<P> Constant quantities: </P> <Ul> <Li> The half - life--t, is the time taken for the activity of a given amount of a radioactive substance to decay to half of its initial value; see List of nuclides . </Li> <Li> The decay constant--λ, "lambda" the reciprocal of the mean lifetime, sometimes referred to as simply decay rate . </Li> <Li> The mean lifetime--τ, "tau" the average lifetime (1 / e life) of a radioactive particle before decay . </Li> </Ul> <Li> The half - life--t, is the time taken for the activity of a given amount of a radioactive substance to decay to half of its initial value; see List of nuclides . </Li> <Li> The decay constant--λ, "lambda" the reciprocal of the mean lifetime, sometimes referred to as simply decay rate . </Li>

What is the term for the amount of time it takes 50 of an element to decay