Conventional vs ams radiocarbon dating
Measuring 14C To obtain the radiocarbon age of a sample it is necessary to determine the proportion of 14C it contains.
Originally this was done by what is known as “conventional” methods, either proportional gas counters or liquid scintillation counters.
Three isotopes of carbon are found in nature; carbon-12, carbon-13 and carbon-14.
Carbon-12 accounts for ~99.8 % of all carbon atoms, carbon-13 accounts for ~1% of carbon atoms while ~1 in every 1 billion carbon atoms is carbon-14.
These newly formed 14C atoms rapidly oxidize to form 14CO..
Photosynthesis incorporates 14C into plants and therefore animals that eat the plants.
Hereafter these isotopes will be referred to as 12C, 13C, and 14C.
Calibration In the 1950s it was observed that the radiocarbon timescale was not perfect.
This brings us to two reasons why a radiocarbon date is not a true calendar age.
The true half-life of 14C is 5730 years and not the originally measured 5568 years used in the radiocarbon age calculation, and the proportion of 14C in the atmosphere is not consistent through time.
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.