The decay of a radioactive substance obeys the rules of first order reaction.
In first order reaction half life time (t1/2) is constant and does not depend on the initial concentration of the substance (t1/2 = 0.693/k) where k is the rate constant.
Half-life time (t1/2) is the time needed for the reaction that the initial concentration of the reactant (substance) reaches its half (reaction be completed by 50 %).
