In the early morning hours of February 27, 1940, chemist Martin Kamen sat in a cold, dark police station. Police officers apprehended the disheveled scientist, too tired to protest, outside of his laboratory at the University of California, Berkeley and hauled him to the station for questioning. They accused him of committing a string of murders that took place the previous evening.
But the police couldn’t pin the crimes on Kamen because the scientist had been locked away in his lab for the past three days, lobbing deuteron particles at a tiny sample of graphite with his colleague, the chemist Samuel Ruben. After he was released, Kamen went home for a brief nap, returned to the lab, and then made one of the most important discoveries of the 20th Century: the carbon-14 isotope.
“All life is made of carbon,” atmospheric chemist Mark Thiemens of the University of California, San Diego, tells Popular Mechanics. “The atmosphere has carbon dioxide. It’s part of the process of photosynthesis—carbon dioxide is used by plants to make oxygen. If you want to understand anything related to biology, you start with carbon.”
Kamen was a child prodigy. Born in Toronto in 1913, he was a remarkably talented musician—easily switching between the violin and viola—and graduated from high school early. To help fund his studies in chemistry at the University of Chicago, he played music in Chicago’s numerous speakeasies. After earning his Ph.D. and yearning for a change of scenery, Kamen took a position at UC Berkeley under the famed physicist E.O. Lawrence.
In Lawrence’s lab, he met Samuel Ruben, a talented chemist and boxer. Ruben was fixated on solving a biochemical conundrum. Scientists knew that, through photosynthesis, plants created oxygen. But what was the source? Was it carbon?
Kamen and Ruben conducted their experiments using a strange-looking machine called a cyclotron. The circular contraption accelerated atomic particles “to a few percent of the speed of light” along a cyclical path in order to create new nuclei and ions, according to John Marra, author of the book Hot Carbon: Carbon-14 and a Revolution in Science. This subatomic coliseum, a set of hollow electrodes called a dee, was sandwiched between two massive electromagnets.
The scientists had to schedule their experiments for the dead of night—the only time the machine was available. During the day, it was used for higher priority projects that sought new treatments for cancer. By irradiating hunks of graphite in the cyclotron, they were able to isolate the isotope, and forever change our understanding of life and its essential building blocks.
Scientists are especially interested in an element’s isotopes: atomic twins containing the same number of protons in their nucleus, but a different number of neutrons. When cosmic rays enter Earth’s atmosphere, they bombard nitrogen—the most common gas in our atmosphere—with neutrons, causing them to lose a proton and turn into different isotopes.
Carbon has three naturally occurring isotopes. Each isotope has a slightly different mass, and is therefore uniquely identifiable. Carbon-12 has six protons and six neutrons in its nucleus. Carbon-13 has an extra neutron. Carbon-12 is the most common isotope, and, along with carbon-13, is completely stable.
Carbon-14, however, is the isotopic black sheep of the carbon family. It is the rarest isotope of carbon, occurring once out of every trillion carbon atoms. It has six protons and eight neutrons, which makes it radioactive and causes it to decay into Nitrogen-14 at an infrequent but measurable rate. The isotope, in essence, acts like a radioactive time-keeper.
Carbon-14 has a half-life of 5,730 years, which means that nearly every 6,000 years, the amount of carbon-14 atoms in a sample of organic material like, say, bone or wood will be cut by half. Because carbon-14’s sister isotope, carbon-12, is so abundant in the atmosphere, it’s processed by plants through photosynthesis, and therefore found in almost all living things, too. Scientists are able to take a sample of material and analyze the ratio of stable carbon-12 particles to decaying carbon-14 particles.
Calculate It Yourself!
Scientists often use the following formula to calculate the rate of radioactive decay for a given sample:
N (t) = N0e -kt
N(t) is the number of carbon-14 atoms left in the sample after time.
N0 is the number of carbon-14 atoms that the sample had when it died.
t is time, the variable we're solving for.
-k is the rate constant for the radioactive decay.
Next, we'll relate the rate constant to the half-life for the first-order kinetics:
Bringing the rate constant to:
k=ln 2/5.73×103=1.21 × 10−4year−1
Therefore, the first equation can be written like so:
N (t)=N0e−ln 2t/t1/2
Tracking this ratio has been critical in unraveling mysteries in the fields of anthropology, archaeology, and paleontology, among countless others. But Kamen and Ruben’s discovery didn’t go mainstream until 1946, when Willard Frank Libby, a professor of chemistry at the University of Chicago, conceived of a way to use carbon-14 to date organic matter.
In his tests, he compared the carbon-14 footprint of two samples: gas extracted from a fossil fuel, which, due to its age, wasn’t expected to have any carbon-14, and a fresh sample of sewage from the city of Baltimore’s sewage system, which was.
Libby received the Nobel prize in Chemistry in 1960.
An Archaeologist's Dream Date
Previously, archaeologists and anthropologists relied on a method called relative dating to interpret how old a specimen might be. They read the geologic record: objects that were buried deeper were likely older than objects buried closer to the surface.
“Suddenly, what was ‘relatively earlier’ or ‘relatively late’ could be tied to points in time, and we could do that with a relatively high degree of accuracy,” Kurin says.
Using this prized isotope, scientists have estimated the age of ancient cave drawings carved into wasp nests; pinpointed when the mysterious Copper Age iceman Ötzi, whose frozen, mummified body was pulled from the Austrian Alps, lived; and tracked the movements of ancient civilizations.
“The general rule is that if you want to measure a process with a radioactive clock, the half life [of the isotope] has to be sort of around the right time scale of what you're measuring,” says Thiemens. “When they wanted to verify if the Shroud of Turin was real or not, carbon-14 was perfect.”
This also illustrates one of carbon dating’s only weaknesses: it only works on samples younger than about 55,000 years. Anything older than that doesn’t have enough quantifiable carbon-14 left.
“If you’re looking for organic remains that are under 50,000 years old, it’s kind of the gold standard,” bioarchaeologist Danielle Kurin of the University of California, Santa Barbara tells Popular Mechanics. “It’s incredible that these residues of the past are available to us.”
Dating in the Modern Age
Most research labs now use an instrument called an Accelerator Mass Spectrometer to identify how many carbon-14 atoms are in a given sample. These highly specialized machines require much less sample material for an accurate reading—just a single milligram compared to the 50 milligrams that were required by previous radiocarbon dating methods.
That can make all the difference with fragile specimens. “When we are sampling organic material, be they feathers or textiles or human remains, we want to do as little damage as possible,” says Kurin. Still, in many cases, they’re prohibitively expensive, and there’s always the concern that carbon could have been leached from its surroundings.
Monumental changes to the chemistry of Earth’s atmosphere can help scientists better constrain the dates provided by radiocarbon dating. Scientists have learned to calibrate their readings with information from other dating techniques. The amount of carbon in the atmosphere hasn’t always been the same. Dendrochronology, which measures time using tree rings, can be used to constrain dates as early as 11,000 years.
When the U.S. and other countries began testing atomic weapons, the composition of the atmosphere changed forever. The explosive nuclear weapons introduced new isotopes, including carbon-14, into the atmosphere. Scientists have used carbon-14 dating to identify bottles of counterfeit Scotch Whisky, which, contrary to what their label says, were produced after the nuclear tests were conducted.
Because we’re releasing more carbon dioxide into the atmosphere than ever before, scientists will have to calibrate future measurements to take this influx of emissions into account.
Dating on Thin Ice
Carbon isotopes have implications in other fields, too. In climate science, for example, it’s incredibly valuable. Scientists like Thiemens look at gas bubbles trapped in ancient ice to better understand ancient environments. Because of their stability and their longer half-lives, carbon’s stable isotopes specifically give scientists clues about what Earth’s climate was like millions of years ago. Even the radioactive, quick-decaying carbon-14 is useful.
As rocks become exposed to Earth’s atmsophere—be it through weathering, glacial recession, or the eruption of volcanic lava—they're also exposed to cosmic radiation and, in turn, carbon-14. Scientists can study how much carbon-14 is found in a particular sample to understand, for example, how quickly a glacier has been receding.
And of course—as Ruben intended—researchers are able to follow the path of the carbon cycle thanks to carbon-14. Scientists can put a plant in a chamber, fill it with a known amount of carbon dioxide, and see how much of that gas the plant absorbs. These experiments help scientists better understand how plants take up carbon from the atmosphere—a critical measurement used to model how climate change will progress in the coming years.
Rise, Fall, Redemption
The importance of Carbon-14 80 years after its discovery is difficult to understate. While the isotope would fundamentally change entire scientific fields and our understanding of the past, its creators didn’t fare quite as well.
Samuel Ruben, sadly, was killed three years after his famed discovery. On his second day on the job at the Office of Scientific Research and Development, he was exposed to deadly phosphine gas after a glass vial containing the toxic fume shattered.
Years later, after dining with a Russian diplomat, Kamen would be accused of yet another crime he didn’t commit—this time, it was espionage. Berkeley fired him, and in 1948, the House Un-American Activities Committee brought him in for questioning. It took Kamen decades to clear his reputation, and he later went on to serve appointments at Washington University, St. Louis, Brandeis University, and the University of California, San Diego. In 1995, he was awarded the prestigious Enrico Fermi award for lifetime scientific achievement by U.S. Department of Energy. He died in 2002.
“Radiocarbon dating allows us to determine when empires rose and when they collapsed, and allows us to tie macro environmental fluctuation to certain moments in time that may be linked to great cultural change,” says Kurin. "We’re so lucky to have this carbon isotope.”
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