Shedding Light on Nuclear Radiation Episode 6: Radiocarbon Dating looks at how scientists use the presence of radioactive carbon-14 atoms in nature to determine the age of ancient artifacts. This dating technique is called radiocarbon dating. The program also explains how carbon-14 is continuously being produced high up in the atmosphere. If it wasn’t for this continuous production, radiocarbon dating just wouldn’t be a thing.
A 5-minute excerpt followed by a 1-minute trailer.
Part A: Introduction
Part B: Radiocarbon Dating with Carbon-14
Part C: How Carbon-14 is Made
Part D: How Radiocarbon Dating Developed…
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Transcript (more or less) (which also serves as a text book)
Part A: Introduction
In today’s lesson, we’re going to look at what’s called radiocarbon dating, which is the technique of finding out how old certain things are using radioactive carbon-14.
So, let’s just say some archeologists discover an ancient site in Greece or Egypt or whatever and in the ancient site they find, say, a preserved skeleton. Using our knowledge of carbon atoms, and in particular radioactive carbon-14 atoms, scientists have actually worked out a way of determining approximately when the person died.
So, let’s look at how radiocarbon dating works.
Part B: Radiocarbon Dating with Carbon-14
This is the mummified body of an adult man found in Egypt in the late 1890s. It’s believed that he died in about 3,400 BC, that is about 5,400 years ago. So how do they know that? Well, it comes down to carbon-14.
As we’ve seen in previous episodes, about 99% of carbon atoms on Earth are carbon-12 atoms, 1% are carbon-13 atoms, but 1 in a trillion or so are carbon-14 atoms.
Carbon-14 is a beta-minus emitter which has a half-life of about 5,730 years. It decays into nitrogen-14, as we can see from the nuclear equation.
While we’re alive, we eat food that is made up of carbohydrates, proteins, fibre, and fats, all of which are made in part of carbon atoms. As I said, about 1 in a trillion of all carbon atoms are carbon-14 atoms, so, right now we all have lots of carbon-14 atoms in us. You may think that 1 in a trillion isn’t many, so let’s look at some numbers.
Each gram of carbon (all 3 isotopes) consists of 5 x 1022 carbon atoms, which is a 5 with 22 zeros after it or 50 billion trillion (50,000,000,000,000,000,000,000). So, if 1 in a trillion of these is a carbon-14 atom, that’s still 50 billion carbon-14 atoms. That’s per gram remember. Now a human body is made up of about 18% carbon atoms by weight. I weigh about 80 kg, so 18% of that is about 14 kilograms, or 14,000 g. Since each gram of carbon has 50 billion carbon-14 atoms in it, we’re talking trillions of them.
(14,000 grams x 50 billion per gram = 700,000 billion or 700 trillion)
We are all very slightly radioactive thanks to the carbon-14 atoms we take in!
Now we consume food that has carbon atoms in it but we’re also constantly getting rid of the carbon atoms that we eat. Our biggest waste product is the carbon dioxide that we produce in a process called cellular respiration; basically, we burn the carbohydrates and fats to get energy:
carbohydrates (and fats) + oxygen –> carbon dioxide + water
The carbon dioxide comes out of our bodies when we exhale. We also lose chemicals that contain carbon atoms when we go to the toilet.
The upshot is, that the ratio of carbon-12 atoms to carbon-13 atoms to carbon-14 atoms in our bodies stays more or less the same throughout our lives. Carbon atoms come into our bodies, and carbon atoms are expelled from our bodies.
However, when a person dies, the intake and the expelling of carbon atoms stops. So, if the body is preserved and doesn’t rot away into the soil or whatever, then the atoms in that body mostly stay as they are.
The carbon-14 atoms though, decay by emitting a beta-minus particle and they turn into nitrogen-14 atoms. The number of carbon-14 atoms will therefore slowly decrease.
Scientists can now take a small fragment of say some bone from someone who died centuries ago and test it to see how many carbon-14 atoms it has compared to carbon-12 and carbon-13 atoms.
Now as I said, carbon-14 has a half-life of 5,730 years. This means that if the ancient bone or whatever that you’re testing started way back then with however many carbon-14 atoms (when it was still part of a living person), then after 1 half-life, it’ll only have ½ as many carbon-14 atoms and then after two half-lives it’ll only have ¼ as many, then an eighth, a sixteenth and so on. So, if the bone is found to have only ½ as many carbon-14 atoms as a bone of a living person normally has (which is about 1 in a trillion carbon atoms), it means that the preserved bone of the ancient person must be about 5,730 years old. If it only has a quarter of the carbon-14 atoms, then the bone must be 11,460 years old, since two half-lives have passed since the person… passed.
The smaller the percentage of carbon-14 atoms it has, the older it is.
Radiocarbon dating is not super accurate but if a bone from a dead person or animal is about 2000 to 3000 years old, they can get an accuracy of about ± 100 years, which is pretty good.
Radiocarbon dating can be used to find out how old lots of things are, as long as they were once part of a living thing and as long as they contain carbon atoms, and as long as they are well preserved. For example, wool is made in part from carbon atoms that are in the sheep’s food and 1 in a trillion of those carbon atoms is a carbon-14 atom. Once the sheep is shorn, and the wool is made into clothing or whatever, no new atoms of carbon are added. The carbon-14 atoms though emit radiation and turn into nitrogen atoms, so really old wool (I’m talking centuries old) has fewer carbon-14 atoms in it than newer wool.
Leather and fur also come from animals that were once alive and both have been used by humans for thousands of years, as seen on this recreation of a prehistoric caveman.
Now plants don’t eat but they too are made in large part of carbon atoms. Plants absorb CO2 (typically through their leaves from the air) and water (through their roots in the ground) and use these ingredients to produce glucose and oxygen using the energy of sunlight. It’s called photosynthesis. Plants then use the glucose as their fuel source AND they use the glucose to make more complex molecules. Once again, 1 1 trillionth or so of all the carbon atoms in the carbon dioxide are carbon-14 atoms which get incorporated into the molecules that make up the plant.
So, cotton for example has been used for thousands of years to make fabrics. Once the cotton is picked, no new carbon atoms are added. The age of ancient items made of cotton can be estimated by measuring how much carbon-14 is present.
Just like they did in the olden days, this woman is using a hand-operated weaving machine called a loom to weave a decorative fabric. This fabric actually contains wool, cotton, and silk fibres.
So you might ask, if carbon-14 has a half-life of 5,730 years, and the Earth is much much older than that, why are there any carbon-14 atoms here at all? Also, was there twice as much carbon-14 on Earth 5,730 years as there is today (since half of it should have decayed over 1 half-life)?
Well, it turns out that new carbon-14 atoms are continuously being made high up in the atmosphere at more or less the same rate as they decay, so the amount of carbon-14 on Earth stays more or less the same. If new carbon-14 atoms weren’t continuously being made, radiocarbon dating just wouldn’t be a thing. So, let’s have a look at how carbon-14 atoms are made.
Part C: How Carbon-14 is Made
To understand how cabon-14 atoms are created in nature, we need to get into some really serious nuclear equations because it’s a multi-step process.
Carbon-14 atoms and lots of other atoms are produced high up in the atmosphere by the action of so-called cosmic rays. So, what are cosmic rays?
Our sun and in fact most stars are mostly made up of hydrogen atoms and helium atoms. Hydrogen atoms make up about 90% of all the atoms in the sun and helium atoms make up about 9%. However, this is a little misleading.
It’s so hot in the sun, that the vast majority of the atoms are completely ionized. The sun is really made mostly of hydrogen nuclei (that is, protons), helium nuclei (which I’m not showing here), and free electrons, all moving at enormous speeds and crashing into each other. It’s like a gas, but it’s called plasma. Plasma is a state of matter like a gas, but it’s so hot that it’s made of really-fast-moving charged particles rather than actual atoms. Actual atoms of hydrogen and helium, that is, nuclei and electrons together, only form in the relatively cooler periphery of the sun.
Now every second, the sun throws out into space vast quantities of hydrogen and helium nuclei and electrons at really high speeds. Other stars do the same. Stars exploding in what are called supernova explosions, cast out even more charged particles with even more energy than stars normally do anyway. All of these charged particles flung out into space (the hydrogen and helium nuclei and the electrons moving at close to the speed of light) are called cosmic rays.
The cosmic rays are travelling so fast that they can collide with atoms in the atmosphere and cause nuclear reactions which create new atoms.
So, for example, high-speed cosmic-ray protons can collide with the nuclei of oxygen-16 atoms and cause them to break apart into protons, neutrons, helium nuclei, and boron-10 nuclei. Various combinations are possible, this is just one example. I’ll recreate the original cosmic-ray proton and the oxygen nucleus just to show you the before and after.
One proton strikes an oxygen nucleus and it fragments into separate bits.
Now as a quick refresher on atomic notation, 16 8 O tells us that this nucleus is made of 8 protons and a total of 16 nucleons (8 of which are neutrons).
Protons (now this is going to sound a little funny but go with me) protons are made of 1 proton and a total of 1 nucleon (because there are no neutrons) so they’re written as 1 1 p, p for proton of course. Neutrons are made of zero protons but 1 nucleon which of course has to be a neutron, so we write 1 0 n.
So that’s step 1 in the creation of carbon-14 atoms. Let’s look at Step 2.
The particles created in this nuclear reaction fly off at huge speeds, since the original cosmic-ray proton is travelling at close to the speed of light. Picture a break when someone’s playing pool but with much higher speeds. The smaller particles often have enough speed to cause further nuclear reactions, one of which involves neutrons and nitrogen-14 atoms.
This is the nucleus of a nitrogen-14 atom. A fast-moving neutron that was created in the previous nuclear reaction can smash into it which sends a proton flying off. What’s left behind is a carbon-14 atom. It has 6 protons, 8 neutrons and a total of 14 nucleons. That’s how carbon-14 atoms are made.
Let me re-draw everything and show you the nuclear equation. A lone neutron crashes into a nitrogen-14 atom which produces a carbon-14 atom and a lone proton.
Notice how we start with 7 protons and end up with 7 protons… though they’re no longer together. We also start with 15 nucleons and end up with 15 nucleons.
Radioactive carbon-14 atoms then join chemically with oxygen atoms to form carbon dioxide, CO2, molecules, which then enter the food chain when they’re absorbed by a plant to use in photosynthesis, and many then end up in animals or humans when the plants are eaten.
As I said earlier, by a strange co-incidence, the amount of carbon-14 being produced in the atmosphere is pretty much exactly the same as the amount that is lost due to natural radioactive decay, so we can use carbon-14 in radiocarbon dating. If carbon-14 wasn’t continuously being produced, radiocarbon dating just wouldn’t be a thing.
So, let’s take a look at how the idea for radiocarbon dating began, and some examples of how it’s used.
Part D: How Radiocarbon Dating Developed…
The idea for radiocarbon dating came from American scientist Willard Libby in about 1945. He and his team developed very accurate Geiger counters, and they then tested things whose age was known. That way, they could measure how accurate radiocarbon dating was.
They drilled into giant sequoia trees that were hundreds of years old and found that the wood fibres taken from the centre of the tree (that had been produced hundreds of years earlier) did in fact contain less carbon-14 than newer wood fibres that were new near the surface of the tree. Since trees grow at different rates in summer and winter, they form rings, so the age of the wood could be determined. The age of the trees found by counting the tree rings tallied very closely with the date they determined using radiocarbon dating. By the way they didn’t have to cut the tree down to count the rings, they used a special drill that extracted a small core where the rings could be counted easily.
Libby also tested samples taken from the tombs of two Egyptian pharaohs called Djoser and Sneferu. These tombs had been dated, based on inscriptions and records, to somewhere between 2,700 BC and 2,550 BC. Using radiocarbon dating, Libby dated the tombs at about 2,800 BC plus or minus 250 years, which is a very good match. Now this was in the late 1940s.
The technique is a lot better now because we can measure the amount of carbon-14 in a sample much more accurately. Radiocarbon dating can reasonably accurately date things as old as about 50,000 years.
Scientists now routinely find the age of ancient things.
In 1991, high up in the snow-covered Ötztal Alps near the border of Italy and Austria, two hikers found the very-well preserved body of a man who, it was quickly realized when experts were called, had lived thousands of years ago.
The man’s body had been preserved for so long thanks to the permanent freezing cold conditions that exist up there.
His clothing and various tools that he had been carrying when he died were also very well preserved. He was nicknamed Ötzi after the mountain range where he was found. By radiocarbon dating some tissue samples, it’s believed that the man died in about 3,200 BC ± about 150 years, in other words about 5,000 years ago.
Otzi’s body and the artifacts that were with him are now on display in the South Tyrol Museum of Archaeology in Northern Italy. Thanks to this amazing find, scientists and archeologists have gained new insights into the lives of the people who lived in the region all those thousands of years ago.
Let’s look at one more example. Between 1946 and 1956, a whole bunch of ancient Jewish scrolls were discovered in some caves near the Dead Sea, not far from the city of Jerusalem. The scrolls had been preserved because the climate there is hot and dry.
These Dead Sea Scrolls as they were named, contain the text of various parts of the Hebrew Scriptures, or what Christians call the Old Testament. The scrolls are mostly made of parchment, which is prepared from the skin of animals, and from papyrus, which is made from the papyrus plant. It’s very similar to paper and in fact the English word paper comes from the word papyrus.
By radiocarbon dating some of the fragments, it was established that the Dead Sea Scrolls were produced at various times between about 200 BC and about 100 AD. One of the reasons that their discovery was major is that they provided more evidence that the biblical text in modern bibles is pretty much exactly the same as it was two thousand years ago, even though all the copies of all of these types of scrolls and books that were passed down were handwritten, so you’d think that errors would have crept in. But there were very few errors.
By the way, the printing press with movable type was invented in 1439 by Johannes Gutenburg. After that, copies of the bible and other books could be printed in huge huge numbers, so the practice of producing long, hand-written manuscripts soon stopped.
So, in summary, all living things contain carbon atoms. One in every trillion of those is a carbon-14 atom. If a person or an animal or plant dies and doesn’t decompose, the carbon-12 and carbon-13 atoms stay where they are, but the carbon-14 atoms decay over time and decrease in number. Scientists can measure the ratio of carbon-14 to carbon-12 and carbon-13 in a sample of, say, ancient bone and determine how old it is. The less carbon-14 it has, the older it is. Brilliant.
Radiocarbon dating has allowed us to better understand human history and, if nothing else, is really important in the tourism industry.
Now so far in this series, we’ve looked at lots of different radioactive substances. In this video, we covered carbon-14 and saw how it’s made. But where do other radioactive substances come from? Well, that’s what we’re going to look at in our next episode. See you then.
Special thanks to
the Benaki Museum, Athens, Greece
the National Archaeological Museum, Athens, Greece
the Temple of Poseidon Archaeological Site, Sounion, Greece
Special thanks to
Maria Gonidou, master weaver, Nafplion, Greece
the people of Rizoma and Theopetra, Trikalon, Greece
the people of Achaea, Greece
Gebelein Man photos via Wikipedia taken by Jack1956 at the British Museum. https://en.wikipedia.org/wiki/File:Bm-ginger.jpg
Carbon-14 simulation by PhET interactive. https://phet.colorado.edu/en/simulations/beta-decay Creative Commons License.
Willard Libby photos via Wikimedia. Creative Commons License. https://commons.wikimedia.org/wiki/File:Willard_Libby_in_Lab.jpg
Tree-Drilling footage from “Cool Tools that Foresters Use – Part 1!” by BeLEAF It or Not! Creative Commons License. https://youtu.be/-CDXKA2NruM?si=nvfFDXZUJjQ-Du1S
Sheep Shearing footage from “Sheep shearing at Lake Metroparks Farmpark” by Lake Metroparks. Creative Commons License. https://youtu.be/jWOKv_jND5A?si=h0JmgGEdPQTaeWbA
Map of Europe by Alexrk2 via Wikimedia Commons. Creative Common License. https://commons.wikimedia.org/wiki/File:Europe_relief_laea_location_map.jpg
All Otzi photos © South Tyrol Museum of Archaeology.
Map of Israel and surrounds by freebibleimages.com
Parchment from Goatskin by Michal Ma?as via Wikimedia Commons. Creative Common License.
Picture of Sneferu taken in the Egyptian Museum, Cairo, Egypt