ACS ‘Chemistry for Life” Radiocarbon Dating and Willard Libby In 1946, Willard Libby developed a method for dating organic materials by measuring their content of carbon-14, a radioactive isotope of carbon, His radiocarbon dating method is now routinely used in archaeology and geology to determine the age of ancient artefacts that originated from living organisms. Libby's discovery, which eamed him the Nobel Prize in Chemistry in 1960, has made it possible to develop more precise historical chronologies across geography and cultures. Willard Libby's concept of radiocarbon dating Willard Libby (1908-1980), a professor of chemistry at the University of Chicago, began the research that led him to radiocarbon dating in 1945. He was inspired by physicist Serge Korif of New York University, who in 1939 discovered that bombardment of Earth's atmosphere by cosmic rays produces neutrons. Korif figured out that these neutrons react with nitrogen-14 ("4N) in the ‘atmosphere to produce a proton ‘and carbon-14, also known as radiocarbon or “C. Libby reasoned that “Cin the ‘atmosphere would find its way into living matter, which would thus be tagged with the radioactive isotope. Theoretically, if he could detect the amount of Gin an object, he could establish the object's age using the halle, or rate of decay, of the isotope. In 1946, Libby proposed this groundbreaking idea in the journal Physical Review. Predictions about “C ‘The concept of radiocarbon dating focused on measuring the National Historic Chemical Landmarks carbon content of organic objects, but in order to prove the idea Libby would need to work out the ebb and flow of carbon on Earth, Radiocarbon dating would be most successful if two important factors were true: that the concentration of “Cin the atmosphere had been constant for thousands of years, and that *C moved readily through the atmosphere, biosphere, oceans and other reservoirs—in a process known as the carbon cycle. In the absence of any historical data concerning the intensity of cosmic radiation, Libby simply assumed that it had been constant. He reasoned that a state of equilibrium must exist wherein the rate of "C production was equal to its rate of decay, dating back millennia. (Fortunately for him, this was later proven to be generally true, at least prior to the era of nuclear weapons.) For the second factor, it would be necessary to estimate the overall amount of #€C and compare this against all other isotopes of carbon. Based on Korff's estimation that just two neutrons ‘were produced per second for each square centimeter of Earth's surface, each forming a Discover more stores and activities about chemistry’ history at wwwacs.orq/landmarks. ¥C atom, Libby calculated a ratio of just one ““C atom per every 10°? carbon atoms on Earth Libby's next task was to study the movement of carbon through the carbon cycle. Ina system where “C moves readily throughout the cycle, the ratio of ¥C to other carbon isotopes should be the same in a living ‘organism as in the atmosphere. However, the rates of movement of carbon throughout the cycle were not then known. Libby and graduate student Ernest, Anderson calculated the mixing of carbon across these different reservoirs, particularly in the Willard Libby (right) with graduate student Ernest Anderson, BeoND yo Asonun,Univer of Chicago ‘oceans, which constitute the largest reservoir, Their results Predicted the distribution of "“C across the carbon cycle and gave Libby encouragement that radiocarbon dating might actually work. Detecting radiocarbon in nature 4G was first discovered in 1940 by Martin Kamen and Samuel Ruben, who created it artificially using a cyclotron accelerator at the University of California Radiation Laboratory in Berkeley Further research by Libby and others estimated its halite as 5,568 years (later revised to approximately 5,730 years), providing another essential factor in Libby's concept. But no one had yet detected C in nature— at this point, Korff and Libby's predictions about radiocarbon were entirely theoretical. In order to prove his concept of radiocarbon dating, Libby needed to confirm the existence of natural “C, a major challenge given the tools then available. At the time, no radiation- detecting instrument (such as @ Geiger counter) was sensitive enough to detect the small amount of $C that Libby's experiments required. Libby reached out to Aristid von Grosse of the Houdry Process Corp. who was able to provide a methane sample that had been enriched in C to a level that could be detected by existing tools. Using this sample and an ordinary Geiger counter, Libby and Anderson established the existence of naturally occurring C, matching the concentration predicted by Korft Although this method worked, it was slow and costly, 50 Libby's group developed an alternative. ‘They surrounded the sample chamber with a system of Geiger counters calibrated to detect and eliminate the background radiation that exists throughout the environment. ‘The assembly was called an “anti- coincidence counter.” When it was combined with a thick shield that further reduced background radiation and a novel method for reducing samples to pure carbon for testing, the system proved to Libby's anti-comcidence counter. The circular ‘be suitably sensitive, arrangement of Geiger counters (center) detected radiation in samples while the thick ‘Finally, Libby had a ‘metal shields on all sides were designed to ‘method to put his reduce background radiation. concept into practice. Glossary Carbon-14: Also known as radiocarbon or ¥C, this radioactive carbon isotope contains eight neutrons, or two more than ¥C, the most ‘common form of carbon. Over time, “C decays into *N (along with an electron and antineuttino), $0 the relative ‘amount of #C remaining in an artefact reveals its age. Cosmic rays: High-energy Particles that move through space al nearly the speed of, light. They produce neutrons when they hit the atmosphere, Haifuife: The time for haf the ‘atoms in a radioactive sample to decay through radiation. For C, the halfife is about 5,730 ‘years, 50 a body that is 5,730 ‘years old would contain half the 4G and be hatf as radioactive as on the day the person died. Isotope: Atoms consist of negatively charged electrons ‘along with a nucleus that ‘contains positively charged protons and neutral neutrons. Different isotopes of a given ‘element — such as C and “C —contain the same number of ‘electrons and protons but a ifferent number of neutrons, ‘The sum of protons and neutrons is known as the mass number, which is 14 in the case of HC. Organic material: Material that ‘contains carbon, Further reading Students may wish to refer to these additional resources: 41960 Nobel Prize in Chemistry httpsu/tinyurl.com/ACS-Nobel Carbon-14 is 7540 Years Old httos:/tinyurl com/ACS- carbont4 How Does Radiocarbon Dating Work? https:iitinyurl.com/ACS- datingThe "Radiocarbon Revolution”) Testing radiocarbon dating coincidence counter to samples whose ages were already known ‘The introduction of radiocarbon The concept of radiocarbon Among the first objects tested dating had an enormous influence dating relied on the assumption were samples of redwood and fir ‘on archaeology and geology, often | that once an organism died, it trees, the ages of which were ‘referred to as the “radiocarbon would be cut off from the carbon confirmed by counting their revolution” Before Libby's cycle. From then on, as a result annual growth rings. The technique was avaiable invest. | of radioactive decay, ts ¥C researchers also sampled Soe de Peta corel Glens, | content would steady ciminish. artifacts fom museums, such as eee coeeataTelnyerteran | Living organisms from the a piece of timber rom Egyptian Sttheologica ste nuhichartiacs. | present day would have the Pharaoh Sesostri II's funerary were found, presuming that the ‘same relative amount of “C as boat, an object whose age was layers had been laid down chrono- | the atmosphere, whereas ancient known from the record of its logically. However, this method sources that were once alive, ‘owner's death ‘simply places events in order ‘such as coal beds or petroleum, without a precise numerical would have no “C left. For In 1949, Libby and Arnold ‘measure. By contrast, radiocarbon | organic objects of intermediate __published thei findings in the dating provided the first objective ages—between a few centuries journal Science, introducing the dating method—the abiltytoattach | and several millennia—an age “Curve of Knowns.” This graph ‘approximate numerical dates to could be estimated by measuring compared the known age of ‘organic remains. the relative amount of “C artifacts with the estimated age presenti the sample and 1s determined by the eee Combining this information with _ radiocarbon dating method, It including the notion that civiization | the known half-life of *c. showed all of Libby's results Crignat in Euene ent cased were similar to the known ages, throughout the word. By dling To test the technique, Libby's thus proving the success of man-made artifacts from Europe, | aroup applied the ant- radiocarbon dating the Americas, Asia, Atica and ‘Oceania, archaeologists estab- lished that civilizations developed in many independent sites across the world, As they spent less time trying to determine artifact ages, archaeologists were able to ask ‘more searching questions about the evolution of human behavior in prehistoric times. 8 Libby's work also contibuted to geology. By using wood samples from trees once buried under Glacial ice, Libby proved the last ice sheet in northem North America receded 10,000-12,000 years ago, not 25,000 years as geologists had | | paren | paste previously estimated. J When Libby first presented a radiocarbon dating to the public, he seca oe or humbly estimated that the method ‘may have been able to measure The “Curve of Knowns” compared the known age of historical artifacts ages up to 20,000 years, With associated with the Bible, Pompeii and Egyptian dynasties with their subsequent advances in the age as determined by radiocarbon dating. (Lower radiocarbon content technology of ¥C detection, the indicates greater age.) The agreement between the two ‘method can now reliably date demonstrated the accuracy of the technique. This version was ‘materials as old as 50,000 years. presented by Libby during his Nobel Lecture in 1960,