|The time taken for the activity of a radionuclide to fall to half its original value is called the half-life, symbol t½ . Put another way, this is the time for half the nuclei in a sample to decay. Each radionuclide has a unique half-life, which can range from fractions of a second to billions of years. For iodine-131 , it is 8 days; caesium-137, 30 years; carbon-14, 5730 years; plutonium-239, 24 000 years; and uranium-238, 4470 million years. In successive half-lives, the activity of a radionuclide is reduced by decay to ½ , ¼ , ⅛ and so on, of its initial value.|| |
This means that we can predict the activity remaining at any future time. As the amount of a radionuclide decreases, the radiation emitted decreases proportionately.