The Search and Discovery of C-14, and the Revolution It Created

Radiocarbon dating helped verify the authenticity of the Dead Sea scrolls. Public Domain

Serge Korff was busy during the 1930s. He earned his doctorate in physics in 1931 from Princeton University (where he also earned his bachelor’s and master’s degrees), and began a long pursuit of study: discovering the secrets of the energetic particles that filtered from space through Earth’s atmosphere. These were called cosmic rays.

His pursuits took him to CalTech and the Mount Wilson Observatory as a research fellow in 1932; the mountains of Peru in 1934 and 1935, as well as Brazil and British Guiana; the Carnegie Institution in Washington in 1936; and, for the remainder of the decade, the Bartol Research Foundation in Delaware as a research fellow.

In 1939, he made a substantial breakthrough. Korff conducted cosmic readings by way of radiosondes, a device carried to the upper atmosphere by way of helium or hydrogen balloon. His device included a proportional counter filled with boron trifluoride (BF3) in order to detect neutrons within the star-produced cosmic radiation. He, along with fellow physicists, Hans Bethe and George Placzek, wrote “On the Interpretation of Neutron Measurements in Cosmic Radiation” for the scientific journal, Physical Review, which was published on April 1, 1940.

Serge Alexander Korff in the 1930s. Los Angeles Times/CC BY 4.0

The prime discovery Korff made in 1939 was that as cosmic rays bombarded the atmosphere, they were producing neutrons. From there, Korff made the prediction that “reaction between these neutrons and nitrogen-14, which predominates in the atmosphere, would produce carbon-14.” A decade before Korff’s prediction, the stable carbon isotope, carbon-13, was discovered by physicists Arthur S. King and Raymond T. Birge. Carbon-14 (C-14), however, had yet to be discovered.

Creating Carbon-14

Since finding C-14 in nature proved impossible with the scientific technology of the day, two physicists at the University of California Radiation Laboratory in Berkeley decided to try to create the radioisotope artificially. Martin Kamen and Samuel Ruben began their experimentation in 1939.

It was during this week in history on Feb. 27, 1940, and after a year’s worth of testing, that Kamen and Ruben discovered the existence of the eight-neutron and six-proton isotope carbon-14.

The two also discovered that C-14 possessed a half-life of about 5,700 years (later revised to 5,730 plus or minus 40 years), which meant that “half the atoms in a sample will change into other atoms, a process known as ‘decay,’ in that amount of time.”

The most common carbon isotope is carbon-12, which accounts for 98.93 percent of the Earth’s carbon. Carbon-13, with its seven neutrons and six protons, makes up 1.1 percent of the Earth’s carbon. Both of these carbon isotopes are stable and not radioactive. Considering the percentages, it would seem that there wouldn’t be any space left on Earth for C-14. But in fact, it accounts for 0.0001 percent of the carbon on Earth, and it is both unstable and radioactive.

Discovering this carbon isotope, however, was only just the beginning. Over the next five years, most of these physicists, among many others, became involved in the Manhattan Project, the World War II effort to create the atomic bomb. Tragically, Ruben died in 1943 while studying the poisonous phosgene gas in a laboratory. The same year of Ruben’s death, Kamen was assigned to the Manhattan Project, but shortly thereafter, his career was ruined for a period of time when journalists falsely reported a Soviet relationship.

C-14 Theories and a Lot of Help

Once the war was over in 1945, numerous physicists returned their focus to C-14, which still proved, for obvious reasons, elusive. One physicist, Willard Libby, of the University of Chicago, was inspired by the research, discoveries, and predictions of the aforementioned physicists, specifically by Korff.

Libby dedicated himself to the study of the carbon isotope and made his own theoretical prediction regarding radioactive decay. He theorized in a 1946 article for Physical Review that since C-14 was in the atmosphere, it could be found in every living thing. Additionally, since C-14 was radioactive and had a very long half-life, and presuming that the rate of concentration of C-14 in the atmosphere was constant over thousands of years, it would be possible to establish the age of preexisting organisms. His proposal was called “carbon dating.”

These theories, however, were still based on a carbon that had yet to be discovered in a natural state. Libby based part of his theory on Korff’s theoretical calculation of C-14’s existence in the atmosphere. Libby’s calculation suggested a ratio of just one C-14 atom per every 10 to the power of 12 carbon atoms. Since there was no technology sensitive enough to detect the specific atom, the professor needed help to solidify his theory. He requested Aristid von Grosse, a nuclear chemist, to provide a sample of methane, enriched in C-14, to conduct an experiment.

Libby, along with his graduate student assistant, Ernest Anderson, tested his theory using Grosse’s sample and a Geiger counter. The experiment worked, proving “the existence of naturally occurring carbon-14.” Additionally, his ratio proved accurate.

No More Coincidences

Willard Libby in the lab, circa 1960s. unknown/CC BY-SA 4.0

With these two theories proven, it was now time to test his theory of radiocarbon dating. Would it be possible to accurately date the lifetime of things? Even things that lived before civilization? Libby and his team at the University of Chicago got to work on building an ultrasensitive counter.

The team constructed an apparatus with a cylinder surrounded by Geiger counter tubes that could also detect outside radiation, which would help indicate whether or not a C-14 atom came from the organism within the cylinder. The apparatus was shielded with steel sheets to help reduce the unwanted outside radiation. Libby called it an “anti-coincidence counter.” After trial and error, the system worked reliably. Now, it was time to use it to test radiocarbon dating.

As a starting point for radiocarbon dating, Libby based the time structure on the carbon cycle, which is “the process that moves carbon between plants, animals, and microbes; minerals in the earth; and the atmosphere.” Libby posited that when something died, the organism was immediately cut off from the carbon cycle, and, therefore, the carbon dating clock immediately began. These organisms would act as a kind of time capsule, or so the theory went.

Testing the Known

Libby’s team decided it best to begin testing things of which they already knew the ages. The first samples were trees: redwood and fir. The ages of trees were easy to calculate by using their annual growth rings. More extravagantly, the team sampled a piece of wood from the funerary boat of Ancient Egypt’s pharaoh Senusret III, who died in 1839 B.C.

The stump of a very old bristlecone pine. Tree rings from these trees (among others) are used in building calibration curves. Public Domain

Libby and his team continued to sample artifacts and write down their findings. In December 1949, the journal Science published the findings of Libby and co-researcher James R. Arnold, a nuclear chemist who had also been part of the Manhattan Project.

The authors explained that their “measurement technique consisted in the combination of about 1 ounce of wood, the collection of the carbon dioxide, its reduction to elementary carbon with hot magnesium metal, and the measurement of 8 grams of this carbon spread uniformly over the 400-square-centimeter surface of the sample cylinder in a screen wall counter.” The authors also published a graph they called the “Curve of Knowns,” which situated those wood samples with the known dates and the radiocarbon date result. The results were groundbreaking.

“Seldom has a single discovery in chemistry had such an impact on the thinking in so many fields of human endeavor,” noted renowned chemist Kenneth Pitzer. “Seldom has a single discovery generated such wide public interest.”

The ‘Radiocarbon Revolution’

Libby’s discovery and method completely revolutionized numerous scientific fields by creating an “invaluable tool for archaeologists, paleontologists, and others looking for reliable dates for organic matter.” The anti-coincidence counter launched what would become the “radiocarbon revolution.”

Over the next decade, there were more than 30 radiocarbon laboratories established around the globe. The revolution was global, and the dating of ancient artifacts could now be reliably and accurately assessed.

As David Mazziotti, a chemistry professor at the University of Chicago, noted, “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 carbon decay.”

Libby was awarded the Nobel Prize in Chemistry in 1960 for his radiocarbon work. During his acceptance speech, he reflected on the inspiring work of Serge Korff and all those who assisted in the creation of the anti-coincidence counter and, therefore, the establishment of the transformative technique of radiocarbon dating.

Over the ensuing decades, the technology has greatly improved upon the foundation that, as Libby noted, had already “lived up to our fondest hopes.” When this dating technique was first introduced, the estimation of accurate dating covered approximately 20,000 years, but can now, through technological advances, like the accelerator mass spectrometer, accurately date formerly living artifacts from as far back as 50,000 years ago.

Measuring C-14 is now most commonly done with an accelerator mass spectrometer. Public Domain

“The dating technique … is something like the discipline of surgery—cleanliness, care, seriousness, and practice,” Libby said. “With these things it is possible to obtain radiocarbon dates which are consistent and which may indeed help roll back the pages of history and reveal to mankind something more about his ancestors, and in this way perhaps about his future.”

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