Half time is the rate at which nuclear decay takes place and is measured in radioisotope's half-life. The half-life of a radioisotope is the time it takes for half the nuclei to decay. For example the radioisotope randon-222 decays into polonium-218 with a half-time of 4 days. Of the remaining nuclei, 25 would decay over the next 4 days, if you waited another 4 days only 12 or 13 randon-222 atoms would remain. The half-time of radioisotopes varies from a fraction of a second to millions of years ahead.
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