Radiocarbon dating, discovered in 1947 by chemist Willard Libby, can be used to work out the age of any material that was once alive, from coal – which comes from trees – to human bones.

All living things contain carbon, which they either absorb from the atmosphere (plants) or take in as their food (animals). Most of that carbon is a stable form called C12 but a tiny proportion is an unstable, radioactive form called C14 or radiocarbon. Radiocarbon decays very slowly at a constant rate.

The proportion of radiocarbon in any animal or plant remains constant throughout its life – some decays but this is replaced by eating or breathing. But once the organism dies, the proportion of C14 starts to fall as the decaying molecules are no longer replaced.

As each atom decays, it changes from carbon to nitrogen and in the process emits a particle called an electron – this is radioactivity. These radioactive emissions can be measured to find out how much radiocarbon has decayed and how much still remains. C14 has a half-life of 5,730 years, which means that the level of radioactivity will fall by half its original amount over that period. So from the proportion of C14 to C12 in the organism, scientists can work out reliably how long ago it died.

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The Dead Sea Scrolls
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