- developed by Willard F. Libby in the late 1940s. Libby worked on the Manhattan project and won a Nobel prize for his effort.
- - Neutrons creted when cosmic rays enter Earth's upper atmosphere
- - Nitrogen 14 atom absorbs a neutron and becomes Carbon14
- - Carbon14 combines with oxygen to form carbon dioxide
- - Carbon dioxide is taken in by plants, and fixed into tissue (also dissolves in water).
- - When plants and animals die they stop taking in CO2 and thus "start the clock"
- - Carbon 14 is radioactive and unstable and thus decays back into N14
- - C14 has a half-life of 5730 +/- 40 years
Assumption was that there was a constant supply on N14 and cosmic radiation; therefore a C14 year is equal to a calander year. However, C14 production rate has not been constant. Differences in solar wind, Earth's magentic field, sun spots, industrial revolution, and nuclear testing in the 1960's changed C14 production. Using dendrochronology, coral reef deposition, glacial ice cores, physicists can correct or calibrate radio carbon dates (b.p.) to approximte calendar dates (b.c.-a.d.)