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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 It might be outdated or ideologically biased.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
References in periodicals archive ?
The halflife of fast [sup.32]P blood clearance was lower for TSP, DICAL and D+F, in relation to RIO and MONTE.
There I was, concentrating on Halo 2 and Halflife 2 that I almost missed out on this gem.
He is the author of fifteen books of poetry, including Chickamauga, which won the 1996 Lenore Marshall Poetry Prize; Black Zodiac (1997), which won the Pulitzer Prize and the Los Angeles Times Book Prize; Country Music: Selected Early Poems (1983), which won the National Book Award; Zone Journals (1988); Appalachia (1998); Negative Blue (2000); The World of the Ten Thousand Things: Poems 1980-1990; and two volumes of criticism: Halflife (1988) and Quarter Notes (1995).
"Patients seem to wake up very well with this drug." In the typical study patient, eszopiclone has a 5- to 7-hour halflife. Polysomnographic studies have shown that after 8 hours--a normal night's sleep--eszopiclone's sedative effects are similar to placebo.
If you want to live this sort of halflife, then fine.
Just as each radioactive element has its own halflife, so every advertising treatment/product!
The estimated speed of convergence to a balanced growth path is 0.013 a year, implying that a halflife on a logarithmic scale of output per worker is approximately 50 years.
Its very long halflife in the human organism causes its accumulation over the lifetime in liver and kidneys.
The mean halflife for the leaf litter in this study is [approximately]219 d, which is similar to the values reported for R.
Sierra has launched a new web site at http://www.halflifefiles.com aimed at halflife fans and providing them with their own search engine.
In this analysis, k is set equal to the expression, -ln(.5)/halflife, where halflife is the number of years before one-half of the damage decays by itself.(3) This allows us to examine the model based on how long a particular pollutant is expected to stay around once it is released into the environment.