The half-life is the time it takes for half of the parent atoms to decay.
Radioactive dating is a method of dating rocks and minerals using radioactive isotopes.
Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes.
The isotopes are then measured within the same machine by an attached mass spectrometer (an example of this is SIMS analysis).
Either a whole rock or a single mineral grain can be dated.
Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer (such as in the argon-argon scheme).