How does carbon dating work?

Before we can answer this question we must understand what an isotope is. As you know, the nucleus of an atom contains the subatomic particles called protons, which have a positive charge, and neutrons, which are neutral and don’t carry a charge. Every element in the periodic table--that’s the neat chart of the elements that you saw in your chemistry classroom--has a distinct number of protons and neutrons in its nucleus. Most elements have known isotopes that have the same number of protons but a different number of neutrons, and some of these isotopes are radioactive, which means that they decay and release radiation in the form of energetic particles.

For example: the element carbon has an atomic number of 6. That means that all of its isotopes have six protons. The most common isotope of carbon is carbon-12, which has six protons and six neutrons. Carbon-13 is an isotope of carbon that has seven neutrons, and carbon-14 has eight neutrons. Actually, the element carbon has fifteen known isotopes, but the one that we’re interested in is carbon-14, and that’s because it is a fairly stable--it hangs around for a long time--radioactive isotope.

Carbon-14, which has six protons and eight neutrons, decays by releasing a beta particle, which is an energetic electron, and an electron antineutrino (this is the antithesis of the neutrino). The result of this decay is the formation of the element nitrogen, which has seven protons and seven neutrons and is stable and not radioactive. The beta particle emitted by the decay of an atom of carbon-14 is not very dangerous and cannot penetrate the human skin, but it can be detected, and that’s the most important fact about this atomic decay.

Radiocarbon dating depends on the radioactive property of carbon-14, but is also depends on the fact that carbon-14 is always being produced in the atmosphere from cosmic ray interaction with atmospheric nitrogen to produce radiocarbon, which is what carbon-14 is called. This radiocarbon reacts with oxygen to produce radioactive carbon dioxide. Carbon dioxide, as you know, is used by plants in the photosynthesis process that ends up making oxygen. Animals, including us, eat plants. What all this means is that the ratio of radiocarbon to normal carbon is maintained as long as the plant or animal is alive, and when it dies, the radiocarbon decays to change the ratio in favor of normal carbon-12.

The half-life of carbon-14 is 5,730 years. What does this mean? A half-life is simply the fact that half of the carbon-14 in a given sample will decay in that many years. Now that we know that, we’re ready to carbon date something.

Hold on there! First we have to determine how to measure the ratio of carbon-14 to carbon-12 in a sample and then look on the plot of beta decay for carbon-14 to find how old the sample is. This is easier said than done. What one has to do is determine the number of beta decays per unit time per weight of the sample. For this we need what is known at a scintillation counter. This is a rather complicated device that uses a solvent (either water or toluene) with a dissolved scintillator, a chemical that emits photons when hit by a beta particle. The sample is dissolved or suspended in this scintillation soup and placed in a refrigerated and well-shielded chamber. A photo detector next to the sample container registers a photon emission as a count. After a given time, the counter tells us how many carbon-14 atoms decayed. Using math and knowing the standard activity (the normal radioactivity of carbon-14), one can calculate the amount of carbon-14 in the sample and thus determine its ratio to carbon-12.

A new method called Accelerator Mass Spectrometry has taken over this task. Mass spectrometry is an analysis concept that can determine the direct ratio of carbon-12 to carbon-14 in a sample. The sample is vaporized and then ionized by bombardment with electrons. The ionized atoms are then attracted into a chamber by means of a charged accelerator. The chamber uses magnetism to bend the accelerated beam of ions in proportion to its mass. What this means is that a beam of ionized atoms of different isotopes of carbon get separated and impact a detector array at different angles. Heavier ions bend less than lighter ions. A carbon-14 atom is more massive than a carbon-12 so one sees a separation in the detector and this can determine the desired ratio.  One should imagine lots of complicated electronics and computer programming being used here.

These explanations are a simplified version of more complicated processes. Needless to say that there is lots of room for error in these determinations. Samples can get contaminated with newer carbon. This happened with the first attempt to date the Shroud of Turin, the purported burial shroud of Jesus. What caused the problem was that the sample was taken from the edge of the shroud where it had been repaired. This introduced newer carbon and the sample indicated that the shroud was less than a thousand years old not two thousand.

There are many other causes for error in carbon dating, but the process, if used correctly, can lead to the dating of important archeological samples. The basic problem with carbon dating is that once the sample is older than eight half-lives or 45,000 years, the method is not reliable. The reason for this is that the very small amount of carbon-14 left in the sample is hard to determine.

There are ways to determine the age of rocks based on other radioactive atoms such as uranium-235, Potassium-40 and Rubidium-47. These methods rely on the idea that the magnum that rocks formed from held a constant amount of the radioactive element from the Earth’s interior. Once a rock cools into a solid form, the radioactive element decays at a known half-life. These methods allow dating of rocks back millions or even a billion years.

Unfortunately, older carbon samples cannot be dated reliably. Someday, a method might be worked out, but until then . . .

Thanks for reading.