Radiocarbon dating—also known as carbon-14 dating—is a technique used by archaeologists and historians to determine the age of organic material. It can theoretically be used to date anything that was alive any time during the last 60,000 years or so, including charcoal from ancient fires, wood used in construction or tools, cloth, bones, seeds, and leather. It cannot be applied to inorganic material such as stone tools or ceramic pottery.

The technique is based on measuring the ratio of two isotopes of carbon. Carbon has an atomic number of 6, an atomic weight of 12.011, and has three isotopes: carbon-12, carbon-13, and carbon-14. (The numbers 12, 13 and 14 refer to the total number of protons plus neutrons in the atom's nucleus. Thus carbon-14 has six protons and eight neutrons.)

Carbon-12 is by far the most abundant carbon isotope, and carbon-12 and -13 are both stable. But carbon-14 is slightly radioactive: it will spontaneously decay into nitrogen-14 by emitting an anti-neutrino and an electron, with a half-life of 5730 years.

The theory behind radiocarbon dating is as follows:

• Terrestrial carbon contains virtually no carbon-14, since any that may have been present would have long since decayed into the stable nitrogen-14.
• Carbon dioxide in the atmosphere, however, contains a mixture of carbon-12 and carbon-14 in known proportions.
• Living plants absorb carbon dioxide out of the air and incorporate it into their structure. The carbon atoms that make up, for example, a tree's annual growth ring, or the cob from an ear of corn, therefore contain the same isotope ratio as does the atmosphere.
• When the plant dies, the carbon-14 starts to decay. Measuring the ratio of the two isotopes therefore determines how long it has been since the constituent carbon atoms were absorbed from the atmosphere.
• The carbon in animals comes entirely from eating plants, or from eating other animals that eat plants. The technique can therefore be used to date human and animal remains.

Why doesn't the carbon-14 in the air decay along with terrestrial carbon? It does. The trick is that radioactive carbon-14 is continually replenished in a complex reaction that involves high-energy cosmic rays striking the upper atmosphere. In this process, nitrogen-14 (7 protons and 7 neutrons) gains a neutron and loses a proton, producing carbon-14 (6 protons and 8 neutrons). The proportion of carbon-14 to carbon-12 in the atmosphere therefore remains relatively stable at about 1.5 parts per billion.

One of the implied assumptions in radiocarbon dating is that levels of atmospheric carbon-14 have remained constant over time. This turns out not to be exactly true, and so there is an inherent error between a raw "radiocarbon date" and the true calendar date. To correct for this, scientists have compared radiocarbon dates from objects who's age is known by other means, such as artifacts from Egyptian tombs, and growth rings from ancient trees. In this way, calibration tables have been developed that eliminate the discrepancy.

Despite its usefulness, radiocarbon dating has a number of limitations. First, the older the object, the less carbon-14 there is to measure. Radiocarbon dating is therefore limited to objects that are younger than 50,000 to 60,000 years or so. (Since humans have only existed in the Americas for approximately 12,000 years, this is not a serious limitation to southwest archaeology.)

Radiocarbon dating is also susceptible to contamination. If the ground in which an object is buried contains particles of coal or other ancient sources of carbon, radiocarbon testing may indicate that the object is far older than it really is. Conversely, contamination by newer plant matter carried by flowing water or intruding plant roots may result in a date that is much too young. Archaeologists are acutely aware of these and other potential difficulties, and take extreme care in the selection and handling of objects to be dated.

Radiocarbon dating was developed by Willard F. Libby in 1949. The original technique was based on counting the number of individual radioactive decay events per unit of time, using a device similar to a Geiger counter. In the 1970s a new technique was developed called Accelerator-based Mass Spectrometry (AMS), which counts the number of carbon-14 atoms directly. This dramatically improves accuracy, and reduces the amount of carbon required from about 10 grams to only a few milligrams. In recent years, dating methods based on cosmogenic isotopes other than carbon (such as beryllium-10 and chlorine-36) have been developed, which allow for the dating of a wider variety of objects over much longer time scales.

There are eight AMS laboratories currently operating in the Unites States. In Arizona, virtually all dating is performed by the Arizona Accelerator Mass Spectrometry (AMS) Laboratory at the University of Arizona in Tucson. On April 26, 2007 this facility celebrated 25 years of operation, during which time it had processed over 75,000 radiocarbon measurements on objects ranging from the Dead Sea Scrolls to the Shroud of Turin. Their commercial rate (in 2008) is $675.00 per sample, which somewhat limits its accessibility to chronically under-funded archeological research projects.

For more information:

• Arizona AMS Lab website: www.physics.arizona.edu/ams/index.htm
• Wikipedia article on radiocarbon dating: http://en.wikipedia.org/wiki/Radiocarbon_dating
• Radiocarbon (the International Journal of Cosmogenic Isotope Research): www.radiocarbon.org