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Radiocarbon dating is used to date

radiocarbon dating is used to date-9

And after 11,460 years (two half-lives), only a quarter of the original carbon-14 atoms are left.

But when we stop eating, or when plants stop photosynthesising, our carbon-14 levels no longer get topped up.(You can read up on radioactivity and isotopes here).Carbon-14, the radioactive version of carbon, is rare — it only makes up one trillionth of all the carbon in the world.But old age isn't the only thing that affects the accuracy of carbon dating.The level of radiocarbon in the atmosphere has varied over time — it was about two per cent higher 3,500 years ago, possibly due to factors affecting cosmic rays (like changes in solar cycles or the Earth's magnetic field).When those speedy protons hit atoms you end up with a few stray neutrons zipping around the place.

And when one of those energetic neutrons hits a nitrogen atom, the nitrogen spits out a proton.

Chemically, carbon-14 is no different from non-radioactive carbon atoms, so it ends up in all the usual carbon places — one trillionth of the carbon atoms in air, plants, animals and us are radioactive.

All radioactive atoms eventually decay into something more stable, and carbon-14 decays into nitrogen.

Radiocarbon dating is used to work out the age of things that died up to 50,000 years ago. As far as working out the age of long-dead things goes, carbon has got a few things going for it. The proteins, carbohydrates and fats that make up much of our tissues are all based on carbon.

Everything from the fibres in the Shroud of Turin to Otzi the Iceman has had their birthday determined the carbon-14 way. There's plenty of hydrogen, oxygen and nitrogen in living things too, but carbon's got something none of them do — a radioactive isotope that can take thousands of years to decay.

From the moment we die the proportion of carbon-14 compared to non-radioactive carbon-12 in what's left of our bodies starts to drop as it gradually turns to nitrogen.