Fifty Years of Radiocarbon Dating

ince the 1350s, when a linen fabric bearing ticles eventually rain down on the earth and full-scale front and back images of what apstrike molecules of atmospheric gas, producing peared to be a crucified man was first displayed neutrons. Some of these neutrons in turn react in a small village in France, many people dehatwith mtrogen, 14N, to form '_C, which quickly ed whether this famous cloth--later enshrined combines with oxygen to form molecules of rain the Cathedral of St. John the Baptist in Tunn, dioactive carbon dioxide 0',CO2). By the lime the Italy--could have once served as the burial radioactive 14CO2reaches the earth's surface, it shroud for Jesus of NaTareth. Arguments raged has mixed fuUy with normal carlton dioxide and for six centuries about the "Shroud of rurin;" accounts for about one molecule in 1012. then, during the 1980s, a group of scientists deThe vast majority of this t4Ceventually enters cided to seek a final answer to the question of the oceans. But 1 or 2 percent goes into the terauthenticity. Investigators in England, Switzerrestrial biosphere, because plants absorb carbon land and the United States analyzed swatches from COs in the air during photosynthesis. Thus from the linen shroud (along with other ancient vegetation, and the animals that feed on it, are tex_e samples of known vintage). They applied tagged with 14C. the now-famous technique for determin/ng the Living things maintain a 14Ccontent that is age of organic materials using the radioactive about equal to the atmospheric concentration carbon isotope t4C---or carbon-14. These meabecause the carbon atoms that undergo mdioacsurements showed that the flax from which the _ve decay within their bodies are continually linen was produced grew sometime in the 13th replaced. But once an organism dies and its or 14th century A.D.--far too recently to have metabolic processes cease, theamount of 1K7behad anything to do with the death of Jesus. gins to diminish, The rate of decline is measured Assigning a reliable age to this medieval arliby the 14Chalf-life, about 5,730 years. fact is just one of a long string of notable accomRecognizing these phenomena, Libby realplishments since Willard F. Libby and his colized that the age of ancient organic matter can leagues at the University of Chicago introduced be found from its residual _4Ccontent, as mearadiocarbon dating in December of 1949 (an sured by the radiation emitted by the matter. R. E.Taylor currently chai_ achievement that brought Libby the 19(K)NoFor the radiocarbon age to correspond with its theDepartment ofAnthtvpolabe] Prize for chemistry). A 1974 article by the actual age, several conditions must be satisfied. gy at theUniversity ofC,al_)rlate Elizabeth IC Ralph and Henry Michael of First, the atmospheric ratio of 14Cto normal carma, Riverstde, where he directs the University of Pennsylvaraa recounted for bon (IrC) must have remained essentially conthe radiocarbon laboratory. Taylor receivedhisPh.D. inanreaders of American Scientist the first quarterstant. Second, the ratio of carbon isotopes in the thropolog'yat the Untversity of century of progress since ta'bbyand his coworkmeastured material must r_othave changed exCalifornia, Los Angeles in ers made their pioneering determinations of age cept by the radioactive decay of t_C---since the 1970.Helmswrittenextenusing 14C.Here I review the entire five decades death of the organism. Third, there should have sively on theapplication ofraof development of this remarkable technique, been rapid and complete mixing of _-KZthrough diocarbon andotherdating one that today requires no more carbon than the various carbon reservoirs. Finally, the methods in archaeology, with what can be found in a few strands of hair. method assumes that the presumed half-life of particular emphasisonisstie_ 14C is correct and that suitable analyses can resurrounding the peopling of the Long Ago and FarAway veal small concentrations of radiocarbon with New World. Address: RadiocarbonLaboratory, Department Radiocarbon dating depends on a chain of natappropriate levels of accuracy and precmion. of Anthropology, Institute of uraI events, some having taken place in deep Libby's means of measuring 14Cemployed Geophysics and Planetary space long ago. The sequence begins in various elemental carbon in amorphous form ("carbon