Half life

The half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve.

OR

The half-life is the time it takes for the count rate, or activity from a sample containing the isotope to fall to half its initial level.

Net decline is the ratio of the number of isotope atoms present in a sample now to the initial number of isotope atoms present.

Net decline = final number of atoms: initial number of atoms

OR

Net decline = final count rate: initial count rate

Count rate → initial count rate/2 to the power of n, where n is the number of half lives