$\lambda = \text{SI} (s^{-1}): \text{ Becquerel }$
$\lambda = \text{Ci}\rightarrow \text{ Curie }$
Decay Rate depends on the Population at that Instant
$-R \equiv \frac{\mathrm{dN}(\mathrm{t})}{\mathrm{dt}} =-\lambda N(t) ; \quad \lambda>0$
$R$ is also called activity.
Units of $R$ : Becquerel (SI);
Curie $(\mathrm{Ci})=3.7 \times 10^{10}$ Becquerel