February 27 – Halfway there

Today’s Factismal: Carbon-14 can be used to determine a person’s age – but not through radioactive decay!

In 1940, one of the great mysteries of biology was “where does the oxygen come from?” It was well known that when plants performed photosynthesis, they too in carbon dioxide (CO2) and water (H2O) and released oxygen. But did the oxygen come from the carbon dioxide, from the water, or from both sources? The biologists were stymied because one oxygen atom looked just like every other oxygen atom.

At least, they were stymied until the physicists came to the rescue. Physicists had succeeded in producing isotopes of various elements; versions of each element with the same number of electrons and protons, which mean that they reacted the same, but with different numbers of neutrons. By making water using a heavier isotope of oxygen, one with ten neutrons instead of eight, Sam Ruben was able to show that the oxygen produced by photosynthesis came from the water and not the carbon dioxide.
But every answer in science leads to a new question. If the oxygen that was being released came from the water, then what was happening to the carbon dioxide? How did carbon move through a plant during its life cycle? In order to determine that, Ruben needed an isotope of carbon that he could track, similar to the heavy oxygen isotope that he had used in water. Chemists had already discovered a carbon isotope with on fewer neutron (carbon 11) but it had a half-life of just 20 minutes, which was far too short a time to be useful.

So Ruben turned to Martin Kamen, who worked in the Berkeley Radiation Laboratory. Ruben used a type of “atom smasher” called a cyclotron to bombard sheets of carbon graphite. After a five day run, Kamen brought the results over to Ruben. They worked through the night and, on February 27, 1940, were finally able to show that the carbon had formed a previously unknown isotope that was radioactive but had a longer half-life.

Unfortunately, the new isotope’s half-life was a little too long for Ruben to use; he lost interest and gave the samples to another chemist who then used it in 1942 to trace how carbon was used in photosynthesis. And in 1949, Willard Libby realized that the long half-life of carbon 14 made it perfect for use in determining the age of artifacts such as the Dead Sea Scrolls and the Shroud of Turin. But it was another series of events in the 1950s and 60s that would make carbon 14 perfect for finding out how old someone is.

Carbon-14 levels in the atmosphere; the decrease is due to the amount of radioactive carbon that was taken up by the biosphere. (Image courtesy )

Carbon-14 levels in the atmosphere; the decrease is due to the amount of radioactive carbon that was taken up by the biosphere. (Image courtesy CDIAC)

From 1945, when the first atomic bomb was tested at Alamogordo, New Mexico, until 1980, when most nations stopped open-air testing, more than 400 atomic weapons with a total yield of nearly 600 megatons of TNT were exploded in the atmosphere. These nuclear tests had an unintended side-effect; they produced a lot of carbon-14. That carbon-14 then went into the biosphere, including the teeth and corneas of children. As a result, by measuring the amount of carbon 14 in a person’s tooth enamel or cornea, forensic scientists can get a very good measure of when that person was born.

If you’d like to help anthropologists as they sort through antiquities, then why not join the Portable Antiquities Scheme? They need help ensuring that newly discovered artifacts aren’t removed by collectors or thieves.

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