Biological half life


physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.
also called biological half-life. 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.
noun
the time taken for half of the atoms in a radioactive material to undergo decay τ
the time required for half of a quant-ty 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)
n.

half-life
(hāf’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.

note: 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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