Radiocarbon dating

• 1. Discovery And Development History

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  In 1939, Martin Kamen and Samuel Ruben of the Radiation Laboratory at Berkeley began experiments to determine if any of the elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research. They synthesized carbon-14 using the laboratory's cyclotron accelerator and soon discovered that the atom's half-life was far longer than had been previously thought. This discovery set the stage for a new method of dating ancient objects. Serge A. Korff, then employed at the Franklin Institute in Philadelphia, predicted that the interaction of thermal neutrons with nitrogen-14 in the upper atmosphere would create radiocarbon. Willard Libby, who was at Berkeley during World War II, learned of Korff's research and conceived the idea that it might be possible to use radiocarbon for dating. In 1945, Libby moved to the University of Chicago, where he began his work on radiocarbon dating. He published a paper in 1946 proposing that the carbon in living matter might include as well as non-radioactive carbon. Libby and several collaborators proceeded to experiment with methane collected from sewage works in Baltimore. After isotopically enriching their samples, they were able to demonstrate that they contained radiocarbon. By contrast, methane created from petroleum showed no radiocarbon activity because of its age. The results were summarized in a paper in Science in 1947, in which the authors commented that their results implied it would be possible to date materials containing carbon of organic origin. Libby and James Arnold proceeded to test the radiocarbon dating theory by analyzing samples with known ages. For example, two samples taken from the tombs of two Egyptian kings, Zoser and Sneferu, independently dated to 2625 BC ± 75 years, were dated by radiocarbon measurement to an average of 2800 BC ± 250 years. These results were published in Science in December 1949. Within 11 years of their announcement, more than 20 radiocarbon dating laboratories had been set up worldwide. In 1960, Libby was awarded the Nobel Prize in Chemistry for this work.

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• 2. Nuclear Physics Principles

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  In nature, carbon exists as three isotopes. Two are stable and not radioactive: carbon-12 and carbon-13; and carbon-14, also known as "radiocarbon", which is radioactive. The half-life of carbon-14 is about 5,730 years, so its concentration in the atmosphere might be expected to decrease over thousands of years, but is constantly being produced in the lower stratosphere and upper troposphere, primarily by galactic cosmic rays, and to a lesser degree by solar cosmic rays. These cosmic rays generate neutrons as they travel through the atmosphere which can strike nitrogen-14 atoms and turn them into carbon-14. The following nuclear reaction is the main pathway by which carbon-14 is created: n + N-14 leads to C-14 + p where n represents a neutron and p represents a proton. Once produced, the carbon-14 quickly combines with the oxygen (O) in the atmosphere to form first carbon monoxide, and ultimately carbon dioxide. Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is taken up by plants via photosynthesis. Animals eat the plants, and ultimately the radiocarbon is distributed throughout the biosphere. The ratio of carbon-14 to carbon-12 is approximately 1.25 parts of carbon-14 to 10^12 parts of carbon-12. In addition, about 1% of the carbon atoms are of the stable isotope carbon-13. The equation for the radioactive decay of carbon-14 is: C-14 leads to N-14 + e minus + ν_e. By emitting a beta particle (an electron, e minus ) and an electron antineutrino (ν_e), one of the neutrons in the nucleus changes to a proton and the nucleus reverts to the stable (non-radioactive) isotope nitrogen-14.

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• 3. Measurement Technology Evolution

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  For decades after Libby performed the first radiocarbon dating experiments, the only way to measure the carbon-14 in a sample was to detect the radioactive decay of individual carbon atoms. In this approach, what is measured is the activity, in number of decay events per unit mass per time period, of the sample. This method is also known as "beta counting", because it is the beta particles emitted by the decaying atoms that are detected. Libby's first detector was a Geiger counter of his own design. He converted the carbon in his sample to lamp black (soot) and coated the inner surface of a cylinder with it. This cylinder was inserted into the counter in such a way that the counting wire was inside the sample cylinder, in order that there should be no material between the sample and the wire. Any interposing material would have interfered with the detection of radioactivity, since the beta particles emitted by decaying carbon-14 are so weak that half are stopped by a 0.01 mm thickness of aluminium. Libby's method was soon superseded by gas proportional counters, which were less affected by bomb carbon (the additional carbon-14 created by nuclear weapons testing). These counters record bursts of ionization caused by the beta particles emitted by the decaying atoms; the bursts are proportional to the energy of the particle, so other sources of ionization, such as background radiation, can be identified and ignored. The counters are surrounded by lead or steel shielding, to eliminate background radiation and to reduce the incidence of cosmic rays. In addition, anticoincidence detectors are used; these record events outside the counter and any event recorded simultaneously both inside and outside the counter is regarded as an extraneous event and ignored. The other common technology used for measuring activity is liquid scintillation counting, which was invented in 1950, but which had to wait until the early 1960s, when efficient methods of benzene synthesis were developed, to become competitive with gas counting; after 1970 liquid counters became the more common technology choice for newly constructed dating laboratories.

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• 4. Atmospheric Calibration Challenges

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  In the early years of using the technique, it was understood that it depended on the atmospheric carbon-14/carbon-12 ratio having remained the same over the preceding few thousand years. To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects. Over time, however, discrepancies began to appear between the known chronology for the oldest Egyptian dynasties and the radiocarbon dates of Egyptian artefacts. Neither the pre-existing Egyptian chronology nor the new radiocarbon dating method could be assumed to be accurate, but a third possibility was that the carbon-14/carbon-12 ratio had changed over time. The question was resolved by the study of tree rings. Comparison of overlapping series of tree rings allowed the construction of a continuous sequence of tree-ring data that spanned 8,000 years. (Since that time the tree-ring data series has been extended to 13,900 years.) In the 1960s, Hans Suess was able to use the tree-ring sequence to show that the dates derived from radiocarbon were consistent with the dates assigned by Egyptologists. This was possible because although annual plants, such as corn, have a carbon-14/carbon-12 ratio that reflects the atmospheric ratio at the time they were growing, trees only add material to their outermost tree ring in any given year, while the inner tree rings don't get their replenished and instead start losing carbon-14 through decay. Hence each ring preserves a record of the atmospheric carbon-14/carbon-12 ratio of the year it grew in. Carbon-dating the wood from the tree rings themselves provides the check needed on the atmospheric carbon-14/carbon-12 ratio: with a sample of known date, and a measurement of the value of N (the number of atoms of carbon-14 remaining in the sample), the carbon-dating equation allows the calculation of N0 , the number of atoms of carbon-14 in the sample at the time the tree ring was formed , and hence the carbon-14/carbon-12 ratio in the atmosphere at that time. Equipped with the results of carbon-dating the tree rings, it became possible to construct calibration curves designed to correct the errors caused by the variation over time in the carbon-14/carbon-12 ratio.

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• 5. Archaeological Revolution Impact

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  Soon after the publication of Libby's 1949 paper in Science, universities around the world began establishing radiocarbon-dating laboratories, and by the end of the 1950s there were more than 20 active research laboratories. It quickly became apparent that the principles of radiocarbon dating were valid, despite certain discrepancies, the causes of which then remained unknown. The development of radiocarbon dating has had a profound impact on archaeology often described as the "radiocarbon revolution". In the words of anthropologist R. E. Taylor, "data made a world prehistory possible by contributing a time scale that transcends local, regional and continental boundaries". It provides more accurate dating within sites than previous methods, which usually derived either from stratigraphy or from typologies (e.g. of stone tools or pottery). Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice age, and the beginning of the Neolithic and Bronze Age in different regions. A sample of the linen wrapping from one of the Dead Sea Scrolls, the Great Isaiah Scroll, was included in a 1955 analysis by Libby, with an estimated age of 1,917 ± 200 years. Based on an analysis of the writing style, palaeographic estimates were made of the age of 21 of the scrolls, and samples from most of these, along with other scrolls which had not been palaeographically dated, were tested by two AMS laboratories in the 1990s. The results ranged in age from the early 4th century BC to the mid 4th century AD. In all but two cases the scrolls were determined to be within 100 years of the palaeographically determined age.

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