September 2009—The development of radiocarbon dating in the 1940s transformed the field of archaeology.Finally, archaeologists could determine the ages of their finds—provided they were organic and between 500 and 50,000 years old.For example, a tree that died 5,730 years ago will only have half as much carbon-14 as a modern living tree does.Measuring the ratio of carbon-14 to stable carbon-12 in an organic sample enables the accurate dating of animal and human bones, mummified bodies, deep-sea sediments, and wood and leather artifacts.The ratio of carbon-14 thus formed to regular carbon-12 in the atmosphere is only about one part in a trillion.Carbon-14 decays radioactively back to nitrogen with a “half-life” of 5,730 years—a half-life is the time it takes for an unstable radioactive sample to decay to half its original amount.
It was developed right after World War II by Willard F.
Most carbon on earth is in its stable form, carbon-12, the nucleus of which contains 6 protons and 6 neutrons.
However, a very small amount of the radioactively unstable isotope carbon-14 (with 6 protons and 8 neutrons) is constantly being formed in the upper atmosphere by cosmic rays from space interacting with nitrogen.
The carbon-14 atoms combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis.
Animals and people take in carbon-14 by eating the plants.
When these energetic neutrons collide with a nitrogen-14 (seven protons, seven neutrons) atom it turns into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons).