<P> The decay rate, or activity, of a radioactive substance is characterized by: </P> <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 inverse 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>

What process uses analysis of the decay of atomic particles to determine exact age