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IGERT program in Archeological Science

Principles of AMS

Radiocarbon dating





Age Calculation


Cosmogenic Radioisotopes



Production of 14C

    In a modern sample, around one carbon nucleus in a trillion contains 2 extra neutrons, giving a mass of 14. This "carbon-14" is radioactive and decays with a half-life of 5730 years. One carbon nucleus in a hundred has a mass of 13 which is a stable nucleus. For historical reasons, uncalibrate radiocarbon measurements are often referred to a half-life of 5,568 years but this discrepancy is corrected during calibration.

Diagram of production of carbon-14 in the upper atmosphere
by reaction of neutrons with nitrogen, and of the subsequent
incorporation of carbon-14 into the biosphere.

    When neutrons from cosmic radiation are captured by nitrogen nuclei in the earth's atmosphere, the long-lived radioisotope 14C is produced. The 14C nuclei so produced combine with oxygen to form 14CO2, and become part of the carbon cycle. All living things achieve an equilibrium concentration of 14C, and when they die, their 14C nuclei decay with a half-life of 5,730 years.

Next page: History of Radiocarbon Dating