The SI unit for activity is Becquerel $(\mathrm{B} q)$ named after the discoverer of radioactivity,

Henry Becquerel.

$(i)$ Activity of a substance having 1 disintegration per second is called 1 Becquerel (B $q$ ). $\therefore 1 \mathrm{~B} q=1$ decay/s

$(ii)$ Curie unit : The activity of a substance in which $3.7 \times 10^{10}$ disintegration per second take place is called l curie $(\mathrm{C} i)$.

$\therefore 1 \mathrm{Ci}=3.7 \times 10^{10}$ decay/s

In practice its smaller units are used,

$1 \mathrm{mC} i=3.7 \times 10^{7}$ decay/s $=10^{-3} \mathrm{Ci}$

$1 \mu \mathrm{C} i=3.7 \times 10^{4}$ decay/s $=10^{-6} \mathrm{C} i$

Curie unit is the old experimental unit.

(iii) Rutherford unit : It is defined as the activity of a quantity of radioactive substance in which $10^{6}$ (ten lakh) nuclei decay per second is called Rutherford $(\mathrm{rd})$. $\therefore 1 \mathrm{rd}=10^{6}$ decay/s