14Cs

14Cs is not a standard isotope designation; the isotope most commonly associated with this symbol is carbon-14, written as 14C. In some contexts, the plural form 14Cs may appear to denote 14C-containing materials (14C-labeled compounds) rather than a single nucleus. Carbon-14 is a radioactive isotope of carbon with six protons and eight neutrons. It decays by beta emission to nitrogen-14 with a half-life of about 5,730 years.

In nature, 14C is produced in the upper atmosphere through interactions of cosmic rays with nitrogen-14 via the reaction 14N(n,p)14C. The produced 14C quickly forms CO2 and is incorporated into the carbon cycle, entering living organisms during photosynthesis and respiration. It is continually replenished in the atmosphere at a low rate and equilibrates with atmospheric reservoirs.

Radiocarbon dating uses the known half-life of 14C to estimate the age of organic material up to around 50,000 years. After death, exchange with the atmosphere ceases and 14C decays, causing the 14C/12C ratio to decline. Modern calibration with tree rings and other archives is used to convert measured 14C activity into calendar years; atmospheric variations due to nuclear testing require calibration (the IntCal curves).

14C concentration can be measured by decay counting methods or by accelerator mass spectrometry (AMS). AMS directly counts 14C atoms in a sample, enabling smaller samples and greater precision.

Limitations include contamination, reservoir effects for marine samples, and post-mortem alteration. Radiocarbon dating has wide applications in archaeology, geology, climatology, and biology; radiolabeled 14C compounds are used as tracers in biochemistry and medical research.

Discovered by Willard Libby in 1949, radiocarbon dating led to the Nobel Prize in Chemistry in 1960.