Radioactivity is a process that is practically uninfluenced by any of the factors, such as temperature and pressure, that are used to control the rate of chemical reactions. The rate of radioactive decay appears to be affected only by the structure of the unstable (decaying) nucleus. Each radioisotope has its own half-life, which is the time it takes for one half the number of atoms present to decay. These half-lives vary from fractions of a second to millions of years, depending only upon the atom. We shall see that the half-life is one factor considered in choosing a particular isotope for certain uses. [Illustration: HALF-LIFE PATTERN OF STRONTIUM-90] Percent of 100 50 25 12.5 6.75 Radioactivity Beginning 1 2 3 4 of Life Half-life Half-lives Half-lives Half-lives 28 years 56 years 84 years 112 years Most artificially made radioisotopes have relatively short half-lives. This makes them useful in two ways. First, it means that very little material is needed to obtain a significant number of disintegrations. It should be evident that, with any given number of radioactive atoms, the number of disintegrations per second will be inversely proportional to the half-life. Second, by the time 10 half-lives have elapsed, the number of disintegrations per second will have dwindled to ¹/₁₀₂₄ the original number, and the amount of radioactive material is so small it is usually no longer significant. (Note the decrease in the figure above.) How Are Radioisotopes Used? A radioisotope may be used either as a source of radiation energy (energy is _always_ released during decay), or as a tracer: an identifying and readily detectable marker material. The location of this material during a given treatment can be determined with a suitable instrument even though an unweighably small amount of it is present in a mixture with other materials. On the following pages we will discuss medical uses of individual radioisotopes—first those used as tracers and then those used for their energy. In general, tracers are used for analysis and diagnosis, and radiant-energy emitters are used for treatment (therapy). Radioisotopes offer two advantages. First, they can be used in extremely small amounts. As little as one-billionth of a gram can be measured with suitable apparatus. Secondly, they can be directed to various definitely known parts of the body. For example, radioactive sodium iodide behaves in the body just the same as normal sodium iodide found in the iodized salt used in many homes. The iodine concentrates in the thyroid gland where it is converted to the hormone thyroxin. Other radioactive, or “tagged”, atoms can be routed to bone marrow, red blood cells, the liver, the kidneys, or made to remain in the blood stream, where they are measured using suitable instruments.[6]