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 the time required for the number of disintegrations per second of a radioactive material to decrease by one-half.
Half-lives are characteristic properties of the various unstable atomic nuclei and the particular way in which they decay. Alpha decay and beta decay are generally slower processes than gamma decay .