half-life

During radioactive decay, the time in which the activity of a radioactive source decays to half its original value (the time taken for half the atoms to decay). In theory, the decay process is never complete and there is always some residual radioactivity. For this reason, the half-life of a radioactive isotope is measured, rather than the total decay time. It may vary from millionths of a second for some radioisotopes to billions of years for others, but each radioisotope has a definite half-life.

To determine a short half-life a Geiger–Müller tube can be used to count the number of particles emitted by a sample. To determine a longer half-life a mass spectrometer is used. Some examples are: sodium-24, 15 hours; carbon-14, 5,730 years; plutonium-239, 24,000 years; and uranium-238, 4,500 million years.