Answer to Question #471 Submitted to "Ask the Experts"

In general, for a process involving a change (transformation, emptying of contents, change of state, etc.), half-life is the time in which half of the original quantity has undergone the change. When dealing with radioactive species, the radiological half-life is the time in which half of the atoms of a particular radionuclide transform into another nuclear form. In one half-life, the number of atoms of the original form (and the amount of radioactivity) decreases by 50 percent; in another half-life, the number of atoms (and activity) decreases by another 50 percent (to 25 percent); and so forth.

Each radionuclide has a characteristic half-life; half-lives vary from millionths of a second to billions of years. For example, ¹³¹I, a radioactive isotope of iodine that is used in medical diagnosis and therapy, has a half-life of eight days while ²³⁸U, which occurs naturally in the earth, has a half-life of 4.47 billion years. Radioactive half-life is also an indication of how rapidly the radioactive transformation is taking place. Radionuclides with a short half-life are undergoing transformation more rapidly; those with a long half-life are undergoing transformation more slowly. For example, in the case of ¹³¹I (eight-day half-life), the atoms are undergoing transformation at the rate of nearly 9 percent per day while for ²³⁸U (4.47 billion year half-life), the atoms are undergoing transformation at the rate of only a few percent every 100 million years!

Charles E. Roessler, PhD, CHP