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This equilibrium continues through plants to herbivores and through them to carnivores.
Once an organism dies, its carbon ceases exchanging with atmospheric carbon but continues decaying with a half-life of about 5730 years.
Thus, measurement of the radiocarbon concentration can give the time that the organism died.
Early measurements were done by counting the beta particles (high energy electrons) liberated in radiocarbon decay.
Furthermore, the instrument itself always introduces a background, similar to most other high sensitivity analytical instruments .
A sample originally containing absolutely no radiocarbon will still give a nonzero measurement from such contributions.
The technique arises from radiocarbon being continually produced in the upper atmosphere by cosmic rays while it is continually decaying, so the atmospheric concentration has reached a fairly steady equilibrium.
Plants are in equilibrium with atmospheric radiocarbon through respiration.
An old sample with in situ contamination cannot generally provide an accurate date.Thus, even if larger samples like RATE’s “on the order of 100 mg”  are submitted to an AMS laboratory, only about 1 mg of carbon will actually undergo analysis.Though Baumgardner calls a 1 mg sample “tiny” , it is generally considered “large” by AMS laboratories [e.g., 5, 7, 8], with enough carbon to provide ion source current for about a day.The second is a set of new samples that the RATE team collected and sent to a leading radiocarbon AMS laboratory to be dated.In both cases, I am convinced that the “intrinsic radiocarbon” is nothing more than contamination and instrument background.
Willard Libby discovered radiocarbon dating in the late 1940s.