When an organism dies, it stops absorbing the radioactive isotope and immediately starts decaying (7).
It is presumed that the proportion of atmospheric C is the same today as it was in 1950 (10), (11) and that the half-life remains the same.
If a radioactivity level comes back as half of what would have been expected if the organism had died in 1950, then it is presumed to be 5,730 years before 1950.
The method developed in the 1940's and was a ground-breaking piece of research that would change dating methods forever. Libby calculated the rate of radioactive decay of the C isotope (4) in carbon black powder.
As a test, the team took samples of acacia wood from two Egyptian Pharaohs and dated them; the results came back to within what was then a reasonable range: 2800BC /- 250 years whereas the earlier independent dates (largely the dendrochronology records) were 2625 /- 75 years (3), (5).
The sample passes through several accelerators in order to remove as many atoms as possible until the C pass into the detector.
These latter atoms are used as part of the calibration process to measure the relative number of isotopes (9).When the half-life was corrected in 1950, the year was taken as a base date from which to calculate all resulting dates.Therefore, any expression of “before present” will mean “before 1950”.This allows researchers to account for variation by comparing the known records of C levels in the tree record, looking for a tree record that has the same proportion of radiocarbon.The overlapping nature of the tree records means this is the most accurate record we have.This does not mean that we have a precise year of 3780BC, it means we then need to calibrate through other methods that will show us how atmospheric concentrations of the C isotope has changed - most typically through the dendrochronology records (tree ring data) (10).