half-life

or half life, half·life

[ haf-lahyf ]

Phonetic (Standard)IPA

noun

, plural half-lives [haf, -lahyvz].

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

/ hăf′līf′ /

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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Word History and Origins

Origin of half-life¹

First recorded in 1905–10; half + life

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Example Sentences

The half-life of a piece of technology these days is very short.

From The Daily Beast

But the half-life of a piece of hardware is very short these days.

From The Daily Beast

Radiation dissipates over time with every isotopic half-life.

From The Daily Beast

It was a small picture, half life-size, and set in an oval frame of black walnut.

From Project Gutenberg

A very beautiful marble group, half life-size, stood in one corner, and gave an air of brightness to the whole room.

From Project Gutenberg

Now the old problem of half-life is taking its toll, and we cant even hope to keep up with the birth rate any more.

From Project Gutenberg

And, finally, Moglaut had warned that the compact power unit pocketed on the left breast had a half-life of only thirteen years.

From Project Gutenberg

Then he knew all hope was over, and the mad passion went out of him, and his arms dropped to his sides as if half life had fled.

From Project Gutenberg

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