Radioactive Dating, the calculation of a substance's age by measurement of the radioactive material it contains or of the amount of natural atomic fission that has occurred in the substance. These methods of calculating age are used in geology, anthropology, archeology, and other fields.
Radioactive dating is based on the fact that a radioactive isotope (form) of an element changes into an isotope of another element at a fixed rate. Each radioactive isotope, or radioisotope, has its own rate, expressed in terms of its half-life. The age of a substance, even if it is billions of years old, can be calculated if the substance contains radioisotopes with half-lives that are sufficiently long.
Radioisotopes used for calculating the age of rocks have very long half-lives. A commonly used radioisotope for this purpose is uranium 238—an isotope of uranium with an atomic weight of about 238. Uranium 238 has a half-life of about 4.5 billion (4,500,000,000) years. It changes into a radioisotope of another element that in turn changes into a series of other radioisotopes before a stable (nonradioactive) isotope is formed. The uranium 238 series ends in lead 206. Since scientists know the rate at which uranium 238 becomes lead 206, they can determine the age of a rock that contains uranium 238 from the ratio of uranium 238 to lead 206 in the rock. They can calculate the age more accurately by measuring all the products in the series.
Other isotopes used in dating rocks include uranium 235, which becomes lead 207; thorium 232, which becomes lead 208; rubidium 87, which changes into strontium 87; and potassium 40, which changes into argon 40.
The age of organic material (material of plant or animal origin) up to some 50,000 years in age can be calculated using a radioactive isotope of carbon. The isotope, called carbon 14, or radiocarbon, has a half-life of 5,730 years. (The most common isotope of carbon is carbon 12, which is stable.) The 1960 Nobel Prize in chemistry was awarded to W. F. Libby for discovering this method of radioactive dating.
Carbon 14 is continually being formed in the atmosphere by the action of cosmic rays on nitrogen in the air. The carbon 14 unites with oxygen, forming carbon dioxide. Plants acquire carbon 14 when they use carbon dioxide to make food through photosynthesis; animals acquire carbon 14 from eating plants. (Only a very small fraction of the carbon dioxide in the air contains carbon 14, so the amount of carbon 14 in a living thing is very small.) When a plant or animal dies, it stops acquiring carbon 14 and the carbon 14 it contains steadily changes back into nitrogen.
Scientists can calculate how long ago a piece of organic material was part of a living plant or animal by comparing the amount of carbon 14 that remains in the material to the amount of carbon 14 that exists in similar material in a living plant or animal. The age calculated requires some adjustment, however, because the concentration of carbon 14 in the air has varied over time. The adjustment required for a given calculated age has been determined by comparing ages calculated with carbon 14 with those obtained by other dating methods.
One technique used to measure the carbon 14 in a material is to measure the radioactivity of the carbon it contains. The accuracy of the technique decreases with the age of the material. Another technique is to measure the amount of carbon 14 in a sample directly. This technique makes use of a particle accelerator to separate carbon 14 from other carbon isotopes.
Other methods depend on effects produced by a radioactive substance in surrounding materials. In one method, called fission-track dating, the age of an object is determined by counting the tiny scars or tracks left in zircon or certain other materials by the fission (splitting) of uranium atoms. It can be used to date materials several tens of thousands to more than 4 billion (4,000,000,000) years old.