Radiocarbon, or carbon 14, is a radioactive isotope of carbon. It is used to determine the age of fossils and other kinds of ancient objects. In addition, researchers use radiocarbon to study certain biological processes. Radiocarbon has a relative atomic mass of about 14 and is heavier than the most common isotope of carbon, which has a relative atomic mass of exactly 12. An isotope’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of an atom of carbon 12.
In nature, radiocarbon forms when high-energy atomic particles called cosmic rays smash into Earth’s atmosphere. Cosmic rays cause atoms in the atmosphere to break down into electrons, neutrons, protons, and other particles. Some neutrons strike the nuclei of nitrogen atoms in the atmosphere. Each of these nuclei absorbs a neutron and then loses a proton. In this way, a nitrogen atom becomes a radiocarbon atom.
All living things contain radiocarbon. In the atmosphere, there is about one atom of radiocarbon for every trillion molecules of carbon dioxide gas. Plants absorb radiocarbon from the carbon dioxide in the air. Human beings and other animals take in radiocarbon chiefly from the food provided by plants.
Radiocarbon dating
is a process used to determine the age of an ancient object by measuring its radiocarbon content. This technique was developed in the late 1940’s by Willard F. Libby, an American chemist. Archaeologists and geologists have used it extensively. They have learned much about prehistoric human beings, animals, and plants that lived up to 50,000 years ago.
Radiocarbon atoms, like all radioactive substances, decay (break down by releasing particles) at an exact and uniform rate. Half the radiocarbon decays, turning back into nitrogen, in about 5,730 years. Therefore, radiocarbon has a half-life of that period of time. After about 11,460 years, a fourth of the original amount of radiocarbon remains. After another 5,730 years, only an eighth remains, and so on.
The radiocarbon in the tissues of a living organism decays extremely slowly, but it is continuously renewed as long as the organism lives. After the organism dies, it no longer takes in air or food, and so it no longer absorbs radiocarbon. The radiocarbon already in the tissues continues to decrease at a constant rate. This steady decay at a known rate—a half-life of approximately 5,730 years—enables scientists to determine an object’s age.
In one method of radiocarbon dating, scientists burn a piece of the object and convert it to carbon dioxide gas. The carbon dioxide is purified, and the amount of radiocarbon in the purified carbon dioxide is measured with radiation counters. These instruments detect the electrons released by the radiocarbon atoms as the atoms change back into nitrogen atoms. The number of electrons emitted indicates the radiocarbon content.
Another method of radiocarbon dating involves the use of certain types of particle accelerators instead of radiation counters (see Particle accelerator ). An accelerator enables scientists to detect and count directly the individual radiocarbon atoms in an extremely small portion of an object. After scientists measure an object’s radiocarbon content, they compare it with the radiocarbon in tree rings whose ages are known. This technique enables them to compensate for small variations of radiocarbon content in the atmosphere at different times in the past. By doing so, scientists can convert an object’s radiocarbon age to a more precise date.
Radiocarbon in biology.
Radiocarbon is used as a “tracer” to study various complex biological processes. In such research, scientists substitute a radiocarbon atom for an atom of a carbon molecule. Then they use a radiation counter to trace the path of the radiocarbon atom through a chemical reaction in an organism.
The radiocarbon used as a tracer is produced artificially in nuclear reactors. Artificial radiocarbon was first discovered in 1939 by two American chemists, Martin D. Kamen and Samuel Ruben.
