Radiocarbon Dating

Radiocarbon (also called 14C) dating was invented in the late 1940s by Walter Libby, who was awarded the 1960 Nobel Prize for his discovery. The technique is typically used on organic remains, such as baskets, leather, dung, plant remains, amino acids, etc. With the development of accelerator mass spectrometry (AMS), beginning in the early 1970s, radiocarbon dating has become an increasingly useful way to measure time for archaeologists as well as scientists in other disciplines

Although the chemistry and physics involved is complex, the basis for the technique is relatively simple. Common nitrogen in the atmosphere (14N) can be changed to radioactive carbon-14 (14C) when bombarded by cosmic rays. Both are taken up by plants and animals along with the nutrients they need to survive. When the plant or animal dies, this process ceases and the 14C present begins decreasing through radioactive decay. Since the rate of decay is a known constant, the ratio of 14C to stable carbon can be used to determine the age of the sample.

The original measuring technique requires about enough material to fill a 35 mm film canister. Using radiometry, which is similar to counting the ticks given off by a Geiger counter, the amount of beta particle (electron) decays per 1000 minutes is measured. The fewer decays the older the sample. The amount of material required often made the technique too destructive to be use on rare or valuable artifacts. AMS dating uses a particle accelerator to count the number of 14C atoms remaining, and can be successful with samples the size of the head of a pin. AMS has reinvigorated radiocarbon dating as a tool for archaeologists.

Both techniques have statistical errors associated with them, expressed as a "+/-" number following the date. The B.P. following the date means "before present." By long standing convention, "present" is defined as A.D. 1950.

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