A radioactive isotope's nucleus is unstable and spontaneously decays, giving off radiation and changing into a different isotope. The rate at which nuclei decay is constant. Half-life describes ...

Radioactive decay is a spontaneous and ... It takes another two days for the count rate to halve again, this time from 40 Bq to 20 Bq. The half-life of this source is 2 days.

A spontaneous process means that it is not possible to say which particular nucleus will decay ... count rate might get very small, it does not drop to zero completely. The half-life of ...