Radiometric Clocks

Introduction In the present nuclear age, the devastating effects of radioactivity have been highlighted by such issues as nuclear weapons testing and radioactive waste disposal, and by the reactor incidents at Three Mile Island and Chernobyl. These events illustrate the vexing problems that can be associated with high levels of radioactivity. However, the presence of radioactivity is not limited to nuclear reactors and atom bombs. We are constantly being showered by very low levels of background radioactivity, which originates from many different sources, some familiar, others distant. A variety of building materials, including cement and rocks, are sources of low-level radioactivity. Certain soil types contain small amounts of radioactive substances. Radioactive Carbon-14 is present in most plants and animals (including our own bodies). Even the air all around us contains a very small proportion of radioactive gasses, like Radon, which has recently become a controversial topic. Cosmic rays, which originate in outer space, also contribute to the background radioactivity. These rays are constantly bombarding the outer atmosphere, crashing into the gas molecules there and creating a mist of secondary radioactivity that reaches the Earth's surface. The cumulative effect of all of these sources results in a continuous shower of low-level radioactivity, referred to as background radioactivity. In addition to these environmental sources of radioactivity, many other localized, man-made sources commonly exist around the home and in the workplace. Many smoke detectors contain radioactive Americium-241. The paint used for the numbers and hands of some old watches and clocks contains radioactive Radium salts. Some other very old paints, used on such items as chinaware, contain Uranium salts. Many radioactive isotopes are also used in medicine, such as radioactive Iodine-129 and Cesium-137. These radioactive sources are local, and are well understood. If handled with care, they do not pose the same dangers as high levels of radioactivity, such as are found in association with nuclear weapons fuel, as well as near the damaged reactor at Chernobyl. This paper seeks to provide some background about the nature of radioactivity, and to present at least one way that people have learned to exploit radioactivity to learn more about the world we live in: radiometric dating of rocks.