Half life is the time taken for half of the radioactive atoms to decay. It is also the time taken for the starting count rate to halve (with the background rate subtracted). This flashcard tutorial explains how to calculate half life of a radioactive nuclide by first identifying and subtracting the background radiation count from the graph. *** NOTE - I made a mistake at 4:15... should be DAYS not hours :) Check out the printable flashcards for this physics tutorial course at: