Carbon 14, radiocarbon

The carbon-14 (C-14) method is used on organic remains and carbonised bodies. While alive, these organisms have a constant level of carbon-14 due to constant exchanges with the atmosphere, and so they are slightly radioactive. When they die, the exchanges cease, there is no longer an addition of carbon-14, and the level then progressively decreases according to a known law. Measuring the residual level of carbon-14 in the sample enables the age of the sample to be deduced; this is the time elapsed since the death of the living organism. In the late 1970s, this dating method underwent a significant advance with the development of accelerator mass spectrometry (AMS). This technique employs a particle accelerator linked to mass spectrometers to isolate a set of carbon-14 atoms and to count the atoms individually in an ionisation chamber. The primary interest with AMS is that it makes it possible to date very small samples: less than a milligram of carbon is sufficient for an analysis, or around 1000 times less than for the classical dating method based on measuring beta radioactivity.