Radiocarbon is a naturally occurring radioactive isotope of carbon. Atoms of radiocarbon behave in the same way as any other carbon atoms except that they weigh slightly more and, after an average of some 5000 years decay to nitrogen.
Because radiocarbon is a radioactive isotope, it only exists on the earth because it has been formed recently. We know from nuclear physics that several possible reactions can result in radiocarbon. These all involve neutrons.
The details are not important in understanding radiocarbon but the possible reactions are:
| Reaction | Q-value | Cross section | Neutron energy |
|---|---|---|---|
| 14N(n,p)14C | 1.81+/-0.05 b | thermal | |
| 16O(n,3He)14C | -14.6MeV | 20+/-2 mb | 40-160MeV |
| 16O(n,pd)14C | -20.1MeV | ||
| 16O(n,n2p)14C | -22.3MeV | ||
Most of the radiocarbon found on earth is formed naturally in the upper atmosphere.
High-energy cosmic rays (from outside the solar system) are constantly bombarding the upper atmosphere. These high-energy particles undergo a whole cascade of nuclear reactions resulting in some slow moving neutrons.
These neutrons react with nitrogen atoms in the atmosphere by the first of the reactions shown above:
14 N(n,p)14C
Because the rate at which cosmic rays strike the earth does not vary greatly, the overall quantity of radiocarbon in the atmosphere is fairly constant. It is, however, only a very small proportion of the total carbon present:
| Isotope | Protons | Neutrons | Proportion | Half life |
|---|---|---|---|---|
| 12C | 6 | 6 | 99% | stable |
| 13C | 6 | 7 | 1% | |
| 14C | 6 | 8 | 0.0000000001% | 5568 years |
The fact that these ratios are fairly constant is important in using radiocarbon as a dating tool. Calibration is necessary because the ratio is not exactly constant.
A small proportion of the cosmic rays reach the surface of the earth (particularly on high mountains) and can form radiocarbon in situ in rocks and ice. This can be used for exposure dating of rocks or ice.
Radiocarbon is also formed (through the processes described above) in man made nuclear reactions. There is some produced in nuclear power generation but this is a relatively small effect. The amount produced in the atmospheric nuclear bomb tests of the 1950's and 1960's was, however, large. For a short period, the abundance in the atmosphere was almost doubled. This means that it is very easy to distinguish pre and post bomb organic material by making a radiocarbon measurement.
The other major artificial effect relevant to radiocarbon dating is the effect of fossil fuel burning. This releases a large amount of carbon dioxide into the atmosphere that contains virtually no radiocarbon (because the organic material from which it derives is so ancient). This reduces the ratio of radiocarbon to stable carbon in the atmosphere. This effect is called the fossil fuel or 'Suess' effect.
Partly because of this, it is very difficult to radiocarbon date material from the period 1650-1950.