Half-life

Half-life is the time in which half the radioactive nuclei in a sample decay, or its activity falls to half its initial value. For a radionuclide, it is a characteristic statistical property of its decay.

Radioactive decay: The spontaneous transformation of an unstable atomic nucleus into a more stable configuration, often releasing radiation. Half-life measures the rate at which this process removes nuclei from a sample.
Radionuclide: An isotope with an unstable nucleus that undergoes radioactive decay. Each radionuclide has its own characteristic half-life.
Radiometric dating: A method of determining age by measuring radioactive parent isotopes and their decay products. Known half-lives let isotope ratios estimate elapsed time.
Biological half-life: The time for a biological system to eliminate half of a substance through physiological processes. It concerns removal from an organism, not nuclear decay.
Exponential decay: A process in which a quantity decreases at a rate proportional to its current amount. A constant probability of decay per unit time produces this pattern in radioactive samples.
Isotope: One of two or more atoms of the same element with different numbers of neutrons. Radioactive isotopes are the nuclei whose changing abundance half-life describes.
Carbon-14 dating: A method for dating once-living material by measuring its carbon-14 content. Carbon-14's 5,730-year half-life sets the method's useful timescale.
Effective half-life: The time for a radioactive substance in a biological system to fall by half through both decay and bodily elimination. It combines physical decay with biological clearance.
Decay constant: The probability per unit time that an individual radioactive nucleus will decay. It determines the half-life through the relation T½ = ln(2)/λ.
Becquerel: The SI unit of radioactive activity, equal to one nuclear decay per second. Activity in becquerels falls as the number of undecayed nuclei falls.

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