Half-life
Definition
The time it takes for half of a sample of radioactive material to decay. After one half-life half remains, after two half-lives a quarter remains, and so on.
Symbol: · Unit: seconds (s), or any unit of time
Technical definition
The time required for a quantity undergoing exponential decay to fall to half its initial value: t(1/2) = ln(2)/λ, where λ is the decay constant. Each radioactive isotope has a characteristic half-life, from fractions of a second to billions of years.
Example
Carbon-14 has a half-life of about 5,730 years. A 1-gram sample of carbon-14 in an ancient bone shrinks to 0.5 g after 5,730 years and 0.25 g after 11,460 years — which is how archaeologists date organic remains.
Half-life is not the time until all of a sample is gone — mathematically, some tiny fraction always remains. It is also a statistical average over many atoms; you cannot predict when one particular atom will decay, only the behavior of a large group.