Willard Frank Libby

Willard Frank Libby

The American chemist Willard Frank Libby (1908-1980) pioneered in radiocarbon dating, for which he received the Nobel Prize.

Willard Libby, a farmer's son, was born on December 17, 1908, at Grand Valley, Colorado. After schooling near Sebastopol, California, he entered the University of California at Berkeley, graduating in 1931, earning his doctorate in 1933, and teaching physical chemistry there until World War II.

Libby's most notable achievement, the method of radiocarbon dating, stemmed from the 1939 discovery that cosmic rays at about 10 miles' altitude interacted with air to give a relatively high density of neutrons. This implied rapid formation of radiocarbon by neutron capture by the abundant nitrogen isotope. Radiocarbon has a long halflife (about 5,730 years), decaying into nitrogen. It may be assumed that after many thousands of years its rate of formation equals its rate of disintegration. If it is rapidly oxidized to carbon dioxide and this enters the biosphere, all living things will have the same specific radiocarbon content, that is, the same proportion of this isotope to the others. But at death carbon absorption stops and thereafter the specific radiocarbon content of the organic remains will steadily diminish with time. Accurate measurements should therefore establish the time that has elapsed since death; a new method of geological dating is thus provided.

During the war Libby worked on isotope separation by the gaseous-diffusion method, and his ideas on radiocarbon dating remained embryonic. In 1945 he moved to the Enrico Fermi Institute of Nuclear Studies, Chicago, and began an extensive study of radiocarbon. The halflife was accurately measured on the artificially produced isotope. The natural isotope was discovered by comparing the radioactivity of methane from sewage and petroleum. The former, only recently out of the biosphere, had a measurably higher activity.

These measurements were made on borrowed equipment by an expensive technique known as isotope enrichment, so Libby decided to devise a simpler method using more sensitive apparatus. Unfortunately, more sensitive counters picked up "background" radiation, much of it due to penetrating cosmic rays. Attempts by Libby to shield the apparatus in various ways met with limited success. The problem was solved by surrounding the counting equipment containing the sample with counters which switched off the central counter whenever an interfering particle (muon) arrived. With this refined apparatus Libby, with E.C. Anderson, made radiocarbon dating a practical possibility. For this work Libby received the Nobel Prize for chemistry in 1960. His method has now become an important routine tool in archeology.

From 1954 until 1959 Libby was research associate in the Geophysical Laboratory of the Carnegie Institute and simultaneously served on the U.S. Atomic Energy Commission. In 1959 he was appointed professor of chemistry at the University of California. In addition to his work on radiocarbon he applied similar considerations to tritium; thus he showed that water remains about nine days in the atmosphere between evaporation and precipitation. Libby was long interested in the behavior of "hot atoms," that is, those whose high energies derive from recoil in nuclear transformations, and used isotopes to study exchange reactions, especially in solution.

In 1966, Libby divorced his wife Leonor and later married Leona Woods Marshall, a professor of environmental engineering at UCLA. Libby remained at the University of California as the Director of the Institute of Geophysics until his retirement in 1976. He died in Los Angeles on September 8, 1980, from complications ensuing from a bout with pneumonia.

Further Reading

A sketch of Libby's life is in Eduard Farber, Nobel Prize Winners in Chemistry, 1901-1961 (rev. ed. 1963). A similar sketch can be found in the H.W. Wilson Company's Nobel Prize Winners (1987). For background see Theodore Berland, The Scientific Life (1962), and Lynn and Gray Poole, Men Who Dig Up History (1968). A more recent biographical sketch is included in the H.W. Wilson Company's Nobel Prize Winners (1987). □