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Half-life is closely related to the property of Radioactive Decay and represents the time taken for half the Atoms in a Radioactive substances to undergo decay and change into another nuclear form (either a radioactive daughter product or a stable form). It is therefore the time taken for the Activity of a radioactive sample to decay by half and is commonly given the symbol t1/2.

Reality is merely an illusion, albeit a very persistent one

– Albert Einstein