History of Science: Discovery of Carbon-14 Opens Window on Past Civilizations — February 27, 1940

On this day in 1940, two scientists discovered an elusive form of carbon, inadvertently opening a window to a lost civilization.

Since the mid-1930s, scientists have predicted the existence of carbon with two extra neutrons in its core, but their lifetimes were thought to be so short-lived that they were impossible to measure.

But Ernest Lawrence, who founded Berkeley Lab, was determined to find out. In 1939, he commissioned chemists Martin Kamen and Samuel Rubin to discover carbon-14. For a year, they were unable to find any trace of this elusive atom.

you may like

And in January 1940, they began a “desperate” experiment to place a piece of graphite (a crystalline form of carbon) inside a cyclotron, one of the first types of particle accelerators. The cyclotron bombarded the sample with deuterons, heavy hydrogen nuclei with one proton and two neutrons. It was hoped that the crystalline form of carbon would absorb extra neutrons and release protons, becoming a “heavy” version of carbon.

They ran the experiment for 120 hours straight. On February 15, a sleep-deprived Kamen stopped irradiating the sample with deuterons and returned home. He was so disheveled that police searching for the escaped murderer briefly questioned him.

After Kamen was released, he returned to the lab, where his colleague Ruben noticed faint signs of radioactivity in the samples. Over the next two weeks, they purified the carbon and converted it into CO2 gas, which was then pumped at right angles through a Geiger counter to measure its radioactivity.

Surprisingly, carbon did not have a short half-life, or the time it takes for half of the radioactive atoms to decay into stable atoms.

“The measured cross-sectional areas and low yields suggest a very long half-life (years),” the researchers wrote in a short paper published in Physical Review Letters on March 15, 1940.

Their measurements showed that it takes about 4,000 years for about half of the carbon-14 to decay to nitrogen-14. (We now know that carbon-14 has a half-life of about 5,730 years.)

Even back then, they recognized the importance of their discovery.

you may like

“Long-lived radiocarbons will be of great importance for many chemical, biological, and industrial experiments,” the researchers wrote in their paper.

Over the next few years, Ruben and Carmen used radioactive carbon and oxygen molecules to elucidate the key steps in photosynthesis. Sadly, Ruben died in a lab accident while working with toxic gases in 1943, and Carmen was fired from Berkeley for associating with musicians and people deemed “left-wing” during the Red Scare. In 1948, he was taken to testify before the House Un-American Activities Committee, and although he was not found guilty of any wrongdoing, he continued to be plagued by unfounded allegations for years.

Although the implications of Carmen and Reuben’s experiment were immediately obvious, it was not until 1949 that University of Chicago chemists James Arnold and Willard Libby demonstrated that the ratio of carbon-14 to stable carbon could be used to estimate the age of carbon-containing artifacts. Libby won the 1960 Nobel Prize in Chemistry for his work on radiocarbon dating.

Archaeologists routinely use radiocarbon dating to estimate the age of ancient human bones and other artifacts up to 50,000 years old. And newer techniques are leading to the analysis of radioactive isotopes of elements such as strontium to determine where ancient humans lived and died, what they ate, and what pollutants they encountered during their lifetimes.