27 February 1940, by Martin Kamen and Sam Ruben at the University of California, although its existence had been suggested by Franz Kurie in 1934. – This is used in chemical and biological research, in a technique called carbon labelling. Carbon-14 atoms can be used to replace nonradioactive carbon, in order to trace chemical and biochemical reactions. – It is produced in the upper layers of the troposphere and the stratosphere, by thermal neutrons absorbed by nitrogen atoms.
Plants fix atmospheric carbon during photosynthesis, so the level of carbon in plants and animals when they die approximately equals the level of carbon in the atmosphere at that time. – Carbon-14 is constantly being made by reaction with nitrogen in the atmosphere. – The amount of 14C in dead organic matter will grow smaller. – Scientists can measure the amount of carbon-14 in a piece of old wood for instance, and say that because there is only a certain amount left, the tree died 1000 years ago.
Carbon-14 atoms are created when cosmic rays from the sun collide with atoms in the atmosphere. Once produced, the atoms diffuse down to the biosphere, where it gets incorporated into all living things.
