Radioactive Dating: Radiocarbon dating is a process used to determine the age of an ancient object by measuring its radiocarbon content. This technique was devel¬oped in the late 1940s by Willard F. Libby, an American chemist. Radiocarbon atoms, like all radioactive substance, decay at an exact and uniform rate. Half of the radiocarbon disappears after about 5,700 years. Therefore, radiocarbon has a half-life of that period of time. After about 11,400 years, a fourth of the original amount of radiocarbon remains. After another 5,700 years, only an eighth remains, and so on. The radiocarbon in the tissues of a living organism decays extremely slowly, but it is continuously renewed as long as the organism lives. After the organism dies, it no longer takes in air or food, and so it no longer absorbs radiocarbon. The radiocarbon already in the tissues continues to decrease at a constant rate. This steady decay at a known rate—a half- life of about 5,700 years—enables scientists to determine an object’s age. After scientists measure an object’s radiocarbon content, they compare it with the radiocarbon in tree-rings whose ages are known. This technique enables them to compensate for small variations of radiocarbon content in the atmosphere at different times in the past. By doing so, scientists can convert an object’s radiocarbon age to a more precise date. Radioisotopes with very long half-lives are used for dating rock specimens such as Uranium-238. Uranium-235 which becomes lead 207; thorium 232, which becomes lead 208; rubidium 87, which changes into strontium 87; and potassium 40, which changes into argon 40 are radio¬isotope which can be used to calculate the age of rocks.