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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.


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.



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


[′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.


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)
References in periodicals archive ?
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.
Being the initial choice as Microsoft's voice assistant could be attributed to her character in Half-Life 2.
Xencor is also advancing a portfolio of biosuperior versions of blockbuster antibody drugs engineered for superior half-life and dosing schedule.
The half-life of DCD in soils was quantified using regression analysis by fitting the DCD concentration in the soil as a function of time to a first-order exponential model as follows (Kelliher et al.
This article presents the concept of half-life analysis of learning curves (Badiru, 2010), using the concept of growth and decay of learning in the acquisition environment.
Episode Two does mark the return of the lengthy vehicle sequences seen in Half-Life 2.
These papers included chemical experiments to measure kinetics in the undergraduate laboratory, half-life experiments using various radioactive isotopes, computer programs to simulate kinetics, calculation methods, classroom activities, and various physical simulations.
The half-life of antibody to CHIKV was calculated by plotting the antibody titer versus age to 18 months on both linear and logarithmic scales.
In emitting green light, the material achieves a brightness half-life of 100,000 hours (a threshold luminance of 1,000cd/m2), 150% longer.
Our XmAb and Xtend platforms confer dramatic improvements to potency and half-life for antibody drugs that can provide key competitive differentiators in the increasingly crowded antibody markets," said Bassil Dahiyat, Ph.
The long-awaited Black Mesa modification, a revamped version of Valve's original Half-Life game from 1998, will be available through Steam's Greenlight service later today.