Have you ever wondered how we know the age of certain objects? When archaeologists discover new cultural heritage objects, how do they figure out how old they are? The answer is radiometric dating, and the most famous radiometric dating method is radiocarbon dating.
The radiocarbon dating method uses one of the carbon isotopes. Isotopes are forms of chemical elements with a slight atomic structure difference from one form to another. They contain the same number of protons in the nucleus, but vary in their neutron numbers. Carbon has three main isotopes. Each carbon isotope has 6 protons in the nucleus, but the number of neutrons differs from one isotope to another. Carbon-12 has 6 neutrons, carbon-13 has 7 neutrons, and carbon-14 has 8 neutrons. Carbon-12 is the most abundant carbon isotope with a natural abundance of about 98.89%, followed by carbon-13 (used in Nuclear Magnetic Resonance) with a much smaller natural abundance of only about 1.1%. These are stable isotopes of carbon. Carbon-14 is not a stable isotope, it is a radioactive isotope, and it is found in trace amounts. This is the isotope from which the name of the method “radiocarbon dating” is coming from.
Radiocarbon: the carbon-14 radioisotope
When the cosmic rays (high-energy particles) interact with particles in the Earth’s upper atmosphere, they can lead to the removal of neutrons from atoms. These neutrons can then be absorbed by the nitrogen atoms that are present in the Earth’s atmosphere. This interaction leads to the formation of carbon-14 atoms. The carbon-14 atoms then interact with the oxygen atoms from the atmosphere, which leads to the formation of carbon dioxide with the carbon-14 isotope in the composition of carbon dioxide. This means that the carbon dioxide, which is then taken up by plants contains also the carbon dioxide formed with the carbon-14 isotope. So now we have the carbon-14 isotope present in the plants. Animals then eat the plants and this is how carbon-14 ends up being present in all living beings. And as long as they are alive, they continue to have the same ratio of carbon-12 to carbon-14 as the atmosphere through a constant exchange with the atmosphere. But this all changes when the organism dies and the exchange with the atmosphere stops. And that’s when we start seeing the decay of carbon-14.
Radioactive beta decay
Carbon-14 decays through a radioactive decay called a beta decay. Through this process, the unstable carbon-14 isotope transforms into nitrogen-14, a stable isotope of nitrogen, and there’s also a release of an electron and an antineutrino during this radioactive decay process.
For radiocarbon dating, we're interested in the decay of carbon-14 into nitrogen-14. The time that it takes for the initial amount of carbon-14 atoms present in the sample to decay to half of that amount is called a half-life. For carbon-14, this half-life is 5730±40 years. After one half-life, that is, after about 5730 years, the number of radioactive carbon-14 nuclei decreased to half. And if we wait another half-life after this, so then after about 11,460 years, the number of radioactive nuclei decreases to half of the previous amount, which is now one quarter of the initial amount. This process keeps on going, and after every new half-life, the amount of radioactive carbon-14 nuclei decreases to half of the previous amount. Thus, by looking at the ratio of carbon-14 to carbon-12 in a sample, we can find out the age of a sample, up to an age limit of about 50,000 years when the amount of carbon-14 is too small to be measured.
Radiocarbon dating in cultural heritage
Radiocarbon dating has many applications in cultural heritage. We can use it to find out the age of any object that contains carbon, up to the age limit of 50,000 year. This means there are a multitude of possible applications of radiocarbon dating in cultural heritage.
One application of radiocarbon dating in cultural heritage involved the dating of the Dead Sea Scrolls. When analyzing the Dead Sea Scrolls, texts of high religious and cultural value, scientists from several laboratories around the world managed to date several scrolls to the period around 400 BCE to around 300 CE, thus pinpointing the age of these precious scrolls.
However, the radiocarbon dating of another artifact of high religious significance revealed something surprising. When analyzing samples from the Turin shroud, the cloth that supposedly covered Jesus’s body after death, radiocarbon dating revealed that the shroud samples that were examined actually date back to the Middle Ages and not to the time when Jesus walked the Erath. But does that mean that the shroud itself was made during the middle ages? Could we use one study alone to reveal everything about the age and origin of the Turin shroud? The answer is no. Due to the Turin shroud's complex history, it is possible that the dating of the samples investigated in this study is not relevant to the entire shroud and more experiments need to be recorded to confirm the age of the Turin shroud.