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The time required for half of the radionuclides of a given material to decay into another element. In other words, the probability of a radioisotope decaying during its half-life is 50 percent. For example, if we take 4 grams of cobalt 60, which has a half-life of 5.26 years, after that time there will be 2 grams of cobalt 60 in the sample. After another half-life, there will be only 1 gram of cobalt 60 in the sample.

The half-life is an important parameter; e.g., in nuclear waste management, it indicates how long the material should be stored before its radioactivity drops to harmless levels. This is usually 10 half-lives.