WordNet (r) 3.0 (2006):

half life
    n 1: the time required for something to fall to half its initial
         value (in particular, the time for half the atoms in a
         radioactive substance to disintegrate) [syn: half life,
         half-life]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "half life":
   activity, artificial radioactivity, contamination, curiage,
   decontamination, fallout, natural radioactivity, nuclear radiation,
   radiant energy, radiation, radioactive radiation, radioactivity,
   radiocarbon dating, radiolucency, radiopacity, radiosensibility,
   radiosensitivity, saturation point, specific activity