Looking into pre-historic life through carbon-14 dating
Radiocarbon dating information
All living things, plants and animals alike, contain the chemical element carbon. Green plants obtain carbon from the carbon dioxide existing in gaseous form in the air. They combine this substance with water in the presence of sunlight and manufacture food. They become the source of carbon for other plants and also for animals, which eat plants or plant-eating animals.
A comparatively insignificant number of the carbon atoms existing in all living things are radioactive; that is, their nuclei disintegrate sooner or later, sometime after a long span of time. Radioactive carbon atoms continue to exist in animals and plants – and in objects made from them – for thousands of years after death has taken place. We find radioactive carbon in the hard parts, soft parts, and manure of animals that died long ago; in peat (derived from dead vegetation); in wooden boats and platforms; In cotton and linen garments; in leather shoes and bags; and in many other plant and animal derivatives.
A Matter of Isotopes
Scientists have found a practical application for the radioactive carbon atoms existing in dead organic matter. By carefully measuring the radioactivity of samples derived from such matter, they have developed a method for dating objects that go back to ancient and late prehistoric times.
The radiocarbon atoms that make possible this dating method are isotopes with mass number 14: carbon-14 atoms, The vast majority of carbon atoms are isotopes with mass number 12. Carbon-12 atoms are not radioactive.
How is radioactive carbon-14 produced? The answer is to be found in the upper reaches of the earth’s atmosphere. This area, filled with tenuous gases, is constantly bombarded by cosmic rays – radiation coming from outer space, probably from beyond our galaxy. As the rays strike the atmosphere, they disintegrate the nuclei, or cores, of some of the atoms that are found there. Among the building blocks that make up the nuclei are the particles called neutrons. When nuclei are smashed by cosmic rays, free neutrons are scattered in the air.
They are very quickly absorbed by the nitrogen atoms, with mass number 14, that make up about 78 percent of the atmosphere by volume. When a nitrogen atom absorbs a neutron, a proton – a particle with a positive charge of electricity – is emitted. The atom keeps its mass number, 14, but instead of remaining a stable nitrogen atom, it becomes an unstable, radioactive carbon atom, with mass number 14.
Radioactive carbon, or radiocarbon, as it is often called, has a half-life of 5,730 years. By that we mean that if we start with 100 atoms of radiocarbon, at the end of 5,730 years we shall have only 50 atoms, or half as many. The rest will have decayed. In the next period of 5,730 years, one half of the remaining 50 atoms – that is, 25 atoms – will also decay and so on.
Cosmic rays have been bombarding the atmosphere for millions of years, and they have been constantly producing new supplies of radiocarbon. At the same time the radiocarbon atoms that have already been formed are steadily disintegrating. It is as if water were being pumped into a tank with a hole on its bottom. A steady level will be reached in the tank when the amount of water entering it will be equal to the amount of water flowing out of it. By analyzing the corresponding “level” of carbon-14 atoms, scientists have calculated that there are 50 metric tons of radiocarbon on the earth at any given time. In each second upon each square centimeter of the earth’s surface, something like 2.4 disintegrations of radiocarbon are taking place – that is, 2.4 atoms of radiocarbon are decaying. This represents a considerable amount of radioactivity,, and it is surprising that it is not noticed ordinarily. The reason for this state of affairs is that the radiocarbon that is produced in the atmosphere is always diluted with a tremendous amount of ordinary carbon.
Where C-14 Goes
What happens to the radioactive carbon on the earth’s surface? We know that under the appropriate circumstances ordinary carbon atoms combine with the oxygen atoms in the air and burn, forming carbon-dioxide molecules, or, if combustion is incomplete, carbon-monoxide molecules. That is what happens, for example, when we set fire to coal, which is made up largely of carbon. We take it for granted, therefore, that the radiocarbon atoms upon the earth will combine sooner or later with oxygen atoms and will produce radioactive carbon-dioxide molecules plus a certain quantity of carbon-monoxide molecules.
As long as plants and animals live, they keep replenishing their stores of radioactive carbon. They can no longer absorb it, however, after they die. The radiocarbon atoms they contain at the moment of death will disintegrate at the rate of 50 percent every 5,730 years, and they will not be replaced.
While the organism is alive, there are 15.3 disintegrations per minute for every gram of carbon that it contains. At 5,730 years from the time of death, there will be 7.65 disintegrations (half of 15.3) per minutes; 11,460 years from the time of death, there will be 3.83 disintegrations (half of 7.65) per minute, and so on. At the end of 20,000 or 30,000 years, the disintegrations will be so few that it will be exceedingly difficult to measure them. By noting the rate at which the radiocarbon atoms in a given sample decay, we can estimate the age of the sample – the time that has elapsed since death. That is the principle of radiocarbon dating.
In order to measure the rate of disintegration, scientists use a special Geiger counter. This instrument is shaped liked a cylinder and has a wire extending down its center. It is filled with gas. An electric potential is maintained between the wire and the cylinder walls, which have been appropriately charged.
As an atom disintegrates, it emits a beta particle. When this particle passes through the gas-filled space of the counter, a tiny spark is produced. At this point, an electrical impulse is recorded by electronic equipment connected to the Geiger counter. The sensitive instrument detects every disintegrated particle that passes through the gas within it.
A small sample of the material to be dated is taken and chemically purified. Purification removes any contamination that entered the material after the death of the organism that had supplied this material. Otherwise the dating will be inaccurate. The sample is then burned into carbon-dioxide gas, which contains the original carbon-14. The gas, also purified, is led onto the counter, which records the carbon-14.