The half-life is the time taken for an amount of a radioactive isotope to decay to half its original value.Because this decay is constant it can be used as a “clock” to measure elapsed time assuming the starting amount is known.
This means that it can be difficult to effectively clean the samples and remove enough contaminating carbon to obtain an accurate date.
In the late 1970s and early 1980s the dating of small samples became possible using Accelerator Mass Spectrometry (AMS; Muller, 1977; Nelson et al., 1977).
The gas counter detects the decaying beta particles from a carbon sample that has been converted to a gas (CO, methane, acetylene).
A liquid scintillation measurement needs the carbon to be converted into benzene, and the instrument then measures the flashes of light (scintillations) as the beta particles interact with a phosphor in the benzene.
He noted some discrepancies indicating that radiocarbon results would need to be “calibrated” to convert them to calendar ages.