Willard F. Libby developed the carbon-14 method of dating old or ancient objects, which became the most important dating technique for measuring the ages of archaeological objects more than ten thousand years old.
Carbon dioxide in the earth’s atmosphere contains a mixture of three carbon isotopes: about 99 percent carbon 12, about 1 percent carbon 13, and approximately one atom in a trillion of radioactive carbon 14. Plants absorb carbon dioxide from the atmosphere during photosynthesis and animals eat the plants, so all living plants and animals contain a small amount of radioactive carbon. When a plant or animal dies, its radioactivity slowly decreases. The half-life for carbon 14 is known to be about fifty-seven hundred years.
Willard F. Libby.
Coal and oil, which were formed from organic matter millions of years ago, have long since lost any carbon-14 activity. Wood samples from an Egyptian tomb or charcoal from a prehistoric fireplace made a few thousand years ago, however, can be dated with good reliability from the left-over radioactivity.
In the early 1930’s, Willard F. Libby was a graduate student at Berkeley working on his Ph.D. in chemistry. For his thesis research, he built a Geiger counter, the first radiation detector of this type constructed in the United States, based on a design that had been published by Hans Geiger in Germany in 1928. Libby’s thesis was an investigation of some rare earth elements that have very low levels of natural radioactivity.
In 1939, the cyclotron group at Berkeley set out on a systematic search to find out if there were any long-lived isotopes of the biologically important elements hydrogen, carbon, nitrogen, and oxygen. Bombardment of nitrogen by neutrons produced a small amount of carbon, which was converted chemically to calcium carbonate and then put inside one of Libby’s Geiger counters as a coating on the wall. The activity caused by carbon 14 was immediately evident. The experimenters wanted to measure its half-life, but this is difficult to do for long-lived activities, because they decrease so slowly. The half-life could be estimated only roughly at this time to be in the range from 1,000 to 100,000 years. World War II brought a halt to basic research as scientists were recruited for various military needs. Libby worked on the atomic bomb project, in particular on the separation of uranium isotopes. In 1945, he returned to academic life as a professor at the University of Chicago, where he would resume a search for radioactive carbon in nature.
During the 1930’s, an investigation of cosmic rays in the upper atmosphere had been conducted by Serge Alexander Korff at New York University. He had developed a special type of Geiger counter filled with boron gas to detect neutrons. Using high-altitude balloons, Korff showed that neutrons were present there. He suggested that the neutrons probably would be absorbed by ordinary nitrogen in the atmosphere to form radioactive carbon 14, which would then enter the biosphere.
In early 1947, Libby started the crucial experiment to test for radioactive carbon in nature. He decided to test samples of methane gas from two different sources. One group of samples came from the sewage disposal plant at Baltimore, Maryland, which is rich in fresh organic matter. The other sample of methane came from an oil refinery, which should contain only ancient carbon from fossils whose activity would have completely decayed. The experimental results confirmed the expectation: The methane from fresh sewage was radioactive but the methane from oil was not radioactive. Evidently, radioactive carbon is present in fresh organics but it decays eventually.
After this initial success, more than two years of intense effort were needed to develop radiocarbon dating into a quantitative method for archaeology. The carbon radioactivity is so weak that elaborate precautions are necessary to shield against natural background radiation.
The first archaeological object analyzed by carbon dating, obtained from the Metropolitan Museum in New York, was a piece of cypress wood from the tomb of King Djoser of Egypt, whose age was estimated based on historical evidence to be about forty-six hundred years. A small sample of carbon obtained from this wood was deposited inside Libby’s counter, giving a count rate that was about 40 percent less than the activity of modern organic carbon. To convert this lesser activity to an age, the half-life of carbon 14 had to be measured in a separate experiment, which was done at Argonne National Laboratory. The resulting age of the wood calculated from its residual radioactivity was about thirty-eight hundred years. Considering that this was the first object to be analyzed, even such rough agreement with the historic age was considered to be encouraging.
To establish the validity of radiocarbon dating, Libby analyzed known samples of varying age. These included tree ring samples from 1072
The validity of radiocarbon dating depends on an important assumption; namely, that the abundance of carbon 14 in nature has been constant over many thousands of years. If carbon 14 was less abundant at some point in history, organic samples from that era would have started with less radioactivity. When analyzed in modern times, their reduced activity would make them appear to be older than they really are. Samples of known age are needed going back many millennia to check how reliable their carbon dates are.
Written historical records, unfortunately, do not extend further back than about 3000
The New York Times on May 30, 1947, gave a brief report that Libby had found radioactive carbon 14 in nature, presumably the result of cosmic rays. The Times went on to say, rather vaguely, that this may provide “a new yard-stick for measuring various earth periods.” In March of 1949, in the journal Science, Libby published his first list of carbon dates for objects of known age, spanning the time period from 1000
Since Libby’s pioneering experiments in the late 1940’s, carbon 14 has established itself as a reliable dating technique for archaeologists and cultural historians. Further improvements are continuing to be developed to increase the precision, to use smaller sample size, and to extend the time scale back to fifty thousand years or older.
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Deevey, Edward S. “Radiocarbon Dating.” Scientific American 186 (February, 1952): 24-28. A series of photographs show how a wood sample is prepared so that its weak radioactivity can be counted. -
Hedges, Robert E., and John A. J. Gowlett. “Radiocarbon Dating by Accelerator Mass Spectrometry.” Scientific American 254 (January, 1986): 100-107. Describes the authors’ work to improve carbon dating by using a mass spectrometer to measure the carbon-14 content of samples instead of radioactive decay. -
Libby, Willard F. Radiocarbon Dating. 2d ed. Chicago: University of Chicago Press, 1965. Shows with numerical calculations how radioactive carbon is formed and distributed around the world. Describes in detail sample preparation and low-level counting procedures. -
_______. “Radiocarbon Dating.” In The Frontiers of Knowledge. Garden City, N.Y.: Doubleday, 1975. An excellent, short description of the carbon-dating technique, written for general readers. Includes photographs of some interesting objects whose age was measured, such as the linen wrapping from the Book of Isaiah Dead Sea Scroll. -
Maschner, Herbert D. G., and Christopher Chippindale, eds. Handbook of Archaeological Methods. 2 vols. Lanham, Md.: AltaMira Press, 2005. A massive, two-volume set that covers all aspects of archaeological practice, including radiocarbon and other methods of dating. Bibliography, index. -
Renfrew, Colin. “Carbon-14 and the Prehistory of Europe.” Scientific American 225 (October, 1971): 63-72. Some prehistoric dates before 4000 b.c.e. obtained with carbon 14 were shown to be in error. Article shows how scientific results are subject to correction as more information is accumulated. -
Taylor, Royal E. Radiocarbon Dating: An Archaeological Perspective. New York: Academic Press, 1987. Written by an archaeologist who is knowledgeable about the physics and chemistry of carbon dating. Gives a broad-ranging perspective over the past forty years of accomplishments in this field. A historical chapter on Libby provides many interesting biographical details. -
Wilson, Ian. The Mysterious Shroud. Garden City, N.Y.: Doubleday, 1986. In the style of a detective story, compiles the best evidence both for and against the idea that the famous Shroud of Turin may have been the burial cloth of Jesus Christ. Includes more than one hundred photographs and an extensive bibliography.
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