Radiocarbon is created when cosmic rays hit atmospheric particles and cause the atmospheric nitrogen to convert into radiocarbon (14C). As a part of the atmospheric carbon dioxide (CO2) the radiocarbon ends up in plants by photosynthesis, in animals by food chain and in the whole environment by the natural cycle of carbon.
When an organism dies, it no longer exchanges carbon with the environment. Therefore, the only way by which the amount of radiocarbon changes, is radioactive disintegration. In 5730 years (the half-life of 14C) half of the original radiocarbon content is left. The radiocarbon is measureable from samples as old as approximately 50 000 years.
The amount of radiocarbon is usually measured using either Liquid Scintillation Counting or Accelerator Mass Spectrometry. The Finnish Museum of Natural History uses at the moment solely the Accelerator Mass Spectrometry, as it is the most sensitive method available.