RADIO CARBON DATING
Radio-carbon dating is a method of obtaining age estimates on organic materials. It has been used to date samples as old as 50,000 years. The method was developed immediately following World War II by Willard F. Libby and coworkers, and has provided age determinations in archaeology, geology, geophysics and other branches of science. Radiocarbon determinations can be obtained on wood; charcoal; marine and fresh-water shell; bone and antler; peat and organic-bearing sediments, carbonate deposits such as tufa, caliche, and marl; and dissolved carbon dioxide and carbonates in ocean, lake and ground-water sources.
Each sample type has specific problems associated with its use for dating purposes, including contamination and special environmental effects. While the impact of radiocarbon dating has been most profound in archaeological research and particularly in prehistoric studies, extremely significant contributions have also been made in hydrology and oceanography. In addition, in the 1950s the testing of thermonuclear weapons injected large amounts of artificial radiocarbon ("Radiocarbon Bomb") into the atmosphere, permitting it to be used as a geochemical tracer.
Radioactive carbon, produced when nitrogen 14 is bombarded by cosmic rays in the atmosphere, drifts down to earth and is absorbed from the air by plants. Animals eat the plants and take C14 into their bodies. Humans in turn take carbon 14 into their bodies by eating both plants and animals. When a living organism dies, it stops absorbing C14 and the C14 that is already in the object begins to disintegrate. Scientists can use this fact to measure how much C14 has disintegrated and how much is left in the object. Carbon 14 decays at a slow but steady rate and reverts to nitrogen 14. The rate at which Carbon decays (Half-life) is known: C14 has a half-life of 5730 years. Basically this means that half of the original amount of C14 in organic matter will have disintegrated 5730 years after the organism’s death; half of the remaining C14 will have disintegrated after another 5730 years and so forth. After about 50,000 years, the amount of C14 remaining will be so small that the fossil can't be dated reliably.
To discover how long an organism has been dead (to determine how much C14 is left in the organism and therefore how old it is), we count the number of beta radiations given off per minute per gram of material. Modern C14 (i.e., from an organism that just died) emits about 15 beta radiations per minute per gram of material, but C14 that is 5730 years old will only emit half that amount, (the half-life of C14) per minute. So if a sample taken from an organism emits 7.5 radiations per minute in a gram of material, then the organism must be 5730 years old. The accuracy of radiocoarbon dating was tested on objects with dates that were already known through historical records such as parts of the dead sea scrolls and some wood from an Egyptian tomb. Based on the results of the Carbon 14 test the analysis showed that C14 agreed very closely with the historical information.