What Are Radioisotopes?
Frederick Soddy
In the 19th Century an Englishman, John Dalton, put forth his atomic theory, which stated that all atoms of the same element were exactly alike. This remained unchallenged for 100 years, until experiments by the British chemist, Frederick Soddy, proved conclusively that the element neon consisted of two different kinds of atoms. All were alike in chemical behavior but some had an atomic weight (their mass relative to other atoms) of 20 and some a weight of 22. He coined the word isotope to describe one of two or more atoms having the same atomic number but different atomic weights.[5]
Radioisotopes are isotopes that are unstable, or radioactive, and give off radiation spontaneously. Many radioisotopes are produced by bombarding suitable targets with neutrons now readily available inside atomic reactors. Some of them, however, are more satisfactorily created by the action of protons, deuterons, or other subatomic particles that have been given high velocities in a cyclotron or similar accelerator.
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.
HALF-LIFE PATTERN OF STRONTIUM-90
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.)