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Carbon 14 is continually being created in the Earth's atmosphere by the interaction of nitrogen and gamma rays from outer space.

Since atmospheric carbon 14 arises at about the same rate that the atom decays, the Earth's levels of carbon 14 have remained constant.

Libby thus reasoned that by measuring carbon 14 levels in the remains of an organism that died long ago, one could estimate the time of its death.This procedure of radiocarbon dating has been widely adopted and is considered accurate enough for practical use to study remains up to 50,000 years old.Because atmospheric carbon 14 arises at about the same rate that the atom decays, Earth's levels of carbon 14 have remained fairly constant.Once an organism is dead, however, no new carbon is actively absorbed by its tissues, and its carbon 14 gradually decays.A very small percentage of carbon, however, consists of the isotope carbon 14, or radiocarbon, which is unstable.

Carbon 14 has a half-life of 5,780 years, and is continuously created in Earth's atmosphere through the interaction of nitrogen and gamma rays from outer space.Plants and animals naturally incorporate both the abundant C-12 isotope and the much rarer radiocarbon isotope into their tissues in about the same proportions as the two occur in the atmosphere during their lifetimes.When a creature dies, it ceases to consume more radiocarbon while the C-14 already in its body continues to decay back into nitrogen.It is naturally unstable and so it will spontaneously decay back into N-14 after a period of time.It takes about 5,730 years for half of a sample of radiocarbon to decay back into nitrogen.Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.