Nuclear physics
Half-Life
The time required for half of a quantity of a radioactive isotope to decay.
Different isotopes have wildly different half-lives: tritium 12.3 years, cesium-137 30 years, plutonium-239 24,100 years, uranium-235 704 million years. Half-life determines fallout persistence: short-lived isotopes dominate early radiation, long-lived isotopes contaminate the environment for decades.
Related terms
Fallout
Radioactive material thrown into the atmosphere by a surface-burst nuclear detonation, which then settles back to earth over hours to weeks.
Tritium
A radioactive heavy isotope of hydrogen with two neutrons, used as fusion fuel and in boosted-fission weapons.
Plutonium-239
A fissile isotope of plutonium produced from uranium-238 in nuclear reactors and used in most modern fission weapons.