All radiocarbon laboratories either standardize to the US National Bureau of Standards Oxalic Acid I (OX-I) which is derived from Sugar Beets in 1955 or a secondary standard NBS OX-II (grown in 1977) or Australian National University Sucrose (ANU), which is sugar from the 1974 growing season in Australia.
Originally this was done by what is known as “conventional” methods, either proportional gas counters or liquid scintillation counters.
When a plant or animal dies it no longer exchanges CO with the atmosphere (ceases to take 14C into its being). 14C decays by emitting an electron, which converts a neutron to a proton, converting it back to its original 14N form.
The History of Radiocarbon Dating Willard Libby invented radiocarbon dating in the late 1940s.
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.