The Discovery of Carbon 14 Dating

By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

The process of carbon 14 dating was discovered by American chemist Willard Libby, a professor at the University of Chicago. Carbon 14 dating, consequently, changed the world of archaeology. Dating objects became relatively easy, although doing it precisely still takes some care.

Figuring out how old something is can be hard. For short times, the number of winters that have come and gone since the event can simply be counted. But this gets trickier over longer times.

Over the timescale of human history, maybe someone turns to their library to study the histories that have been written. From that, they can piece together how long ago something happened.

But things are not always that easy. What if something happened before writing was invented? What if a skull is discovered in a backyard while someone is digging a trench? How do they know whether it is a person who ran afoul of Don Corleone or a grave of archaeological significance?

There are a number of ways, mostly looking at indirect clues. However, there is one method that works extremely well for biological samples, which is to say things that were once alive. This method is called carbon dating.

The Role of Carbon in the Human Body

Every living thing is made of carbon, along with other elements. Carbon is a great element from which to make life. The reason is that carbon can make lots of connections, what scientists call bonds, to other atoms. Because of this, organic chemistry, which is the chemistry involving the element carbon, is far more complex than other forms of chemistry.

At an atomic level, though, one does not need to know any of that. One just needs to know that humans, and every living thing, have a lot of carbon. In fact, if the human body is broken down into its constituent elements, carbon is the second most prevalent in terms of mass. Oxygen makes up more, but that is because oxygen is found in water, and H2O or water is a big component of blood and tissues. The bottom line is that people have a lot of carbon in them, about 18%, or 30 pounds for a 150-pound person.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.

Different Types of Carbon

It might surprise people to learn that there are several different kinds of carbon. Carbon is an element that contains six protons at the center of the atom. And that is always true. An atom with six protons is carbon. The most common kind of carbon also contains six neutrons and is called carbon 12. The 12 comes from the number of protons and neutrons because six plus six equals 12.

But not all forms of carbon contain just six neutrons. There is carbon 13, which consists of six protons and seven neutrons. And there is also carbon 14, which consists of six protons and eight neutrons.

From a chemical point of view, there is no substantive difference between carbon 12, carbon 13, and carbon 14. But, from a nuclear physics point of view, these different versions of carbon are what scientists call isotopes. Carbon 12 and carbon 13 are stable and exist forever. However, carbon 14 is slightly radioactive and decays.

Learn more about exposing the truth about radiation.

The Decay of Carbon 14 Isotope

Carbon 14 decays into nitrogen 14 by beta decay. This means that a neutron in a carbon 14 atom turns into a proton and an electron, and the electron escapes. If that decayed electron is seen, it is possible to know that a carbon 14 atom has decayed.

The half-life of carbon 14 is approximately 5,730 years. Remember that half-life is the amount of time it takes for half of the substance to decay into the daughter product. In two half-lives, what will remain is a quarter of the original substance because half of a half is a quarter, and so on.

This process of carbon 14 decay, which starts after the death of a living thing, can be thus utilized to determine the age of the dead organic matter.

Learn more about what’s inside atoms.

The Discovery of Carbon 14 Dating

Carbon 14 dating changed the world of archaeology. Dating objects became relatively easy, although doing it precisely takes some care. But credit should be given where it is due. Carbon 14 dating was brought to the world from the brain of American chemist Willard Libby, a professor at the University of Chicago. He had worked on the Manhattan Project and developed uranium purification techniques that eventually became widely used in the industry.

He calculated that for a gram of carbon, with a concentration of about one part per trillion of that carbon being carbon 14, then about 14 disintegrations per minute could be expected. If only 7 disintegrations per minute were seen, then the conclusion was that the object was one half-life, or 5,730 years old.

Now it turns out that the electron that is emitted in carbon 14 decay, when it turns into a nitrogen atom, has a very low energy. It is really very hard to detect it. Libby had his original idea about 1939, when he first heard about carbon 14 being created in the atmosphere, but that whole war thing got in the way.

Publishing of Carbon 14 Dating by Willard Libby

He tabled the idea after World War II when he formally published his idea in 1946 in the journal Physical Review. Because of the difficulty at the time of detecting such low energy of an electron, it took him a couple of years to develop the proper technology and techniques. In 1949, he published his first results in the journal Science. For his work, he received the 1960 Nobel Prize in Chemistry.

Dendrochronology or Counting the Tree Rings by Willard Libby

Libby validated his work in several ways. He dated the death of a sequoia by exploiting a technique called dendrochronology. This is performed by taking borings of ancient living trees and counting the tree rings.

One takes borings of trees that died many hundreds of years ago and matches them up with currently living trees. Using this technique, someone can bootstrap their way back more than 1,000 years. It is really a very clever technique. When Libby did that, he found he could accurately determine the age of the piece of wood.

Libby’s first big triumph came with the dating of the last surge of glacial growth before the current warming period. Prior to carbon dating, geologists dated the destruction of a forest as a glacier surged over it as about 25,000 years B.C. But he was able to use wood from the destroyed forest and found that the trees died about 11,400 years old, with an uncertainty of about plus or minus 350 years. That is a broad range, to be sure, but definitely not 25,000 years old.

Common Questions About the Discovery of Carbon 14 Dating

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