Half-Life

Definition

The time required for half of a radioactive substance to decay. The remaining fraction follows N(t) = N₀ × (1/2)^(t/t½). Carbon-14's 5,730-year half-life powers radiocarbon dating of archaeological specimens up to roughly 50,000 years old. In pharmacology, drug half-life determines dosing intervals: caffeine's 5-hour half-life means an 8 a.m. coffee still leaves roughly 25% of the dose active by 6 p.m. Five half-lives reduce the starting quantity to about 3%, which clinicians treat as effectively cleared.