If you were given a piece of radioactive material, you would probably wonder if the activity of the material would always remain the same or if the activity would decrease from day to day. In fact, early studies of radioactive materials showed that the activity of these materials decreases at different rates depending on the radioactive nuclides forming the material. For some materials the activity decreases very rapidly but for others the activity decreases very slowly. If, however, the activity of a piece of material decreases by some fraction over, say, a day, then it decreases by the same fraction each succeeding day. It was convenient to introduce a quantity called the half-life or half-period to describe this behaviour. The half-life was defined to be the time required for half the radioactive nuclides in any piece of radioactive material to decay. Half-lives were found to vary from fractions of a second to millions of years depending on the particular radioactive nuclide studied. The half-life of any particular radioactive nuclide, characterized by its mass number, was however always the same. A measurement of the half-life is very useful in identifying the active nuclides in any piece of radioactive material.
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