Radiocarbon, or Carbon 14, naturally occurring radioactive isotope of carbon. With an atomic weight of 14, it is heavier than ordinary carbon, which has an atomic weight of 12. Radiocarbon is produced when cosmic rays disturb nitrogen atoms in the upper atmosphere, causing them to gain a neutron and lose a proton. Radiocarbon, found in 1.1% of CO2 (carbon dioxide) molecules, is absorbed by plants in CO2 gas and passed on to animals and humans. Radiocarbon dating, developed in the 1940s by U.S. chemist Willard F. Libby, can calculate the age of organic matter to about 50,000 years, by comparing its remaining amount of radiocarbon to that in a contemporary radiocarbon sample. Radiocarbon breaks down by releasing particles over a period measured by the half-life (the time it takes half the isotope to decay)—5,700 years. Counting radiocarbon involves burning a portion of it to release the CO2 gas; today a particle accelerator and a magnetic field are used to separate out the carbon 14 atoms. Artificially produced radiocarbon is used medically as a “tracer” to study biological functions. The artificial isotope was first produced (1939) in the United States by chemists Martin D. Kamen and S. Ruben.