# Radiocarbon dating of the iceman

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.And the longer dead things lie around, the lower the carbon-14 levels get.

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.

With an extra neutron and one less proton, that's no longer a nitrogen atom — six protons plus eight neutrons spells carbon-14.

The newly formed carbon-14 atoms end up in carbon dioxide, which ends up in plants, which end up on our dinner plates as fruit, veg or a highly processed version of plants known as meat.

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).If you know the rate that carbon-14 decays at, and how much of the carbon in a shroud, **iceman** or piece of old wood or bone is radioactive, you can work out how long ago they stopped breathing or photosynthesising. We know that on average it takes an atom of carbon-14 a little over 8,000 years to decay to nitrogen (although you never know when an individual atom is going to decay — it's completely random). But the value that's used to calculate the age of an object isn't an absolute figure, it's a statistical term called half-life.We even know that in a gram of carbon, 14 carbon-14 atoms turn into nitrogen every minute. The half-life of a radioactive isotope is the amount of time it takes for half of the atoms in a sample to decay. That means that no matter how many carbon-14 atoms were present when something died, after 5,730 years only half of them are left — the rest have decayed to nitrogen.So the proportion of carbon-14 inside living things is the same as the proportion of carbon-14 in the atmosphere at that time.But when we stop eating, or when plants stop photosynthesising, our carbon-14 levels no longer get topped up.And nuclear reactions have seen a leap in carbon-14 activity since 1945.