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Carbon dating using mass spectroscopy

Most carbon-14 atoms don't decay while you're watching

Carbon dating involves measuring very small radioactivities.

This means the sample needs to be quite big, typically a few grams, otherwise the radioactivity is too small to measure.  It’s only good if you don’t mind burning quite a lot of the object you’re trying to date.

The problem with measuring radioactivity is that you need the carbon-14 atoms to actually decay in order to detect the radiation.

You need around a billion carbon-14 atoms to see 1 decay a second.

Just count the carbon-14 atoms regardless of whether they decay

But there’s a better way that doesn’t rely on detecting the actual decays, which can date samples up to 100 000 years old.  As we’re about to see, radioactivity decreases with time because there are fewer undecayed nuclei around TO decay.

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A technique called mass spectroscopy enables the carbon-14 and carbon-12 atoms to be counted directly.  No radiation needs to be detected.

By finding the proportion of carbon-14 atoms remaining you can calculate the age of the sample.  For example if there are only 25% of the carbon-14 atoms to carbon-12 atoms that there are in the atmosphere then the sample must be 2 half-lives old.

This technique is better because you use all the carbon-14 atoms, not just the ones that happen to be decaying at the time.  Because of this you can use a much smaller sample: as little as a few milligrams.

back to Lesson 15: Half-life part 2