Materials technology
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last modified
November 2002

Accelerator Mass Spectrometry

The giant 14UD particle accelerator, is involved in a range studies on the isotopic composition of materials that allow their age or origin to be determined. The process is known as Accelerator Mass Spectrometry, and the measurements being made by researchers in the Department of Nuclear Physics are unbelievably sensitive.

When cosmic radiation from space interacts with the atmosphere and the surface layers of Earth, minute quantities of radioactive nuclei (such as carbon, aluminium and chlorine) are produced. These isotopes decay sufficiently slowly to be useful in dating human artefacts or natural features in the environment.

For example, if you measure the amount of decayed carbon 14 in an object and compared this with the amount of undecayed carbon 14 (or radiocarbon) you can estimate how old that object is (the older the object, the greater the ratio of decayed to undecayed carbon 14). This can be determined indirectly by measuring the radiation emitted by the carbon 14 as it decays. The process is called carbon dating. Unfortunately, you need to use a large sample to get an accurate ratio, something that's not always possible.

A more accurate method would be to count directly how many radiocarbon atoms are in a sample, though this is quite difficult because the radioactive nuclei are present in vanishingly tiny concentrations: less than 1 atom in over a million billion atoms. However, this is what can be achieved with Accelerator Mass Spectrometry (AMS).