Radioactive tracers are substances that contain a radioactive atom to allow easier detection and measurement. (Radioactivity is the property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.) For example, it is possible to make a molecule of water in which one of the two hydrogen atoms is a radioactive tritium (hydrogen-3) atom. This molecule behaves in almost the same way as a normal molecule of water. The main difference between the tracer molecule containing tritium and the normal molecule is that the tracer molecule continually gives off radiation that can be detected with a Geiger counter or some other type of radiation detection instrument.
One application for the tracer molecule described above would be to monitor plant growth by watering plants with it. The plants would take up the water and use it in leaves, roots, stems, flowers, and other parts in the same way it does with normal water. In this case, however, it would be possible to find out how fast the water moves into any one part of the plant. One would simply pass a Geiger counter over the plant at regular intervals and see where the water has gone.
Industry and research. Radioactive tracers have applications in medicine, industry, agriculture, research, and many other fields of science and technology. For example, a number of different oil companies may take turns using the same pipeline to ship their products from the oil fields to their refineries. How do companies A, B, and C all know when their oil is passing through the pipeline? One way to solve that problem is to add a radioactive tracer to the oil. Each company would be assigned a different tracer. A technician at the receiving end of the pipeline can use a Geiger counter to make note of changes in radiation observed in the incoming oil. Such a change would indicate that oil for a different company was being received.
Another application of tracers might be in scientific research on plant nutrition. Suppose that a scientist wants to find out how plants use some nutrient such as phosphorus. The scientist could feed a group of plants fertilizer that contains radioactive phosphorus. As the plant grows, the location of the phosphorus could be detected by use of a Geiger counter. Another way to trace the movement of the phosphorus would be to place a piece of photographic film against the plant. Radiation from the phosphorus tracer would expose the film, in effect taking its own picture of its role in plant growth.
Medical applications. Some of the most interesting and valuable applications of radioactive tracers have been in the field of medicine. For example, when a person ingests (takes into the body) the element iodine, that element goes largely to the thyroid gland located at the base of the throat. There the iodine is used in the production of various hormones (chemical messengers) that control essential body functions such as the rate of metabolism (energy production and use).
Suppose that a physician suspects that a person's thyroid gland is not functioning properly. To investigate that possibility, the patient can be given a glass of water containing sodium iodide (similar to sodium chloride, or table salt). The iodine in the sodium iodide is radioactive. As the patient's body takes up the sodium iodide, the path of the compound through the body can be traced by means of a Geiger counter or some other detection device. The physician can determine whether the rate and location of uptake is normal or abnormal and, from that information, can diagnose any problems with the patient's thyroid gland.