| Half life | Average life |
| (i) The time required for half of the radioactive substance to decay is called as half life of that substance. (ii) It is denoted by $$t _{\frac{1}{2}}$$ (iii) $$t_{\frac{1}{2}} = $$ half life $$= \left(\dfrac{\ln 2}{\lambda} \right)$$ where $$\lambda$$ is decay constant. | (i) The expected life for the radioactive substance within a sample is called as average life of that substance. (ii) It is denoted by $$\tau$$. (iii) $$\tau =$$ average life $$= \dfrac{1}{\lambda}$$ |