Activity of Population or Specific Activity
It is the measure of radioactivity of a radioactive substance. It is defined as ' the number of radioactive nuclei, which decay per second per gram of radioactive isotope.' Mathematically, if 'm' is the mass of radioactive isotope, then
where N is the number of radioactive nuclei which undergoes disintegration.
Radioactive equilibrium: Suppose a radioactive element A disintegrates to form another radioactive element B which in turn disintegrates to still another element C.
B is said to be in radioactive equilibrium with A if its rate of formation from A is equal to its rate of decay into C.
It is important to note that the term equilibrium is used for reversible reactions but the radioactive reactions are irreversible, hence it is preferred to say that B is in a steady state rather than in equilibrium state.
At a steady state,
Thus at a steady state (at radioactive equilibrium), the amounts (number of atoms) of the different radioelements present in the reaction series are inversely proportional to their radioactive constants or directly proportional to their half-life and also average life periods.