Atom used in radiocarbon dating
Since the carbon in these fuels was ancient, it contained no radiocarbon. Therefore, prior to atmospheric bomb testing, the proportion of radiocarbon to C was relatively low, giving relatively old ages. Ocean corals, dated by another radiometric method - Uranium-Thorium dating - have also helped to extend the calibration curve beyond the age of the most ancient treering chronologies. A., Kromer, B., Mc Cormac, G., van der Plicht, J., Spurk, M. INTCAL98 radiocarbon age calibration, 24,000-0 cal BP. Therefore, radiocarbon dates are calculated to a "pre-bomb" age of 1950 A. Material which died after 1950 has such high amounts of radiocarbon its age is reported as "percent modern (1950)" (example 180% modern). This bomb radiocarbon has been gradually removed from the atmosphere by by natural processes, but the "bomb spike" can be shown through the dating by means such as comparing the bottle date and radiocarbon age of wines. The ratios are consistent among species, and the slight (1-3%) differences can also be calculated from the ratio of C) decreases as the radiocarbon decays. Libby determined, one gram of pure carbon should produce about 14 (13.56) radioactive decays per minute.
The rate of bombardment is greatest near the poles, where the Earth's magnetic field is dipping into the Earth and therefore does not deflect incoming cosmic rays.
As far back as there were organisms who were able to consume food containing both ordinary carbon and an isotope of carbon; the comparison of the ratios - which pinpoints the time when an organism stopped eating - gives rise to carbon dating.
Carbon dating is one type of radiometric dating, there are others.
Other forms of radioactive dating are more broadly applicable. It decays to Nitrogen 14 and has a half life of 5,730 years.
Typically, Carbon 14 is used for fossil dating for fossils 100-30,000 years old, but older than that, Potassium 40 or Rubidium 87 are more effective. Carbon dating is the radio-activity of Carbon 14 which is unstable so it emits protons once in a while in order to become a more stable isotope.