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radiocarbon dating

radiocarbon dating is an accurate method for determining the age of things that have a biological origin such as bones, cloth made from natural fibers, artifacts made of wood, and charcoal.

All plants and animals are made up of chemicals that contain three types of carbon atoms or isotopes. The most common isotope is carbon 12. It generally accounts for approximately 99% of all the carbon atoms found in a plant or animal. Another isotope is carbon 13 that comprises about 1% of the total. The third isotope is carbon 14. It is found in only small amounts compared to carbon 12. In living plants or animals there are only about one carbon 14 atoms for every trillion carbon 12 atoms.

Carbon (C12, C13 and C14) is acquired by plants from carbon dioxide (CO2) in the atmosphere. Animals get carbon by breathing carbon dioxide and by ingesting plant material and other animals. The result is that all living things have a specific ratio of carbon 12 and carbon 14 isotopes. Generally speaking, it is the ratio found in the atmosphere. The ratio begins to change when a plant or animal dies and it is no longer taking on carbon from nature. Carbon 14, unlike carbon 12 and carbon 13, is radioactive. This means that over time the carbon 14 atoms will decay. When the isotope carbon 14 decays it gives off a beta particle and in doing so becomes nitrogen 14. The amount of carbon 12 and carbon 13, however, remains constant.

The half-life of different isotopes range from fractions of a second to millions of years. The half-life of carbon 14 is 5730 years. This means that in 5730 years, half of the carbon 14 atoms will have become nitrogen 14. In the next 5730 years, half of the remaining carbon 14 will have decayed

Thus, if we are able to measure the percentage of carbon 14 that remains in a sample, we can determine its age. The sample may be from a human bone, an animal antler, a piece of line made from flax fibers, a wooden tool or charcoal from an ancient fire. All of these things were once part of something that was alive.

Two radiocarbon Methods

Two radiocarbon methods are commonly used for determining how much carbon 14 remains:

• Radiometric: The radiation emitted by the carbon 14 in a sample is measured. The lower the radiation level, the lower the amount of carbon 14 that remains. Both gas proportional counters and liquid scintillation systems are used for radiometric carbon 14 dating.
• Accelerator Mass Spectrometry (AMS): A particle accelerator is used to sort ions according to their mass so they may be counted with special detectors. The advantage of AMS is that only a small sample of a few milligrams is required for radiocarbon dating.

Both methods produce accurate radiocarbon results. The radiometric method is simpler and less costly but requires larger samples. AMS only requires very small samples but it is costly because of the complex equipment used.

Both methods require that the sample be destroyed to convert the carbon into a usable form for measurement.

radiocarbon testing is only accurate for material that is less than 50,000 years old.

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© 2005 Daniel R. Porter, Bronxville, New York