# Half-Life

The rate of radioactive decay is characteristic of each radionuclide. Scientists talk about this rate as a radionuclide's radioactive half-life, commonly referred to as just half-life. It is the time required for the disintegration of one-half of the radioactive atoms that are present when measurement starts. It does not represent a fixed number of atoms that disintegrate, but a fraction.

For example, if there are hundred atoms of a radionuclide that has a half-life of one minute, there will be one half that number, or fifty atoms of the original radionuclide left one minute later. After the second minute, there will be 25 atoms of the original radionuclide left. The fact that this simple example points to the existence of 12.5 radioactive atoms after three minutes, illustrates that half-life is intended to be used for the very large number of atoms that are found in even small samples of radioactive materials. 100 atoms aren't going to give off much radiation!

The half-life tells how quickly the radioactivity from the radionuclide will decrease. The number of curies tells how active it is now.

### Biological Half-Life

The biological half-life is the time an organism takes to eliminate one half the amount of a compound or chemical on a strictly biological basis. Thus, if a stable chemical compound were given to an individual and half of it were eliminated by the body (perhaps in urine) within three hours, the biological half-life would be three hours.

### Effective Half-Life

The effective half-life incorporates both the radioactive and biological half-lives. It is used when health physicists calculate the dose received from an internal radiation source. To determine the effective half-life of a radionuclide in a human, one needs to know the radioactive half-life as well as the biological half-life of the radionuclide.