• noun the time required for an organism to eliminate naturally half the amount of a substance that has entered its body
  • The time taken for half of the atoms of an unstable isotope to decay. Half-lives can be under a second (0.3 seconds for Sodium 20) or millions of years (about 4.5 billion years for Uranium 238).
  • The time interval that elapses for a given radioactive substance to lose half of its radioactive intensity. That is, it is the time required for half the atoms of a given sample of identical nuclides to undergo decay. The half-life of 50V (an isotope of vanadium), for instance, is approximately 1.4 × 1017 years, while that for 6Be (an isotope of beryllium) is approximately 5.9 × 10-21 seconds. Also called radioactive half-life.

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