Give the different units of radioactivity and define them.

(N/A) The $SI$ unit for activity is Becquerel $(Bq)$,named after the discoverer of radioactivity,Henry Becquerel.
$(i)$ Becquerel $(Bq)$: The activity of a substance having $1$ disintegration per second is called $1$ Becquerel $(Bq)$. $\therefore 1 \text{ } Bq = 1 \text{ decay/s}$.
$(ii)$ Curie $(Ci)$: The activity of a substance in which $3.7 \times 10^{10}$ disintegrations per second take place is called $1$ curie $(Ci)$. $\therefore 1 \text{ } Ci = 3.7 \times 10^{10} \text{ decay/s}$. In practice,its smaller units are used: $1 \text{ } mCi = 3.7 \times 10^{7} \text{ decay/s} = 10^{-3} \text{ } Ci$ and $1 \text{ } \mu Ci = 3.7 \times 10^{4} \text{ decay/s} = 10^{-6} \text{ } Ci$. Curie is the older experimental unit.
$(iii)$ Rutherford $(rd)$: It is defined as the activity of a quantity of radioactive substance in which $10^{6}$ (one million) nuclei decay per second. $\therefore 1 \text{ } rd = 10^{6} \text{ decay/s}$.