Radioactive decay helps reveal age of artifacts

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arbon dating might sound like a matchmaking service for the scientifically minded but it is not.

It is one of the techniques scientists have for determining the age of an object. There are a variety of such techniques, each of which have their own advantages and disadvantages. Carbon dating is particularly effective at dating human artifacts and long dead organisms from the last 50,000 years but would not be applicable to determining the age of a dinosaur bone.

Determining the age of the Earth and all of the geological layers we have uncovered over time has been one of the major tasks in science. Determining a timeline for the past has been one of the truly great accomplishments of the past century. We now know a great deal about the history of our planet.

This is, in part, due to our understanding of the atom and the discovery of radiation by Henri Becquerel in 1896. Prior to his work, both the concept of an atom and the structure were still matters of speculation. Becquerel helped to start a revolution, which has ultimately resulted in our modern understanding of atomic structure and quantum theory.

Atoms are composed of a nucleus surrounded by an electron cloud. Each nucleus is composed of protons and neutrons (except hydrogen which has only a proton) and it is the number of protons in a nucleus that determines an element’s identity.

However, the number of neutrons can vary resulting in the generation of isotopes. For carbon, the most common isotope, making up 98.93 per cent of all carbon atoms, is carbon-12. It has a nucleus consisting of six protons and six neutrons. Carbon-13, on the other hand, still has six protons but has seven neutrons giving it the extra mass.

Carbon-13 atoms make up the almost all of the remaining 1.07 per cent of carbon atoms.

Other than the slight difference in mass – which does have some effects with regard to chemical reactions – these two isotopes of carbon are pretty much the same. They are both “stable isotopes” and typically found in approximately the same abundance throughout the environment.

Carbon-14 – consisting of six protons and eight neutrons – is an example of an “unstable isotope”. It undergoes radioactive decay and breaks down to a nitrogen atom (seven protons and seven neutrons) by the emission of a beta particle and an electron antineutrino.

This decay is first order ,which means it has a constant half-life of 5,730 years. In practical terms, this means half of the carbon-14 atoms in a sample will decay to nitrogen over this time period. That is, if you started with precisely 10 grams of carbon-14, after 5,730 years there would be exactly five grams left. And after another 5,730 years, the amount would be reduced by half again, leaving exactly 2.5 grams and so on.

Understanding radioactive decay rates and mechanisms was a big part of science in the early 20th century but by the 1940s, much of the science had been worked out. In the late 1940s, Willard Libby realized measuring the amount of carbon-14 remaining in a sample could be used to determine its age. That is, if there was half of the original amount of carbon-14 in a sample, then the sample was 5,730 years old.

One of the critical points in using carbon-14 to date samples is it only works on samples which were once living tissues freely exchanging carbon with the atmosphere but are now dead.

Carbon-14 is constantly being created in the upper atmosphere through the bombardment of nitrogen atoms by cosmic rays. Hence, the concentration of carbon-14 is relatively constant over time at 0.0000000000015 per cent.

This might not seem like a lot but we are talking about atoms and there are an awful lot.

These newly created carbon-14 atoms quickly form carbon dioxide and are then incorporated into plant life. If the plants are eaten by other organisms, then these organism incorporate carbon-14. As long as the organism is alive, the reservoir of carbon-14 in its body is constantly replenished. It is only once the organism has died that the amount of carbon-14 starts to diminish.

But measuring the amount of carbon-14 in relationship to carbon-12 and carbon-13 allows scientists to determine how long ago the organism was exchanging carbon with its environs. This can then be used to date the artifact.

There are many other aspects to the process of carbon dating. For example, it required a calibration curve to be developed as the levels of carbon-14 in the atmosphere have fluctuated over time. Fossil fuels have no carbon-14 so the recent combustion of ancient carbon has impacted the atmospheric composition. But most of the issues have been resolved over time.

Carbon-14 dating is one of the most significant advances in archeology as it has allowed scientists to better understand our past.