The half life of carbon-14 is about 5,700 years, so if we measure the proportion of C-14 in a sample and discover it's half a part per trillion, i.e.half the original level, we know the sample is around one half life or 5,700 years old.It cannot be applied to inorganic material such as stone tools or ceramic pottery.The technique is based on measuring the ratio of two isotopes of carbon.The theory behind radiocarbon dating is as follows: Why doesn't the carbon-14 in the air decay along with terrestrial carbon? The trick is that radioactive carbon-14 is continually replenished in a complex reaction that involves high-energy cosmic rays striking the upper atmosphere.In this process, nitrogen-14 (7 protons and 7 neutrons) gains a neutron and loses a proton, producing carbon-14 (6 protons and 8 neutrons).As the Earth's upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon - carbon 14 (C-14).
The trouble is that after 40,000 years there is under 1% of the original C-14 left, and it becomes too hard to measure it accurately.
The ensuing atomic interactions create a steady supply of c14 that rapidly diffuses throughout the atmosphere.
Plants take up c14 along with other carbon isotopes during photosynthesis in the proportions that occur in the atmosphere; animals acquire c14 by eating the plants (or other animals).
Because it reacts identically to C-12 and C-13, C-14 becomes attached to complex organic molecules through photosynthesis in plants and becomes part of their molecular makeup.
Animals eating those plants in turn absorb Carbon-14 as well as the stable isotopes.