Which isotope is used to perform radiocarbon dating
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).
These latter atoms are used as part of the calibration process to measure the relative number of isotopes (9).
When an organism dies, it stops absorbing the radioactive isotope and immediately starts decaying (7).
As previously mentioned, the half life of the C isotope is 5,730 years - this means that it takes 5,730 years to reach half the radioactivity that the organism had at the point of death, another 5,730 years to reach 25% radioactivity it had at the point of death and so on.
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.AMS counts the quantity of C isotope is constantly formed in the upper atmosphere thanks to the effects of cosmic rays on nitrogen-14 atoms.