Radiocarbon dating is a product of the nuclear age. The work is based upon research by Willard F. Libby, who was looking at the effects cosmic rays have upon the atmosphere (Aitken 1990:56). Cosmic rays ejected from the sun strike the earth in the upper atmosphere, the majority of earth’s atmosphere is nitrogen and it was this reaction Libby examined. He theorised that the rays would react with the nucleus of a nitrogen atom, converting a proton in the neutron. The loss of the proton would mean the atom would become a carbon atom. The transmutation of elements has been the goal of many an alchemist searching for gold. The value of the cosmically generated carbon atoms is certainly more valuable than gold for archaeologists as it has a couple of properties essential for timekeeping.
Cosmically generated carbon is known as carbon-14, because of its atomic weight. The six protons and eight neutrons give it the weight of 14 AMU (atomic mass units). Normal carbon has six protons but only six neutrons, giving it a lighter weight of 12 AMU. It’s this mass difference that gives us a way to distinguish which form of carbon we have. The most important thing about carbon-14 however, is that it’s not stable. The instability is due to the imbalance between the number of protons and neutrons in the nucleus. In stable carbon the two factions balance, but in carbon-14 there are two extra neutrons, which leads to stresses in the atom.
The atom will decay at some random time by spitting out an electron from the nucleus and converting a neutron back into a proton. This turns the carbon back into nitrogen. The chances of a carbon-14 atom decaying by 5730 years into the future are 50: 50. The 50: 50 ratio means that if we have a means of measuring time, if we have enough atoms to iron out statistical problems. The problem is that we would need to know how much carbon-14 was in the sample in the first place to know how much is missing. Chemists have found a way of side-stepping this problem.
We are all, to an extent, radioactive. We breathe in carbon-14 from the atmosphere and eat animals and plants that have carbon-14 in their bodies. Inside us it decays back into nitrogen, the lost carbon is replenished from the world around us. So the ratio of carbon-12 to carbon-14 remains constant, reflecting the background radiation of the planet. However, when we die we cease to take on more carbon-14. This means the carbon which decays inside us is no longer replaced and the ratio of carbon-12 to carbon-14 increases. Six thousand later the ratio of carbon-12 to carbon-14 has doubled, because the amount of carbon-14 has halved. So provided the atmosphere stays constant we can compare past artifacts to modern measurements to get a date for the artifact. The proviso is that the artifact must be made of once living material, so that it can take in carbon-14.
The problem is that the production of carbon-14 in the atmosphere is dependent upon solar activity.which varies, and the Earth’s magnetic field, which isn’t constant either. In the past there has tended to be more carbon-14 than the present and so the quality of the dates given by radiocarbon dating vary, producing dates that get be too young the further back you go. This is how dendrochronology can help radiocarbon dating.
The trees used for dendro-dating are already of a known date. If we use radiocarbon dating to re-date the trees the error in dating will be shown. The difference between the radiocarbon date and the dendro-date will be the error.
The experiments have revealed the ‘wiggles’ in radiocarbon production and now allow us to date with reasonable accuracy back to the mesolithic period, about ten thousand years ago.
Radiocarbon dating has been used extensively across the globe as an independent dating device, but the most controversial example is the dating of the ‘Turin Shroud’.
The shroud is believed to have been used by Son of God while he was dead, the proof being the imprint, in negative, of his image. The Catholic Church has never formally claimed the shroud as a holy relic, but it nevertheless the image and the artefact still carries great power. The act of taking a material sample to be destroyed was therefore a great gamble for the church. Because the dates provided by radiocarbon dating are a matter of probability it makes little sense to trust in one date. The figure, if quoted to one standard deviation, has only a 2/3 chance of being within that range. Even if quoted to two deviations, the figure is still only accurate to 19/20. The samples were therefore sent to three labs.
The figures returned were all in the range of the thriteenth and fourteenth centuries (Damon et al 1989), the period when the shroud first went on display. The figures would therefore suggest that the shroud is a supremely clever fake. However another suggestion is that the scientists are incompetent, as they got the wrong date and the shroud should be tested and re-tested until they finally get the right answer (for an example see Walsh 1999).
