HALF-LIFE Definition & Meaning

half-life

or half life, �󲹱��·��ڱ�

[ haf-lahyf ]

Phonetic (Standard)IPA

noun

plural half-lives

Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
Also called bi·o·log·i·cal half-life [bahy, uh, -, loj, ik, uh, l , haf, -lahyf]. Pharmacology. the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.
Informal. a brief period during which something flourishes before dying out.

half-life

noun

the time taken for half of the atoms in a radioactive material to undergo decay τ
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 )

half-life

/ ��ă��′��ī��′ /

The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity. For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.

half-life

In physics , a fixed time required for half the radioactive nuclei in a substance to decay. Half-lives of radioactive substances can range from fractions of a second to billions of years, and they are always the same for a given nucleus, regardless of temperature or other conditions. If an object contains a pound of a radioactive substance with a half-life of fifty years, at the end of that time there will be half a pound of the radioactive substance left undecayed in the object. After another fifty years, a quarter-pound will be left undecayed, and so on.

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Notes

Scientists can estimate the age of an object, such as a rock, by carefully measuring the amounts of decayed and undecayed nuclei in the object. Comparing that to the half-life of the nuclei tells when they started to decay and, therefore, how old the object is. ( See radioactive dating .)

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51�Թ� History and Origins

Origin of half-life¹

First recorded in 1905–10; half + life

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Example Sentences

Examples have not been reviewed.

The medical examiner said the ketamine in Perry's system could not have been from the infusion therapy because of the drug's short half-life.

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Methane warms the planet 86 times more powerfully than carbon dioxide but has a much shorter half-life.

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"We found that starvation had the greatest impact on the intermediate group of proteins in muscular cells, which have a half-life of a few hours, causing the breakdown to shift and accelerate," Collins said.

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A laboratory measurement method can then be used to determine how much 14C is still contained in a particular tree ring and thus calculate the tree's approximate time of death over the known half-life.

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One molecule of methane has more heat-trapping power than CO2, and its half-life in the atmosphere is just a decade, meaning its levels can fluctuate.

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