half-life

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half-life

half-life measure of the average lifetime of a radioactive substance (see radioactivity ) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay. For example, the half-life of a particular radioactive isotope of thorium is 8 minutes. If 100 grams of the isotope are originally present, then only 50 grams will remain after 8 minutes, 25 grams after 16 minutes (2 half-lives), 12.5 grams after 24 minutes (3 half-lives), and so on. Of course the 87.5 grams that are no longer present as the original substance after 24 minutes have not disappeared but remain in the form of one or more other substances in the isotope's radioactive decay series. Individual decays are random and cannot be predicted, but this statistical measure of the great number of atoms in the sample is very accurate. The half-life of a radioactive isotope is a characteristic of that isotope and is not affected by any change in physical or chemical conditions.

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half-life

The Oxford Essential Dictionary of the U.S. Military | 2001 | Â© The Oxford Essential Dictionary of the U.S. Military 2001, originally published by Oxford University Press 2001. (Hide copyright information) Copyright

half-life n.the time required for the activity of a given radioactive species to decrease to half of its initial value due to radioactive decay. The half-life is a characteristic property of each radioactive species and is independent of its amount or condition. The effective half-life of a given isotope is the time in which the quantity in the body will decrease to half as a result of both radioactive decay and biological elimination.

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half-life

A Dictionary of Nursing | 2008 | Â© A Dictionary of Nursing 2008, originally published by Oxford University Press 2008. (Hide copyright information) Copyright

half-life (hahf-lyf) n.
1. the time taken for half the atoms of a radioactive isotope to decay: a measure of the radioactivity of the isotope.

2. (in pharmacology) the time taken for the body to excrete half a given amount of a drug.

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