half-life

or half life, half·life

[ haf-lahyf ]

noun

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

1. Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
2. 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.
3. Informal. a brief period during which something flourishes before dying out.

half-life

noun

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

half-life

/ hăf′līf′ /

1. 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

1. 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.

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 .)

Word History and Origins

Origin of half-life¹

First recorded in 1905–10; half + life

Example Sentences

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.

From BBC

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

From Science Daily

"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.

From Science Daily

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.

From Science Daily

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.

From Science Daily