About one in a trillion atoms of carbon is the radioactive isotope 14C. It is a ‘cosmogenic isotope’ produced by the interaction between atmospheric nitrogen (N) and cosmic rays. The half-life of 14C is 5,730 years, so every 5,730 years the ratio of 14C to 12C will halve. ‘Isotopic clocks: the persistence of carbon’ describes how scientists Willard Libby, Hessel de Vries, Hans Seuss, and their contemporaries established the radiocarbon dating technique. Radiocarbon has become one of the most valuable isotopic tools available to scientists looking to date materials formed over the past 50,000 years or to trace and apportion different sources of carbon.