbon dating is a nuclear process call radioactive decay. To explain how radioactive decay works we need to explain what is matter.
Anything that you could ever touch (solids), drink (liquids) or breathe (gases) is made up of atoms. Atoms are made up of three different subatomic particles: protons and neutrons, which are found in the centre of the atom, and electrons, located on the edge of the atom, orbiting the nucleus (a bit like planets around the Sun).
The number of protons determines what element the atom is; for example, hydrogen has 1 proton, helium has 2, carbon 6, nitrogen 7, oxygen 8, gold 79, and uranium has 92 (this is how elements are ordered on the periodic table). The number of neutrons determines what isotope the atom is of the element. The most common form of hydrogen has no neutrons, but it’s possible to have hydrogen with 1 proton and 1 or even 2 neutrons.
There are three different isotopes or carbon, which have 6, 7 or 8 neutrons that are called carbon-12 (12C), carbon-13 (13C) and carbon-14 (14C) respectively. 12C and 13C are quite stable; however 14C is not and radioactively decays into nitrogen-14 (14N) by turning a neutron into a proton and emitting radiation (a beta particle). Around 99% of the carbon on Earth is 12C, about 1% is 13C, and only one in every trillion carbon atoms is 14C.
The key to radioactive dating is knowing how quickly an unstable isotope decays. In the case of 12C it takes 5730 years for half of the original amount to decay into nitrogen – this timescale is called the half-life. So starting with 1 kilogram of 14C, after waiting 5,730 years there will be 500 grams of 14C, after waiting another half-life (a total of 11460 years) there will be 250 grams, and after total of 17,150 years there will be 125 grams of 14C left.
Through photosynthesis, plants absorb all isotopes of carbon from the carbon dioxide in the atmosphere. When an organism dies, it contains a ratio of 14C to 12C, but, as the 14C decays with no possibility of replenishment, the ratio decreases at a regular rate (the half-life of 14C). The measurement of 14C decay provides an indication of the age of any carbon-based material.
Other radioactive elements, such as uranium, can be used for dating non-organic objects such as rocks. Uranium-lead dating is very common as it occurs via two processes which allow for cross checking; uranium-235 decays into lead-207 with a half-life of around 700 million years, and uranium-238 decays into lead-206 with a half-life of around 4.5 billion years.
Did you know…
Scientists have using radioactive dating to calculate the age of the Earth, which is 4.54 billion years old.