A naturally occurring radioactive isotope of carbon with a half-life of 5,730-year (+/- 40 years) years and a mass number of 14, commonly used in radiocarbon dating archaeological materials and in demonstrating the metabolic path of carbon in photosynthesis. Its known rate of decay is the basis of radiocarbon dating. Willard Libby discovered natural carbon-14. Libby showed the essential uniformity of carbon-14 in living material and went on to measure the radiocarbon level in organic samples dated historically - materials as old as 5,000 years from sources such as Egyptian tombs. Libby's conclusion, with allowance for radioactive decay, was that over the past 5,000 years the carbon-14 level in living materials has remained constant within 5 percent precision of measurement. His work made this dating method available to scientists.