Plants and animals assimilate carbon 14 from carbon dioxide throughout their lifetimes.When they die, they stop exchanging carbon with the biosphere and their carbon 14 content then starts to decrease at a rate determined by the law of radioactive decay.
The CRA conventions include (a) usage of the Libby half-life, (b) usage of Oxalic Acid I or II or any appropriate secondary standard as the modern radiocarbon standard, (c) correction for sample isotopic fractionation to a normalized or base value of -25.0 per mille relative to the ratio of carbon 12/carbon 13 in the carbonate standard VPDB – Cretaceous belemnite formation at Peedee in South Carolina, (d) zero BP (Before Present) is defined as AD 1950, and (e) the assumption that global radiocarbon levels are constant.
Standard errors are also reported in a radiocarbon dating result, hence the “±” values.
The new standard, Oxalic Acid II, was proven to have only a slight difference with Oxalic Acid I in terms of radiocarbon content.
Over the years, other secondary radiocarbon standards have been made.
This scintillator produces a flash of light when it interacts with a beta particle.