• physical: time interval in which half of the nuclei of a radioactive nuclide have decayed. Short half-lives lead to a high radiation activity, long half-lives yield a low radiation activity.
  • biological: time interval in which a biological system, e.g. a human being or an animal, excretes (e.g. via the urine) half of the amount of a certain substance taken up in the body or a certain organ.
  • effective: time interval in which the amount of a radionuclide in a biological system is reduced by half, as a result of a coaction of radioactive decay and excretion.