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Radioactive decay is a change in an atom's nucleus that happens to individual nuclei spontaneously (by chance), which occurs at a fixed rate dependent upon the particular nuclide. The presence of repeated decays in such manner within some material is termed radioactivity.
If you had a given number of a particular radioactive nuclide, and each time one decayed, you replaced it with another (non-decayed), the long-term rate of decays (average count of decays per second) would remain constant. Without such replacement, the number of nuclides not yet decayed (parent nuclides) is constantly falling exponentially so the rate of decays falls with it. Given this, decay rates are often characterized by half-lives, the time it takes for half the original parent nuclides to decay into the resulting nuclides (daughter nuclides), or alternately, by a radioactive decay timescale, the time in which the count of parent nuclides falls by a factor of e. Types of radioactive decay:
Radioactive decay generates heat (radioactive heating), can damage surrounding material due to particles expelled in the process (possibly making some surrounding material radioactive) and most types of decay change the material's chemical composition (as atomic numbers change). In some cases, such changes can be observed at a distance through spectroscopy: for example, some supernova produce radioactive elements and observable spectral lines for the parent and daughter nuclides allow the decay and its rate to be detected.
A material's composition can show signs of radioactivity through isotopes in otherwise-unexpected ratios. By extrapolating backward, knowing in what manner the radioactivity was produced, the time-interval since it was made radioactive can be determined (radioactive dating).