Martin Kamen and Samuel Ruben's discovery of the radioactive isotope carbon-14 in 1940 helped usher in a new era of dating artifacts from past civilizations.
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Milestone: Carbon-14 discovered
Who: Martin Kamen and Samuel Ruben
On this day in 1940, two scientists discovered an elusive form of carbon — and inadvertently opened a window into lost civilizations.
Since the mid-1930s, scientists had predicted the existence of a form of carbon with two extra neutrons in its nucleus, but they thought it would be so short-lived that it would be impossible to measure.
But Ernest Lawrence, who founded the Berkeley Laboratory, was determined to find it. In 1939, he tasked chemists Martin Kamen and Samuel Rubin with discovering carbon-14. For a year, they found no hint of the elusive atom.
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Then, in January 1940, they launched a "desperation" experiment, in which they placed a piece of graphite (a crystalline form of carbon) inside a cyclotron, one of the first types of particle accelerators. The cyclotron bombarded their sample with deuterons — nuclei of a heavy form of hydrogen with one proton and two neutrons. The hope was that the crystalline form of carbon would absorb the extra neutrons, emit a proton, and become a "heavy" version of carbon.
They ran the experiment for 120 hours straight. On Feb. 15, a sleep-deprived Kamen stopped bombarding the sample with deuterons and headed home. He was so disheveled that police, who were looking for an escaped murderer, briefly questioned him.
When Kamen was released, he returned to the lab, where his colleague Ruben noted faint signs of radioactivity in the sample. For the next two weeks, they purified the carbon, converting it into a CO2 gas that could be pumped at the right angle at the Geiger counter to measure its radioactivity.
Surprisingly, the carbon did not have a short half-life — the time it takes for half the radioactive atoms to decay into a stable atom.
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"The measured cross section coupled with low yield suggests the half-life to be very long (years)," the researchers wrote in a short paper published March 15, 1940, in the journal Physical Review Letters.
Their measurements indicated it would take around 4,000 years for about half the carbon-14 to decay into nitrogen-14. (We now know the half-life of carbon-14 is about 5,730 years.)
Even at the time, they recognized the significance of their find.
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- 'It is the most exciting discovery in my 40-year career': Archaeologists uncover evidence that Neanderthals made fire 400,000 years ago in England
"Long-lived radio-carbon will be of great importance for many chemical, biological, and industrial experiments," the researchers wrote in the paper.
In the next few years, Ruben and Kamen used radioactive carbon and oxygen molecules to elucidate the key steps in photosynthesis. Sadly, Ruben died in 1943 in a lab accident while working with a poisonous gas, and Kamen was fired from Berkeley after having social interactions with musicians and other people considered "leftists" during the Red Scare. In 1948, he was hauled up to testify in front of the House Un-American Activities Committee, and although he was never found guilty of any wrongdoing, he was dogged by unfounded allegations for years.
While the implications of Kamen and Ruben's experiments were immediately apparent, it wasn't 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 ages of carbon-containing relics. Libby would earn the 1960 Nobel Prize in chemistry for his work on radiocarbon dating.
