Note 4: Nuclear Half-Life

The concept of "half-life" is basic to an understanding of radioactive decay of unstable nuclei.

Unlike physical "systems"—bacteria, animals, men and stars—unstable isotopes do not individually have a predictable life span. There is no way of forecasting when a single unstable nucleus will decay.

Nevertheless, it is possible to get around the random behavior of an individual nucleus by dealing statistically with large numbers of nuclei of a particular radioactive isotope. In the case of thorium-232, for example, radioactive decay proceeds so slowly that 14 billion years must elapse before one-half of an initial quantity decayed to a more stable configuration. Thus the half-life of this isotope is 14 billion years. After the elapse of second half-life (another 14 billion years), only one-fourth of the original quantity of thorium-232 would remain, one eighth after the third half-life, and so on.

Most manmade radioactive isotopes have much shorter half-lives, ranging from seconds or days up to thousands of years. Plutonium-239 (a manmade isotope) has a half-life of 24,000 years.

For the most common uranium isotope, U-238, the half-life is 4.5 billion years, about the age of the solar system. The much scarcer, fissionable isotope of uranium, U-235, has a half-life of 700 million years, indicating that its present abundance is only about 1 percent of the amount present when the solar system was born.