14C

14C, or carbon-14, is a radioactive isotope of carbon with six protons and eight neutrons. It is produced in the Earth's upper atmosphere when cosmic rays convert nitrogen-14 into 14C, which quickly oxidizes to carbon dioxide and becomes part of the carbon cycle. In living organisms, 14C is incorporated through photosynthesis, so the 14C/12C ratio in living matter matches that of the atmosphere. After death, 14C decays to nitrogen-14 by beta decay with a half-life of about 5,730 years.

Radiocarbon dating uses the remaining 14C in an organic sample to estimate age, assuming a known initial 14C level and closed-system conditions. The technique is effective for organic material up to about 50,000 years old, with calibration curves (such as IntCal) used to correct for past variations in atmospheric 14C due to solar activity and human impacts.

Production and atmospheric abundance have fluctuated since preindustrial times. Fossil fuels contain no 14C, diluting atmospheric 14C when burned (the Suess effect). In the 1950s–1960s, nuclear weapons testing created a pronounced 14C bomb peak later recorded in tree rings and other archives.

Measurement methods include traditional beta counting and accelerator mass spectrometry (AMS), the latter allowing much smaller samples. Applications span archaeology, geology, paleoclimatology, and environmental tracing. Limitations include contamination, reservoir effects in marine samples, and the need for calibration; radiotoxicity is low due to the long half-life and low activity.