What radioactive isotope of carbon is used in radiocarbon dating?

Carbon-14 is the radioactive isotope of carbon used in radiocarbon dating.

Carbon-14 contains six protons and eight neutrons, unlike stable carbon-12 and carbon-13. Cosmic rays help create it in the atmosphere when neutrons interact with nitrogen-14. The resulting carbon-14 becomes part of atmospheric carbon dioxide, which plants absorb through photosynthesis; animals acquire it through the food chain.

When an organism dies, it stops exchanging carbon with its environment. Its carbon-14 then decays into nitrogen-14, with a half-life of about 5,730 years. By measuring the remaining amount, scientists can estimate when the organism lived, usually for samples no older than roughly 50,000–60,000 years.

A common mix-up is confusing carbon-14 with carbon-12 or carbon-13, which are stable isotopes. The method was pioneered by Willard Libby and colleagues in the late 1940s, earning Libby the 1960 Nobel Prize in Chemistry. Radiocarbon dates are calibrated using records such as tree rings because atmospheric carbon-14 levels have varied over time.