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measure of the average lifetime of a radioactive substance (see radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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) 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 isotoperadioactive isotope
or radioisotope,
natural or artificially created isotope of a chemical element having an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached.
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 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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The time taken for the number of atoms of a radioactive isotope to be reduced, by radioactive decay, to one half. The mean life is the average time before decay of a large number of similar elementary particles or atoms of a radioisotope. Mean life is equal to 1.44 times the half-life.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the average time required for the number of radioactive nuclei in a sample of a radioactive substance to be halved. If N0 radioactive nuclei exist at a time t = 0, the number of nuclei N decreases in time according to the law

N = N0eλt

where λ is the disintegration constant. The quantity τ = 1/λ is called the mean life of the radioactive nuclei. The half-life T1/2 is related to λ and τ by the equation

T1/2 = τ 1n 2 = (1n 2)/λ = 0.693/λ

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


[′haf ‚līf]
The time required for one-half of a given material to undergo chemical reactions.
The average time interval required for one-half of any quantity of identical radioactive atoms to undergo radioactive decay. Also known as half-value period; radioactive half-life.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. the time taken for half of the atoms in a radioactive material to undergo decay.
2. the time required for half of a quantity of radioactive material absorbed by a living tissue or organism to be naturally eliminated (biological half-life) or removed by both elimination and decay (effective half-life)
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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Through its shifting narrative and quirky, engaging characters, The Half-Life of Everything balances humor,A candor, melancholy, and warmth.
The age was significantly associated with FVIII half-life (t[sub]1/2 =5.76 + 0.59 x age, R 2 = 0.32, P < 0.01) [Figure 2]a.
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The drug could cause human fetal harm based on the animal data, mechanism of action, relatively low molecular weight (about 556), and terminal half-life (17 hours).
If the half-life is shorter than 45 million billion years, the probability is high that the scientists would have seen some hint of the process.
They developed a static population-based pharmacokinetic (PPK) model, called "CSTD half-life tool" (http:// www.sust-chem.ethz.ch/downloads) specifically for the post-ban period that explains the relationships between [t.sup.in.sub.1/2], [t.sup.elim.sub.1/2], and [t.sup.CSTD.sub.1/2].
This Early Access release also offers multiplayer, with deathmatch and team deathmatch modes on six "completely reimagined" maps from the original Half-Life currently available.
Under the proposed agreement, Ontario Power Generation expects to receive about 290 cubic feet of intermediate-level nuclear waste per year at the deep geologic repository, including carbon-14, with a half-life of about 5,000 years; nickel-59, with a half-life of 76,000 years; nickel-63, with a half-life of 100 years; iodine-129, with a half life of 15.7 million years; chlorine-36, with a half-life of about 300,000 years; and many other isotopes with half-live exceeding 30 years.