The nuclei of a radioactive isotope are unstable and decay. The activity of a source is the number of nuclei that decay each second, and it is measured in becquerel, symbol Bq. As time passes, fewer undecayed nuclei are left, so the activity of the source falls.
The half-life of an isotope is the time taken for half of the undecayed nuclei to decay. It is also the time taken for the activity of a source to fall by half. For example, a source with an activity of 800 Bq and a half-life of 2 hours will have an activity of 400 Bq after 2 hours and 200 Bq after 4 hours.
Different isotopes have very different half-lives, from a fraction of a second to billions of years. The half-life of an isotope is the same however big the sample is.
