Although C decays fairly quickly, heavier isotopes (such as uranium-238) decay much more slowly.
Because the present decay rates of these heavier isotopes are so small, the assumption that these rates have always been constant naturally leads to age estimates of millions and even billions of years.
Because these radioisotope methods yield age estimates of many millions of years for igneous rocks, it is thought that sedimentary rocks are also millions of years old, as well as the organic remains found within them.
Yet this assumption leads to a contradiction: If these organic samples really are many millions of years old, then they should be radiocarbon “dead.” But they aren’t! Evolutionists have attempted to blame these surprising results on a number of mechanisms. Furthermore, laboratories take great pains to keep contamination to a minimum, and researchers have found that, provided a sufficiently large testing sample is used (in the ballpark of 100 milligrams or so), the amount of such possible lab contamination is negligible compared to the C already present within the specimen.
Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.
Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.
Carbon 14 is continually being created in the Earth's atmosphere by the interaction of nitrogen and gamma rays from outer space.