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Other radio-dating techniques are used to date ancient rocks.We can plot a graph of radioactivity against time for our sample that had a half-life of 10 years.We’re going to see what 'half-life' means and why radioactivity changes with time. It doesn’t depend on the size of the sample and it doesn’t change with time. So we imagine going in forward one half-life at a time from ZERO years: 10 years, 20 years, 30 years, etc.

For example, it might take 10 years for the count rate to drop from 80 Bq to 40 Bq; another 10 years to drop from 40 B to 20 Bq; another 10 years to drop from 20 Bq to 10 Bq and so on. The half-life of a particular isotope is always the same. The unit of radioactivity is named after Henri Becquerel, who discovered it. A given isotope always takes the same amount of time for the count rate to decrease by a half.Remember that the carbon-14 decays all the time whether the thing's alive or not.It's just that when it's living the carbon-14 is constantly replaced so the overall radioactivity stays constant. We don’t just stick a Geiger counter in front of it and hope for the best.An equilibrium is reached whereby about one in a trillion carbon atoms in the atmosphere is carbon-14.

Carbon-14 in living things decays all the time but is replaced by carbon-14 in food. In fact, all living things are rearranged food and NOTHING else.

The carbon dioxide is separated out from the other gases.

It is mostly carbon-12 with tiny amounts of the radioactive carbon-14. For example, a sample with a count of only 25% of atmospheric carbon dioxide must be two half-lives old: 100% - 50% takes 1 half-life 50% - 25% takes a second half-life If the half-life is 5600 years then the sample must be 5600 x 2 = 11 200 years old.

We measure the radioactivity of the carbon dioxide in a special chamber to shield it from background radiation. You can use a much smaller sample of the material you want to test if you count the carbon-14 atoms directly rather than having to wait for them to decay.

We can then compare it with the radioactivity of the same amount of carbon dioxide from the atmosphere. Even this kind of carbon dating can only be used to date things that were once alive and died less than about 60 000 years ago.

So you can pick any period of time, say 1 minute, and measure how much the radioactivity drops to in that minute. After another minute it will drop to 63% of this value. Now 63% isn't a very obvious number so we pick a time where the drop is something simple, like 50%, or a half.