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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 ?
In particular, there can be no assurance that the research results are indicative of results to be obtained in further studies needed to demonstrate extended antibody half-lives in humans, or that PDL can obtain intellectual property rights related to these discoveries.
Noker and Gorman (2003a, 2003b) reported mean ( [+ or -] SD) terminal elimination half-lives, ranging from 88 to 146 days (132 [+ or -] 13 days for males and 110 [+ or -] 26 days for females) for PFOS and 49 to 200 days (141 [+ or -] 52 days for males and 87 [+ or -] 47 days for females) for PFHS, after intravenous dosing of three male and three female cynomolgus monkeys in separate experiments, with no significant difference between males and females or between the two compounds.
Our analysis provides a transparent interpretation of the half-lives derived from exponential fitting of CSTD.
Previously reported data in both primates and human patients have documented that PDL's Human Anti-CMV Antibody and Human Anti-Hepatitis B Antibody have half-lives of between two and three weeks.
2011) recently applied a pharmacokinetic model to cross-sectional biomonitoring data to estimate intrinsic elimination half-lives of PCBs in humans.
By binding to the patient's serum albumin the half-life of the therapeutic compound is dramatically enhanced, meaning that drugs with natural half-lives of minutes can be converted into AlbudAb-drug conjugates with half-lives well over a week.
But some compounds have much longer half-lives, and waste products have to be kept in storage for years and years.
based in Mountain View, CA, is a privately held biotechnology company focused on the discovery and development of novel therapeutics with improved in vivo half-lives.
Most empirical estimates of human elimination kinetics for persistent chemicals reflect apparent elimination half-lives that represent the aggregated effects of intrinsic elimination, ongoing exposure, and changes in body weight.
By measuring the proportions of radioactive elements and their daughters, researchers can determine how many half-lives have passed since the formation of the radioactive elements in such objects as stars.
13 AlbudAb Technology Platform - Versatile Albumin Binding Domains for the Development of Therapeutics with Tunable Half-Lives