# half-life

(redirected from*biologic half-life*)

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## half-life,

measure of the average lifetime of a radioactive substance (see radioactivity**radioactivity,**

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.

**.....**Click the link for more information. ) 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 isotope

**radioactive 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.

**.....**Click the link for more information. 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.

## half-life

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.

## Half-Life

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

*N* = *N*_{0}*e*^{λt}

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

*T*_{1/2} = *τ* 1n 2 = (1n 2)/*λ* = 0.693/*λ*

## half-life

[′haf ‚līf] (chemistry)

The time required for one-half of a given material to undergo chemical reactions.

(nucleonics)

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.

## 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**)