Radioactive decay
= Radioactive decay
{title2=$\overline N(t)=N_0e^{-\lambda t}$}
{wiki}
= Radioactivity
{synonym}
<Radioactive decay> is the spontaneous transformation of an unstable <atomic nucleus> or nuclear state. Its timing can be unpredictable for an individual <atomic nucleus> while a large population has a reproducible average behavior. A constant <nuclear decay constant> $\lambda>0$ gives the familiar <exponential decay> law. The <half-life> describes how quickly the parent population falls, while <radioactive activity> describes its rate of transformations.<University Physics Volume 3 by Samuel J. Ling and others/10.3 Radioactive Decay>[University Physics Volume 3 by Samuel J. Ling and others; Radioactive Decay Law, equations 10.7–10.20: decay constant, half-life and activity]{ref}
This connection between randomness and a regular population law makes <radioactive decay> useful for measuring elapsed time and for understanding sources of <radiogenic heating>. It also distinguishes two questions in an experiment: what the source emits, and what an instrument records. <Becquerel's uranium-radiation experiment> used a <photographic plate>; the <Curie ionization-current measurement> measured an electrical consequence of radiation. Their readouts reveal different aspects of the same investigation.