By David Nashed
Have you ever wondered how archaeologists ascertain the age of certain fossils and artifacts? Well look no further, for the answer is right here in carbon dating!
Carbon dating is a technique that has been used by archaeologists and various other life scientists to determine the approximate age of carbon-based materials. These materials can either originate from living matter or be inanimate objects.
Now, let’s talk about how carbon dating works. Carbon, alongside nitrogen and phosphorus, is one of the biochemical building blocks of all living matter on earth. However, there isn’t just one type of carbon, there are actually three: carbon-12, carbon-13, and carbon-14.
The main difference between these three types of carbon is the number of neutrons they possess. Carbon-12 contains 6 neutrons. Carbon-13 contains 7, and carbon-14 contains 8. However, all types of carbon contain 6 protons. Since both carbon-13 and carbon-14 contain more neutrons than protons, they’re referred to as isotopes. Isotopes partake in the same chemical reactions as their base element; just at different rates.
However, carbon-14 differs slightly from carbon-12 and carbon-13, as it is chemically unstable. Furthermore, it is slightly radioactive and is constantly undergoing radioactive decay. Because of this, it is known as a radioisotope. All radioisotopes decay at a fixed-rate known as a half-life. The half-life of a radioisotope describes how long it takes for half of the atoms in the given mass to decay.
Carbon-14’s half-life is 5730 years! So every 5730 years, the quantity of carbon-14 in any given mass will halve itself. For example, only half of the original carbon-14 in a given mass will remain after 5730 years. After another 5730 years, only a quarter will remain. This will continue until there is no carbon-14 left in the given mass. Once this happens, the sample is said to be radiocarbon dead.
That was just the chemistry behind carbon dating. Now, let’s talk about how it ties into biology.
Essentially, as living organisms progress through their lives, they assimilate carbon-14 from carbon dioxide. This quantity of carbon-14 grows continuously as the organism lives, and only stops when the organism is dead. Once this happens, the organism’s carbon-14 levels begin to decay at half-rate. The residual quantity of carbon-14 is what carbon dating measures.
There are three techniques used to measure the quantity of carbon-14 in any given matter: gas proportional counting, liquid scintillation counting, and accelerator mass spectrometry.
Gas proportional counting is the standard method of carbon dating. To use this method, scientists must first convert the carbon sample into carbon dioxide gas. This is done so that measurements can be taken in gas proportional counters.
Essentially, this technique revolves around the process of counting the number of beta particles emitted by a given sample. Carbon-14 decay emits beta particles, and the number being emitted can be used to determine the quantity of carbon-14 currently in the sample.
Liquid scintillation counting is another carbon dating technique. However, it is somewhat outdated, and has been overshadowed by gas proportional counting and accelerator mass spectrometry. In this method, the carbon-14 sample is in liquid form and a scintillator is added to it. Scintillators are substances—typically liquids—which produce light when struck by ionized radiation rays. In the case of liquid scintillation counting, these rays are beta particles.
Once the scintillator has been added to the carbon-14 sample, a vial containing the mixture is then passed between two photomultipliers, which are detectors of electromagnetic light.
After the scintillator interacts with a beta particle, it produces a flash of light. The two photomultipliers then register the flashes made, as well as the intensity of them. This intensity is then used to determine the energy of the beta particles, which will provide a rough estimate of the quantity of carbon-14 in the sample.
Both of these methods accomplish the same task: determining the quantity of carbon-14 in a sample. Once this quantity has been determined, it is then compared to the quantity of carbon-14 that the sample began with. This initial quantity can be determined by analyzing modern versions of the sample.
However, while these two methods are viable, they are not the most efficient.
That title goes to the third and final method of carbon dating: accelerator mass spectrometry. Accelerator mass spectrometry is the newest method of carbon dating. In this method, the quantity of carbon-14 in the sample is directly measured, and is then compared to the quantity of carbon-12 and carbon-13 in the sample.
This method does not count beta particles, and instead relies on the proportional relation between the three types of carbon within the sample. This proportional relation allows scientists to determine how much carbon-14 the sample began with, which is then used to determine its approximate age.
Accelerator mass spectrometry is not only the most efficient and accurate of the three, but is also the simplest.
The Lawrence Livermore National Labratory’s Accelerator Mass Spectrometer
Now that we’ve covered how carbon dating works, let’s talk about what it’s used for!
Carbon dating can be used on most, if not all, living matter, assuming that they have assimilated carbon-14 at some point in their life. Inorganic matter, on the other hand, is much more restrictive. In general, most inorganic matter, such as metals, cannot be tested.
Of course, there are some exceptions to this. However, these exceptions must have absorbed carbon-14 at some point during their initial formation.
Overall, carbon-14 dating is an incredible technique which has roots in both chemistry and biology. It has had an undeniable impact on geology, hydrology, geophysics, and biomedicine.
One could consider it one of the most significant discoveries of the 20th century. No other scientific method has managed to restructure man’s understanding of the present and past to such an extent. One can only wonder how it will develop in the future, and how it will change our perception of life on Earth.
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Questions:
Q: Why isn’t carbon-13 a radioisotope, even though carbon-14 is?
A: Essentially, even though carbon-13 does have an imbalance of neutrons and protons, the imbalance is not great enough to cause the atoms to become radioactive. Carbon-13 is still a stable isotope, as the imbalance is too miniscule to actually offset anything. The imbalance in carbon-14 however, is enough to make it unstable, and in turn, radioactive.
This is a great example of how chemistry doesn’t necessarily work in absolutes, and how it can actually have a margin of error.
Q: How far can carbon dating be used for?
A: Carbon dating can be used to determine the age of substances as old as 60,000 years old. Once the substances pass this threshold, carbon dating can no longer be used to determine its age, as by that point, the substance will already be “radiocarbon-dead.”