How long does radiation last?

Radiation is emitted as an unstable atom decays to become a lighter element. Radiation, therefore, persists for as long as the unstable isotope remains present. Unstable isotopes emit radiation only once per decay, though they can decay several times.

The persistence of radiation depends on the time it takes for an unstable isotope to decay. Different isotopes have different half-lives, which is the time it takes for 50 percent of a sample to decay. For example, carbon-11 is so unstable that it has a half-life of only 20 minutes, while the half-life of tellurium-128 is over 2 septillion years. Isotopes that decay in this manner are called radioactive isotopes.