Half-life is a term used to describe the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable atoms survive. The term is also used more generally to characterize any type of exponential (or, rarely, non-exponential) decay. For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body.
In the context of radioactivity, half-life refers to the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay. This means that if a sample of a radioactive substance has a half-life of 10 years, then after 10 years, half of the original sample will have decayed, and after another 10 years, half of the remaining sample will have decayed, and so on. Half-lives are characteristic properties of the various unstable atomic nuclei and the particular way in which they decay.
Half-lives can range from millionths of a second to billions of years, depending on the stability of the nucleus. The radioactive isotope cobalt-60, which is used for radiotherapy, has a half-life of 5.26 years. Half-lives for beta decay range upward from one-hundredth of a second and, for alpha decay, upward from about one one-millionth of a second.
In summary, half-life is a term used to describe the time required for a quantity to reduce to half of its initial value. It is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable atoms survive. Half-lives are characteristic properties of the various unstable atomic nuclei and the particular way in which they decay.
