C14

Carbon-14, or C-14, is a radioactive isotope of carbon with six protons and eight neutrons, giving it an atomic mass of 14. It decays by beta minus emission to nitrogen-14, with a half-life of about 5,730 years. The natural abundance of 14C is extremely low, roughly one atom per trillion carbon atoms in the pre-industrial atmosphere.

C-14 is produced in the upper atmosphere by cosmic rays, mainly through the reaction N-14 + n → C-14 + p. The newly formed 14C enters the global carbon cycle as carbon dioxide and is incorporated by plants during photosynthesis, passing through the food chain to living organisms. When an organism dies, 14C no longer cycles in and its quantity decreases according to its half-life.

Radiocarbon dating uses the known 14C/12C ratio to estimate the time since death of organic material. It is effective for dating samples up to about 50,000 years old. Dates are calibrated against other records, such as dendrochronology, because atmospheric 14C levels have varied over time and due to reservoir effects that can skew results. The mid-20th century nuclear bomb tests created a pronounced increase in atmospheric 14C (the bomb pulse), which has since declined as levels re-equilibrate with reservoirs and fossil fuel combustion dilutes 14C in the atmosphere (the Suess effect).

Measurement methods include accelerator mass spectrometry (AMS), which counts 14C atoms directly and requires only small samples, and beta-counting techniques, which measure decay events but are less sensitive. 14C is widely used for dating organic materials in archaeology, geology, and paleoclimatology, and 14C-labeled compounds are used as tracers in biological and medical research. Safety concerns are generally low for external exposure, but ingestion or inhalation of 14C-bearing material can pose radiological hazards.