Radioactive Dating

In the nineteenth century, prominent scientists such as Charles Lyell, Charles Darwin, Sir William Thomson (Lord Kelvin), and Thomas Huxley, were in continual debate about the age of the earth. The discovery of the radioactive properties of uranium in 1896 by Henri Becquerel subsequently revolutionized the way scientists measured the age of artifacts and supported the theory that the earth was considerably older than what some scientists believed.

There are several methods of determining the actual or relative age of the earth's crust: examination of fossil remains of plants and animals, relating the magnetic field of ancient days to the current magnetic field of the earth, and examination of artifacts from past civilizations. However, one of the most widely used and accepted method is radioactive dating. All radioactive dating is based on the fact that a radioactive substance, through its characteristic disintegration, eventually transmutes into a stable nuclide. When the rate of decay of a radioactive substance is known, the age of a specimen can be determined from the relative proportions of the remaining radioactive material and the product of its decay.

In 1907, the American chemist Bertram Boltwood demonstrated that he could determine the age of a rock containing uranium-238 and thereby proved to the scientific community that radioactive dating was a reliable method. Uranium-238, whose half-life is 4.5 billion years, transmutes into lead-206, a stable end-product. Boltwood explained that by studying a rock containing uranium-238, one can determine the age of the rock by measuring the remaining amount of uranium-238 and the relative amount of lead-206. The more lead the rock contains, the older it is.

The long half-life of uranium-238 makes it possible to date only the oldest rocks. This method is not reliable for measuring the age of rocks less than 10 million years old because so little of the uranium will have decayed within that period of time. This method is also very limited because uranium is not found in every old rock. It is rarely found in sedimentary or metamorphic rocks, and is not found in all igneous rocks. Another method for dating the rocks of the earth's crust is the rubidium-87/strontium-87 method. Although the half-life of rubidium-87 is even longer than uranium-238 (49 billion years or 10 times the age of the earth), it is useful because it can be found in almost all igneous rocks. Perhaps the best method for dating rocks is the potassium-40/argon-40 method. Potassium is a very common mineral and is found in sedimentary, metamorphic, and igneous rock. Also, the half-life of potassium-40 is only 1.3 billion years, so it can be used to date rocks as young as 50,000 years old.

In 1947, a radioactive dating method for determining the age of organic materials, was developed by Willard Frank Libby, who received the Nobel Prize in Chemistry in 1960 for his radiocarbon research. All living plants and animals contain carbon, and while most of the total carbon is carbon-12, a very small amount of the total carbon is radioactive carbon-14. Libby found that the amount of carbon-14 remains constant in a living plant or animal and is in equilibrium with the environment, however once the organism dies, the carbon-14 within it diminishes according to its rate of decay. This is because living organisms utilize carbon from the environment for metabolism. Libby, and his team of researchers, measured the amount of carbon-14 in a piece of acacia wood from an Egyptian tomb dating 2700-2600 B.C. Based on the half-life of carbon-14 (5,568 years), Libby predicted that the concentration of carbon-14 would be about 50% of that found in a living tree. His prediction was correct.

Radioactive dating is also used to study the effects of pollution on an environment. Scientists are able to study recent climactic events by measuring the amount of a specific radioactive nuclide that is known to have attached itself to certain particles that have been incorporated into the earth's surface. For example, during the 1960s, when many above-ground tests of nuclear weapons occurred, the earth was littered by cesium-137 (half-life of 30.17 years) particle fallout from the nuclear weapons. By collecting samples of sediment, scientists are able to obtain various types of kinetic information based on the concentration of cesium-137 found in the samples. Lead-210, a naturally occurring radionuclide with a half-life of 21.4 years, is also used to obtain kinetic information about the earth. Radium-226, a grandparent of lead-210, decays to radon-222, the radioactive gas that can be found in some basements. Because it is a gas, radon-222 exists in the atmosphere. Radon-222 decays to polonium-218, which attaches to particles in the atmosphere and is consequently rained out—falling into and traveling through streams, rivers, and lakes.

Radioactive dating has proved to be an invaluable tool and has been used in many scientific fields, including geology, archeology, paleoclimatology, atmospheric science, oceanography, hydrology, and biomedicine. This method of dating has also been used to study artifacts that have received a great deal of public attention, such as the Shroud of Turin, the Dead Sea Scrolls, Egyptian tombs, and Stonehenge. Since the discovery of radioactive dating, there have been several improvements in the equipment used to measure radioactive residuals in samples. For example, with the invention of accelerator mass spectometry, scientists have been able to date samples very accurately.