(redirected from Half-lives)
Also found in: Dictionary, Thesaurus, Medical, Financial.


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.
..... 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 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.
..... 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.


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 ?
A clear conceptual distinction between apparent and intrinsic half-lives is required to reduce the uncertainty in elimination half-lives of persistent chemicals.
Most elimination half-lives reported in the literature are estimated from concentration declines in LD for particular individuals.
b) Atthese high concentrations, intrinsic elimination half-lives are not representative for the general population.
The second factor that has been reported to influence CSTD-based half-lives is the rate of elimination of a substance from the body by all possible pathways, which we refer to as the "human elimination half-life.
The first is to clarify the interpretation of half-lives derived from CSTD fitted with an exponential function by using a transparent and mathematically explicit model of the relationships among a) CSTD-based half-life, b) time trend of intake, and c) the human elimination half-life, in a postban period.
In particular, we estimated human elimination half-lives of p, p'-DDT and p, p'-dichlorodiphenyldichloroethylene (p, p'-DDE) by applying Equation 7 to CSTD from Sweden (Glynn et al.
CONCLUSIONS: Children and adolescents have shorter TCDD half-lives and a slower rate of increase in half-life than adults, and this effect is augmented at higher body burdens.
Shorter elimination half-lives for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) have been reported in human infants (Kreuzer et al.
Thus, all data pairs occurring before July 1977 were excluded (nine positive half-lives excluded: 0.
2004) recently presented estimated half-lives based on experimental data for a number of brominated flame retardants, including tetra- to hexa-BDEs, but not for any higher BDEs.
The objective of the present study was to calculate the apparent half-lives for BDE-209 and other higher BDEs in human serum, using data from occupationally exposed workers sampled before, during, and after a vacation period.
These isotopes have half-lives 100 to 500 times longer than predicted by theory--sufficiently long to permit detailed studies of the chemical properties of lawrencium, the last member of the actinide series.