half-life

The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance, such as a drug in serum, whose quantity decreases exponentially with time. Cf.:half-time.

- biologic half-life the time required for one-half of an amount of a substance to be lost through biologic processes.

- effective half-life the time required for the body burden of an administered quantity of radioactivity to decrease by half through a combination of radioactive decay and biologic elimination.

- physical half-life the time required for half the atoms of a radionuclide to undergo disintegration.

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half-life -.līf n the time required for half of something to undergo a process: as

a) the time required for half of the atoms of a radioactive substance to become disintegrated

b) the time required for half the amount of a substance (as a drug or radioactive tracer) in or introduced into a living system or ecosystem to be eliminated or disintegrated by natural processes <the serum \half-life is about 2 days (Jour. Amer. Med. Assoc.)>

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(hafґlīf″) 1. the time required for the decay of half of a sample of particles of a radionuclide or elementary particle, equal to 0.693 divided by the decay constant; symbol t_1/2 or T_1/2. 2. half-time.