How can scientists date things such as the Dead Sea Scrolls so confidently

A.� By using a chemical dating technique called radiocarbon dating.

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Q.� When was radiocarbon dating discovered

A.� By a team of scientists led by the late Professor Willard F. Libby of the University of Chicago after the second world war.

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Q.� How does radiocarbon dating work

A.� Carbon has three different isotopes, one of which, carbon 14, usually abbreviated to C14, is unstable and reactive.

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C14 is created in the atmosphere and absorbed by all living things via photosynthesis after which it enters the food chain. However, once an organism dies, it stops absorbing carbon 14 and the unstable isotope begins to decay, doing so at a constant rate.

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Q.� How fast does C14 decay

A.� After 5730 years, half the C14 in the original sample will have decayed and after another 5730 years, half of that remaining material will have decayed, and so on. So, based on the amount of remaining C14 in a sample in comparison with modern concentrations, scientists can determine how long it is since an organism died, indicating its age.

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Q.� What are the age limits for C14 dating

A.� Because of the rate of decay any dead organism will have no C14 left after around 60,000 years and material older than this has to be dated using other techniques.

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Also, it is difficult to reliably date materials less than 300 years by the radiocarbon method as they give false results. The last 300 years have seen large amounts of radiocarbon released into the atmosphere, mainly through the burning of fossil fuels and nuclear testing, leaving more modern samples with artificially high levels of C14.

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Q.� What can radiocarbon dating be used for

A.� Any organic material, including bone, wood and plant material, which brings us back to our original question: The Dead Sea Scrolls are made from papyrus and so archaeologists are able to say when the reeds from which the scrolls were made were cut.

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If you're interested in how another dating technique, dendrochronoloy, works, click here. Or why not ask a question about any other scientific technique here.

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by Lisa Cardy