Among all radioactive isotopes, tellurium-128 has the longest measured half-life.
Tellurium-128 undergoes an extremely rare form of radioactive decay called double beta decay. Its half-life is about 2.2 × 10²⁴ years, vastly longer than the age of the universe, which is about 1.38 × 10¹⁰ years. A half-life is the time required for half of a sample’s radioactive nuclei to decay, not the time for the sample to disappear.
Tellurium-128 is a naturally occurring isotope of tellurium, element 52. Its decay is so slow that detection requires extremely sensitive experiments and large quantities of material. The result is often cited in discussions of the longest directly measured radioactive half-lives.
This fact is easy to confuse with the longevity of uranium-238 or thorium-232, whose half-lives are measured in billions of years. Those isotopes are long-lived, but their half-lives are still enormously shorter than tellurium-128’s.