It was while working in the Kent Laboratory building in the 1940s that Prof. Willard Libby and his UChicago associates developed radiocarbon dating—an innovative method to measure the age of organic materials. Scientists soon used the technique on materials ranging from the dung of a giant sloth from a Nevada cave; seaweed and algae from Monte Verde, Chile, the oldest archaeological site in the Western Hemisphere; the Shroud of Turin; and the meteorite that created the Henbury Craters in northern Australia.
Now the American Chemical Society has designated the discovery of radiocarbon dating as a National Historic Chemical Landmark. The society will officially recognize the achievement at 4 p.m. Oct. 10, with the unveiling of a plaque in the foyer of the Kent Chemical Laboratory building at 1020 E. 58th St.
This year marks the 70th anniversary of Libby’s first publication on radiocarbon dating, which appeared in the June 1, 1946 issue of Physical Review. The work earned Libby the 1960 Nobel Prize in chemistry “for determinations in archaeology, geology, geophysics and other branches of science.”
The technique, which measures materials’ content of carbon-14, quickly made an impact on archaeology and geology. Archaeologists testing the ages of artifacts from multiple sites across the Eastern and Western hemispheres found that civilization originated simultaneously around the world rather than in Europe. And Libby himself, when he analyzed wood samples from trees once buried beneath glacial ice, documented that North America’s last Ice Age ended approximately 11,000 years ago—not 25,000 years ago as previously believed.
“This radiocarbon dating method was a transformative advance to archaeology and historical studies, allowing the determination of the age of archeological sites and objects without reliance on a knowledge of local customs and history,” said Viresh Rawal, professor and chair of the Department of Chemistry.
Transformative research
The designation of UChicago as a National Historic Chemical Landmark joins the University’s 2006 designation by the American Physical Society as an historic physics site to commemorate the work of Robert Millikan, who received the 1923 Nobel Prize in physics for experiments conducted at the Ryerson Physical Laboratory building, 1100 E. 58th St. A plaque commemorating that work hangs in the first-floor lobby of the Kersten Physics Teaching Center, 5720 S. Ellis Ave.
Two scientists working at the University of California Radiation Laboratory in Berkeley discovered carbon-14 in 1940. The only previously known radioactive carbon isotope at the time was carbon-11, which had a half-life of only 21 minutes (half the isotope’s radioactivity will decay in that time).
Radiocarbon dating depended upon the discovery cosmic rays, which constantly bombard Earth and turn some carbon atoms in living tissue into radioactive isotope carbon-14. The isotope has a half-life of approximately 5,600 years, which means that during this period, half the number of radioactive carbon atoms in any once-living substance will convert to nitrogen. By this means, scientists may date objects as much as 50,000 years old.
Minute radioactivity levels
With his first graduate student, Ernest Anderson, and others, Libby determined that the expected minute level of radioactivity in organic material actually existed. This work enabled Libby and postdoctoral associate James Arnold to publish a carbon-14 atomic calendar in the Dec. 23, 1949 issue of the journal Science. They documented the viability of the technique with this article, which compared the ages of samples of known age with the ages as determined by their radiocarbon content.
The University announced Libby’s results in a news release issued in connection with the Science article. Libby collaborated extensively with Oriental Institute archaeologist Robert Braidwood in conducting C-14 tests on artifacts of known age from Mesopotamia and Western Asia, including wood from an Egyptian mummy’s casket.
Other tested samples included part of the deck of a funeral ship placed in the tomb of Sesostris III of Egypt, the heartwood of one of the largest redwood trees ever cut, and the linen wrapping one of the Dead Sea Scrolls. The second edition of Libby’s Radiocarbon Dating, published by the University of Chicago Press in 1955, lists 27 pages of objects for which he had obtained radiocarbon dates before the fall of 1954.
“Libby’s method remained the only way to measure carbon-14 in samples for several decades and was long considered the most accurate means of dating by carbon decay,” said David Mazziotti, a UChicago chemistry professor who submitted the formal nomination of the site as a historic chemical landmark to the American Chemical Society. “Within 10 years of Libby’s 1949 Science paper, there were 20 radiocarbon dating laboratories around the world.”
—Prof. Jacqueline Stewart
