The mean life of a radioactive substance is

Concept:

• The half-life of a radioactive element (T1/2): The time interval in which the mass of a radioactive substance or the number of atoms reduced to half of its initial value.
• The expression for the half-life is

Where λ = is the decay rate constant

• Mean life (τ): The average lifetime of all the nuclei of a particular unstable atomic species is called mean life.

where τ is mean life, and λ is radioactive decay constant

• The relation between the half-life and mean life is given by

⇒ T1/2 = 0.693 × τ 

Explanation:

The mean life (τ ) of a radioactive substance is reciprocal of disintegration constant λ.