Archaeology

The Basic Physics of Radio Carbon Dating



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Radio carbon dating is a method for estimating the age of archaeological samples by comparing the amount of unstable carbon 14 found in the sample with the amount of stable carbon 12 found in the sample. A team of University of Chicago researchers led by chemistry professor Willard Libby developed the technique of radio carbon dating in 1949. Professor Libby won the 1960 Nobel Prize for chemistry for his groundbreaking work in developing radio carbon dating. 

Carbon 14 is an unstable, radioactive isotope of carbon. Carbon 14 forms when nitrogen atoms are bombarded by cosmic rays in the Earth’s upper atmosphere. The nucleus of nitrogen atoms contains 7 protons and 7 neutrons. Cosmic rays interact with nitrogen atoms and cause 1 proton to become a neutron. This interaction produces carbon 14. 

Carbon 14 reacts identically to other carbon isotopes. Accordingly, carbon 14 becomes part of complex organic molecules which make up living organisms. Organic compounds made with carbon 14 are unstable. The carbon 14 molecules deteriorate over time with one of the neutrons eventually being transformed into a proton. When this transformation is complete, the atom is no longer a carbon atom. Instead, it has become a nitrogen atom. 

Carbon 14 deteriorates at a predictable rate. One half of the carbon 14 molecules in a sample deteriorate every 5,730 years. This is the half life of carbon 14. Three fourths of the carbon 14 molecules in a sample will have deteriorated after 11,460 years. Virtually all of the carbon 14 molecules in a sample will have deteriorated after about 50,000 years. 

Living organisms continue ingesting compounds containing carbon 14, as well as compounds containing other carbon isotopes, for as long as they are alive. Once the plant or animal dies, it ceases to take in additional carbon compounds. After the living organism dies, the carbon 14 compounds will begin to deteriorate. The compounds made from stable carbon isotopes do not deteriorate over time. 

Scientists use radio carbon dating to estimate the age of archaeological samples. Scientists calculate the ratio of carbon 14 compounds to other carbon compounds in living organisms. Scientists then use this calculation to assist them in estimating the age of a sample. In order to estimate the age of a sample, scientists first must determine the amount of carbon 14 found in the sample. The scientists then compare the amount of carbon 14 found in the sample to the amount of other carbon isotopes found in the sample. The resulting ratio is compared to the ratio of carbon 14 to other carbon found in living things. If the ratio of carbon 14 to other carbon in the same is half of the ratio of carbon 14 to other carbon in living organisms, then scientists can estimate that the sample is approximately 5,730 years old. 


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  • InfoBoxCallToAction ActionArrowhttp://www.pbs.org/wgbh/nova/tech/radiocarbon-dating.html
  • InfoBoxCallToAction ActionArrowhttp://archserve.id.ucsb.edu/courses/anth/fagan/anth3/Courseware/Chronology/08_Radiocarbon_Dating.html