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Carbon dating is a method of determining the age of certain materials. The upper atmosphere of the Earth is constantly bombarded by cosmic rays from events in space. This bombardment creates a rare and unstable isotope of carbon, 14C, from the nitrogen which comprises 80% of the atmosphere. Naturally occurring carbon has two stable isotopes, 12C and 13C. 14C, however, is unstable, and decays to nitrogen with a half life of 5,700 years. The term half life indicates the time that it will take half the atoms in any sample to decay. Thus a sample containing, say, 32 atoms of 14C, after 5,700 years will probably contain 16 atoms, in 11,400 years (2 × 5,700) there will be 8 atoms, and only after 100,000 years will the last atom decay. Of course, usually far more than 32 atoms are available.
All living things, and some soils, exchange carbon with the air, and therefore will contain the same percentage of 14C as the atmosphere. This is constantly replenished as fast as it decays by the cosmic ray bombardment. As soon as a living organism dies, it ceases this exchange, and no more 14C enters its body. The 14C that it contains will decay, without being replenished. So if we examine a fossil that contains half as much 14C as an equivalent living animal, we can say that it is probably 5,700 years old. Thus fossils may be dated by their 14C content.
There are two ways of measuring the 14C content of a sample. One method consists of waiting for the atoms to decay and detecting the decay products. This is not very useful for small samples which contain only a few atoms, as they would take a very long time to decay. A better way, high energy mass spectrometry, involves detection of the atoms themselves before they decay; with this method samples weighing only a few milligrams may be dated. This involves ionizing the atoms so they acquire charge, accelerating them, and deflecting them with a magnetic field. The path they take when deflected varies depending on their mass, and is used to detect the quantity of 14C present.
We have assumed that the levels of 14C have remained constant throughout history. In fact, this is not quite true. It is certainly not true for the last century: future archaeologists will notice a very significant increase from the 1960s onwards due to nuclear tests. Concentrations of 14C have varied in the past as well, probably due to changes in the Earth\'s magnetic field. But as long as we know how the concentrations of 14C varied in the past, we may still accurately date samples. This data is provided by tree-rings in the bristlecone pine tree, which can live to the astonishing age of several thousand years. Each ring contains the concentration that existed in that year. From bristlecone pine studies, we may confidently date samples up to 6000 Â BCE. JJ
See also archaeology. |
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