Radiocarbon dating

Radiocarbon dating is a scientific method of dating organic material based on the amount of carbon-14 (14C) it contains. Cosmic radiation creates carbon-14 in the atmosphere which is absorbed by plants through photosynthesis. When animals eat plants the carbon-14 is absorbed into its system. As carbon-14 stops being absorbed when an organism dies. Carbon-14 is a radioactive isotope with a half-life of 5,730 years; its long half-life means it can be used to accurately date organic material up to 70,000 years old. Radioactive decay is a steady process, and the half-life is the time taken for half of a radioactive sample to decay. It is the most commonly used method in archaeology for providing absolute dates.

The process of radiocarbon dating was invented by American chemist Willard Libby in the 1940s and received a Nobel Prize for his research. Carbon-14 has a nucleus of six protons and eight neutrons, making it unstable. As it decays into a more stable form it releases beta particles, which is detected by a Geiger counter. This used to be the common method for measuring the radiation from a sample, but now accelerator mass spectrometry is becoming more common as it can be used on smaller samples. This change in methodology means that pollen samples can be radiocarbon dated.

The amount of carbon-14 in the atmosphere (and therefore plants and animals) varies over time, so dates need to be calibrated. The need for calibration was demonstrated when dendrochronological dates for some samples did not match the radiocarbon dates. As tree rings can be securely dated, this allows the equations for radiocarbon dating to be corrected.