<P> Radiocarbon dating is also simply called Carbon - 14 dating . Carbon - 14 is a radioactive isotope of carbon, with a half - life of 5,730 years, (which is very short compared with the above isotopes) and decays into nitrogen . In other radiometric dating methods, the heavy parent isotopes were produced by nucleosynthesis in supernovas, meaning that any parent isotope with a short half - life should be extinct by now . Carbon - 14, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near - constant level on Earth . The carbon - 14 ends up as a trace component in atmospheric carbon dioxide (CO). </P> <P> A carbon - based life form acquires carbon during its lifetime . Plants acquire it through photosynthesis, and animals acquire it from consumption of plants and other animals . When an organism dies, it ceases to take in new carbon - 14, and the existing isotope decays with a characteristic half - life (5730 years). The proportion of carbon - 14 left when the remains of the organism are examined provides an indication of the time elapsed since its death . This makes carbon - 14 an ideal dating method to date the age of bones or the remains of an organism . The carbon - 14 dating limit lies around 58,000 to 62,000 years . </P> <P> The rate of creation of carbon - 14 appears to be roughly constant, as cross-checks of carbon - 14 dating with other dating methods show it gives consistent results . However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon - 14 and give inaccurate dates . The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon - 14 by a few percent; conversely, the amount of carbon - 14 was increased by above - ground nuclear bomb tests that were conducted into the early 1960s . Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon - 14 created in the atmosphere . </P> <P> This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium - 238 impurities . The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons . This causes induced fission of U, as opposed to the spontaneous fission of U . The fission tracks produced by this process are recorded in the plastic film . The uranium content of the material can then be calculated from the number of tracks and the neutron flux . </P>

Radiometric age dating works best and is most useful in rocks that are