The Carbon-14 Method of Age 

 Determination ^ 



By Frank H, H. Roberts, Jr. 

 Associate Director, Bureau of American Ethnology 



During the period immediately following World War II an impor- 

 tant byproduct of research on cosmic rays was the development of a 

 method whereby the age of certain objects can be determined by 

 laboratory tests. The latter are based on the carbon-14 content of the 

 objects, and their results undoubtedly will be extremely useful in ar- 

 cheology, several branches of geology, oceanography, meteorology, and 

 related fields where chronology is essential to the solving of many prob- 

 lems. Previous types of "calendars," such as tree-ring dating, pollen 

 analysis, and glacial varves, were helpful in restricted areas but were 

 not universally applicable. This latest method of age determination 

 does not suffer from that handicap. For the first time it now appears 

 that prehistoric dates that are virtually precise can be obtained from 

 samples from any region in the world. The method has some limita- 

 tions and an occasional test goes awry, but as the techniques are 

 improved the age determinations unquestionably will become more 

 accurate. 



Carbon 14, a radioactive heavy form of carbon with an atomic 

 weight of 14 in contrast to the normal, stable carbon atomic weight of 

 12, is continually being formed in the upper atmosphere of the earth. 

 It results from the bombardment of nitrogen-14 atoms by cosmic rays, 

 streams of neutrons flowing toward the earth from outer space. The 

 new carbon-14 atoms thus formed, commonly called radiocarbon, 

 begin an immediate spontaneous disintegration but enough remain to 

 combine with oxygen to form carbon dioxide which eventually mixes, 

 in the air that surrounds the earth, with the much larger proportion 

 of carbon dioxide containing ordinary carbon. All living things 

 which absorb carbon dioxide from the atmosphere take in some of the 

 carbon 14 as well as the carbon 12. The proportions between the two 



1 Revised by permission, with the addition of new material, from the article "Carbon 

 14 Dates and Archeolosy," in Transactions of the American Geophysical Union, vol. 33. 

 No. 2, pp. 170-174, 1952. 



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