Radiocarbon dating reservoir effects and calibration

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.

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.

His first publication showed the comparisons between known age samples and radiocarbon age (Libby et al, 1949; Libby, 1952). For the first time it was possible to obtain ages for many events which occurred over the past ~50,000 years.

In 1960 Libby was awarded the Nobel Prize for chemistry for this contribution.

This method needs less than 1 mg of carbon and directly measures the abundance of the individual ions of carbon (14C, 12C and 13C).

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.

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.

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.

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