Radiocarbon dating. Carbon 14 is produced naturally in the earth’s atmosphere through the interaction of cosmic rays and Nitrogen 14. A neutron comes along and strikes a 14 N nucleus, knocking off a proton and creating a 14 C atom. This atom now has an affinity for oxygen and quickly oxidizes as a 14 CO2 molecule, which has many of the same chemical properties as regular CO2. Through photosynthesis, the 14 CO2 molecules work their way into the plant system, and from there into the food chain. The ratio of 14 C to regular carbon in living things is the same as the ratio of these carbon atoms in the earth’s atmosphere, which is fairly constant, being in a state of equilibrium. When a living being dies, it no longer ingests 14 C and the existing 14 C in the now defunct life form begins to decay. In 1949, Willard F. Libby and his associates at the University of Chicago measured the half-life of this decay at 5568 ± 30 years, which to this day is known as the Libby half-life. We now know that the half-life is closer to 5730 years, called the Cambridge half-life, but radiocarbon dating labs still use the Libby half-life for technical and historical reasons. Libby was awarded the Nobel prize in chemistry for his discovery.
(a) Carbon 14 dating is a useful dating tool for organisms that lived during a specific time period. Why is that? Estimate this period.
(b) Suppose that the ratio of 14 C to carbon in the charcoal on a cave wall is 0.617 times a similar ratio in living wood in the area. Use the Libby half-life to estimate the age of the charcoal.
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