Research Profile

by Luisa Bonolis

Willard Frank Libby

Nobel Prize in Chemistry 1960
"For his method to use carbon-14 for age determination in archaeology, geology, geophysics, and other branches of science".

Toward Nuclear Chemistry
Willard Frank Libby was born on December 17, 1908, in Grand Valley, Colorado and grew up in a ranch north of San Francisco. After graduating from high school, in 1927 he entered the University of California at Berkeley and enrolled as a mining engineer, but during his studies he became interested in chemistry and thus enrolled in chemistry, physics, and mathematics courses. After receiving his BS degree in 1931, he continued his university work at Berkeley, also studying under physical chemist Gilbert Newton Lewis, and learned to build Geiger-Müller counters which could be used to detect radioactive isotopes emitting low-energy radiation. Using this technique he discovered, independently of the work of G. de Hevesy and M. Pahl, the natural alpha-particle radioactivity of samarium. The experimental skill acquired through the study of weakly radioactive substances laid the basis for his future research activity.
Libby received his PhD degree in 1933 and joined the Berkeley faculty where he rose through the ranks until he became associate professor in 1945. His investigations in natural and induced radioactivity, isotopic-tracer techniques and hot-atom chemistry were among the earliest work in the field of nuclear chemistry in the US.
In the meantime, after the United States entered World War II, in 1942 Libby was asked to join the the top secret Manhattan Project and worked with Nobel chemistry laureate Harold C. Urey, at Columbia University to develop methods for separating uranium isotopes by gaseous diffusion, an essential step in the creation of the nuclear bomb. This work definitely led to Libby's interest in nuclear science.
In October 1945, at age thirty-six, Libby accepted a position as full professor of chemistry at the University of Chicago, in the Department of Chemistry and the Institute for Nuclear Studies (now the Enrico Fermi Institute for Nuclear Studies) thus becoming the youngest full professor in that university.

The Discovery of Carbon-14
The concept of dating inorganic materials using decay rates of radioactive elements present in the sample, went back to early studies of radioactivity. In 1908, after William Ramsay and Frederick Soddy had determined the rate at which radium produces alpha-particles, Rutherford himself proposed that he could determine the age of a rock sample by measuring its concentration of helium. The discovery of radioactivity actually upset previous assumptions on which most estimations of the age of earth were based, providing a basis for new calculations in the form of radiometric dating on a geologic time scale.
On the other hand, the use of isotopes as tracers in biological cycles pioneered by George de Hevesy in the early 1920s, had provided a concrete foundation to reason on the possibility of detecting the presence of minutes traces of active elements in living matter.
Both these properties found a new unexpected application in the early 1940s, in studying the fate of carbon-14, an isotope of carbon produced by primary cosmic-rays in the higher layers of the atmosphere.
The neutron-induced transmutation of atmospheric nitrogen to carbon-14 was discovered at the Lawrence Radiation Laboratory in Berkeley in the early 1930s. In 1934 Franz N. D. Kurie, investigating neutron-induced disintegration of light elements with a cloud chamber, proposed that when nitrogen was exposed to fast neutrons, a less frequent reaction could be observed, leading to the transformation of the abundant ordinary nitrogen-14 into carbon-14.

Kurie also suggested that some of the tracks he was observing might arise even from helium-2 (deuterium) and helium-3 (tritium), the two isotopes of hydrogen. The first one had been recently discovered by Harold Urey, while tritium had been just obtained by Rutherford, M. L. Oliphant, and P. Harteck, bombarding deuterium with high-energy deuterons (nuclei of deuterium atoms).
Kurie's work and later research of T. W. Bonner, W. M. Brubaker, W. E. Burcham and M. Goldhaber, definitely clarified that the irradiation of air in a cloud chamber with neutrons induced the reaction: neutron + nitrogen-14 = hydrogen-1 + carbon-14.
The neutrons producing the reaction appeared to be of thermal, or of near thermal energy. During the period 1934-1936, the thermal-neutron absorption cross-section for nitrogen was found by Fermi's group in Rome to be quite large, compared to most materials. This property indicated in fact that thermal neutrons in air would be expected to react with nitrogen essentially converting it into carbon-14 by this kind of reaction.
In the physical sense, the discovery of carbon-14 had thus been established by these studies, but the establishment of its existence in the chemical sense, which was to reveal the most important one, was delayed by many obstacles.
The table of isotope showed that there was a blank at the position mass 14 in the element carbon, so that it seemed reasonable to try to produce it by irradiating nitrogenous materials with neutrons. In US it was natural to look toward the Radiation Laboratory at Berkeley, where future Nobel physics laureate Ernest O. Lawrence was assembling a group of physicists attracted by the cyclotron, a much stronger particle source than any then existing. But detection of carbon-14 was not an easy task.
The new 60-inch cyclotron was in operation on a full schedule when the discovery of nuclear fission burst on the world in January 1939. At that time Lawrence decided that both cyclotrons must be diverted to a full-time effort to determine definitely whether long-lived isotopes of hydrogen, carbon, nitrogen, and oxygen did or did not exist. Lawrence asked Martin Kamen to organize a complete and systematic campaign in this direction and in early 1940, Kamen and Samuel Rubens - who had previously worked under Libby as a doctoral student - succeeded in obtaining enough radiocarbon by bombarding graphite with a strong deuteron beam from the new Berkeley cyclotron. They were also able to make a rough estimate of its surprisingly long half-life, remarkably close to the disintegration value now accepted of about 5,730 years.
In the meantime, the cosmic-ray physicist Serge Alexander Korff of New York University had found in 1939 that cosmic rays produce showers of neutrons when they strike atoms in the top of the atmosphere.
At this point the question was: What will million-electron-volt neutrons do once liberated in the air? Oxygen is essentially inert to neutrons, but the atmosphere contains about 78 percent nitrogen, and the abundant isotope nitrogen-14 was known to be quite reactive. Korff answered to this question pointing out that the main part of the total number of neutrons produced as byproduct of cosmic radiations, results in the formation of carbon-14 in the atmosphere.
After the war, from the data of Korff and B. Hammermesh it was in fact possible to estimate that, on average, one or two atoms of carbon-14 would be produced in this way each second for each square cm of the earth's surface.
In 1941 Libby and R. Cornog had also shown that fast neutrons on nitrogen make tritium and carbon-12, in analogy to the above mentioned reaction by which slow neutrons on nitrogen make ordinary hydrogen of mass 1 and carbon-14. On this basis, in June 1946 Libby called thus the attention to a possible explanation of the tenfold greater abundance of helium-2 (as decay product of hydrogen-3) in atmospheric helium, as compared to helium from gas wells. Such traces of tritium could be thus used as a tracer for atmospheric water. By measuring tritium concentrations, Libby later developed a method for dating well water and wine, as well as for measuring circulation patterns of water and the mixing of ocean waters.

The Radiocarbon Clock
At the same time, Libby suggested that the newly formed carbon-14 has high energy at the moment of its formation, so that it rapidly oxidises to carbon dioxide, which spreads out and distributes itself evenly in the atmosphere.
Active and non-active carbon dioxide are dissolved in a constant ratio in the water of the seas and lakes where they are converted into carbonate and bicarbonate, and they are assimilated by trees and plants during photosynthesis, and finally also by all forms of terrestrial life that consume vegetation contain the same proportion of carbon-14. Since the age of the earth - remarked Libby - is much greater than the life of radiocarbon, a radioactive equilibrium must exist in which the rate of disintegration is equal to the rate of production. It appeared that in this great span of time there is adequate opportunity for the carbon dioxide to form, for the atmosphere to mix, for the oceans to mix, and for the biosphere to cycle many times. All this formed a grand system which is continually stirred.
Assuming that the intensity of the cosmic radiation has been constant during the last few tens of thousands of years, the rate of formation of carbon-14 would be equal to the rate at which it disappears to reform nitrogen-14. The average lifetime of carbon-14 should be sufficiently long to allow for the formation of a stationary state in the concentration of this isotope not only with reference to the atmosphere, but also to the hydrosphere and biosphere as well. The continuous labelling of the biosphere and living matter must be thus characterized by an expected number of disintegrations per second per gram of carbon.
At that time, tracer methodology, an offspring of nuclear science, was already providing essential support for the ever widening and deepening knowledge of structure and function in biological systems. Now it was on the verge of increasing by large factors the already large total benefit which up to that time had been derived from it.
It was clear from the previous set of assumptions that organic matter, while it is alive, is in equilibrium with the cosmic-radiation; that is, all the radiocarbon atoms which disintegrate in living bodies are replaced by the carbon-14 contained in the food.
Since the neutron intensity at sea level is negligible, Libby and his collaborators were thus led to the prediction that the exchange of carbon with its surroundings by living bodies would cease at the time of death. At death isolation occurs, and the radiocarbon clock starts ticking.
The radioactive disintegration process takes over in an uncompensated manner as no additional carbon-14 is incorporated into tissues, and, according to the exponential law of radioactive decay, the ratio of carbon-14 to the stable and most abundant carbon-12 constantly changes at a known rate. By measuring the remaining activity, it should be thus possible to determine the time elapsed since death by direct comparison of the specific activity of the specimen with that of living matter in general.
The first steps to be taken to develop the idea were to look for natural radiocarbon in living systems and to measure the half-life accurately, a value on which the precision of radiocarbon dating depends to a high degree.
Libby and his collaborators started by analysing living matter to see whether it was true that about one atom in 1000000000000 of the carbon in living systems was radiocarbon. The estimate of the amount of diluting carbon was difficult, for it was not known at that time whether the oceans really mixed in the lifetime of radiocarbon. Only later they demonstrated that this condition is fulfilled within the accuracy of approximately 10 percent. At the moment it was thus assumed that the mixing was rapid, and it was hoped that this crucial assumption was correct, because the stability of the reservoir that is given by the ocean could not have been obtained in any other way, as the ice ages came and went, and the amount of living matter on earth changed considerably. It is the essence of the radiocarbon dating method that at the instant of death the radiocarbon concentration is known accurately.
The experience in the measurement of feeble amounts of radioactivity which the group had gained on other connections over the years, as well as some fortunate circumstances, such as the possibility of having at disposal enriched methane from sewage gas concentrated by Aristid V. Grosse in his thermal diffusion column, helped in the difficult detection of natural carbon-14 in the early stage of this research. In fall 1947 the group was able to report evidence for the existence of radiocarbon in living matter, in about the expected amount.
After having verified that natural radiocarbon did exist, they went to work on the problem of how to increase the sensitivity of detection of the low-energy, short-ranged radioactive carbon radiation by two or three orders of magnitude, without the cumbersome process of enrichment. An important key was finding an effective way to distinguish between background radiation and the relatively weak and infrequent beta-decay from carbon-14 in natural samples, by a clever use of a shield of counters in anticoincidence with the radiocarbon dating counter. The latter was a screen wall-counter (a Geiger counter with a grid between the centre wire and the outer-tubing) which had been developed by Libby himself.

Having acquired a sufficiently sensitive and practical technique, they began to test the main assumption on which radiocarbon dating is based: the natural distribution and concentration of radiocarbon on earth. This problem was Ernest Anderson's doctoral thesis project. He took wood samples collected about the turn of the century from widely dispersed places, as well as seal meat and oil from Antarctica, coming from exploratory expeditions. All gave the same result. The next step was to try the dating method, for which they were joined by James R. Arnold, a physical chemist of Princeton.
During the next two years, as efforts were made to establish the efficacy of the radiocarbon dating system, Libby and his two assistants, began to collaborate with archaeologists in an attempt to provide that discipline with more precise chronologic specification. However, how could they expect a museum keeper to give precious invaluable materials which would be destroyed to determine the carbon-14 content? A group of recognized experts helped in giving advise and in acquiring the materials for them.
Libby and his group then verified the accuracy of the radiocarbon dating technique by applying it to samples of fir and redwood trees whose exact ages had already been determined by counting their annual rings. The method was extended to historical artifacts of known age. Wood from 12th Dynasty Egyptian Pharaoh Sesostris III's funerary boat was estimated to be 3,261 years old; its known age was actually 3,750 years.
In December 1949 they were able to demonstrate the inverse relation between the carbon-14 content and age for a series of known-age samples, and published their first “Curve of Knowns.”
Later Libby also dated linen wrappings from the Dead Sea Scrolls, bread from Pompei buried in the eruption of Vesuvius (AD 79), charcoal from a Stonehenge campsite, and corncobs from a New Mexico cave, and he showed that the last North American ice age ended about 10,000 years ago, not 25,000 years ago as previously believed by geologists. The most publicized and controversial case of radiocarbon dating is probably that of the Shroud of Turin, which believers claim once covered the body of Jesus Christ but which Libby’s method applied by others shows to be from a period between 1260 and 1390.
In 1960 Willard Frank Libby was awarded the Nobel Prize in Chemistry “for his method to use carbon-14 for age determination in archaeology, geology, geophysics, and other branches of science.” In nominating Libby for the Nobel Prize, one scientist stated: “Seldom has a single discovery in chemistry had such an impact on the thinking in so many fields of human endeavour. Seldom has a single discovery generated such wide public interest.”
Certainly one of the most sensational was the discovery in September 1991 of a well preserved find of a human in the melted ices of South Tyrol Alps, immediately dated by an archaeologist to be at least four thousand years old. Until this discovery, such a well preserved find of a human several thousand years old - fully clothed and with numerous personal belongings - had never before been seen anywhere in the world. Carbon-14 analysis performed by four different scientific institutions provided indisputable proof of the authenticity and extraordinary age of the iceman and his possessions. The results were unequivocal: the iceman lived between 3,350 and 3,100 BC, and was therefore walking and hunting along the valleys and peaks of the Alps some 5,000 years ago.


Bibliography

G. B. Kauffman (2008). Libby Willard Frank, Complete Dictionary of Scientific Biography, Vol. 22. Detroit: Charles Scribner's Sons: 295-300
G. T. Seaborg (1981). Willard Frank Libby, Physics Today 34(2), 92-95
W.F. Libby (1967). History of Radiocarbon Dating, University of California, Los Angeles (United States). Department of Chemistry and Institute of Geophysics and Planetary Physics, Report Number IAEA-SM--87/40, http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0336&numPages=23
W.F. Libby (1972). Radiocarbon Dating, Memories, and Hopes, Report Number Conf-721019--(P1), http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0338&numPages=17&fp=N
M. D. Kamen (1963). Early History of Carbon-14, Science 140 (3567): 584-590
E. K. Ralph and H. N. Michael (1974). Twenty-five Years of Radiocarbon Dating: The long-lived bristlecone pines are being used to correct radiocarbon dates, American Scientist 62 (5): 553-560
R. E. Taylor (2000). Fifty Years of Radiocarbon Dating, American Scientist 88: 60-67



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