half-life,  in radioactivity, the interval of time required for
one-half of the atomic nuclei of a radioactive sample to decay
(change spontaneously into other nuclear species by emitting
particles and energy), or, equivalently, the time interval
required for the number of disintegrations per second of a
radioactive material to decrease by one-half.  The radioactive
isotope cobalt-60, which is used for radiotherapy, has, for
example, a half-life of 5.26 years. Thus after that interval, a
sample originally containing 8 g of cobalt-60 would contain only
4 g of cobalt-60 and would emit only half as much radiation.
After another interval of 5.26 years, the sample would contain
only 2 g of cobalt-60. Neither the volume nor the mass of the
original sample visibly decreases, however, because the unstable
cobalt-60 nuclei decay into stable nickel-60 nuclei, which
remain with the still-undecayed cobalt.  Half-lives are
characteristic properties of the various unstable atomic nuclei
and the particular way in which they