Radiocarbon Dating
This method of scientific dating is used for samples which were once alive (eg in bone, charcoal, leather). All of these contain carbon, a proportion of which is radioactive 14C, an isotope that is continuously being formed in the upper atmosphere. As living organisms take up radiocarbon along with other carbon atoms, the ratio between the two forms remains constant. However when they die the radiocarbon decays and is not replaced. Since it decays at a known constant rate, the decreasing concentration of 14C can be measured and the date when the material died estimated.
The radiocarbon in a sample may be counted in two different ways: either by Liquid Scintillation Counting or by Accelerator Mass Spectrometry.
Unfortunately the amount of radiocarbon in the atmosphere has varied over time, and so the radiocarbon ages produced by the measurement process must be converted into actual calendar years by calibration.
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