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Materials that originally came from living things, such as wood and natural fibres, can be dated by measuring the amount of carbon-14 they contain.
The results showed that Ötzi died over 5000 years ago, sometime between 33 BC. It is found in the air in carbon dioxide molecules.
By measuring the ratio of Carbon-14 in a sample and comparing it to the amount in a recently deceased sample its date can be determined.
It was one of the earliest techniques to be developed, during the 1940s. In the figure right, the production of radio-active carbon is demonstrated. Here, 7 protons and 7 neutrons (N) plus one neutron form an isotope of carbon, with 8 neutrons and 6 protons. Radiocarbon dating works because an isotope of carbon, C, is constantly formed in the atmosphere by interaction of carbon isotopes with solar radiation and free neutrons. Living organisms absorb carbon (for example, we breathe it in).
By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.