The unit for the radioactive decay constant is:

$t_{1/2}$ of $^{232}Th$ is $1.39 \times 10^{10} \ \text{years}$. Calculate the number of $\alpha$-particles emitted by $1.0 \ \text{g}$ of $^{232}Th$ per second.

The half-life period of a radioactive material is $15 \ minutes$. What $\%$ of radioactivity of that material will remain after $45 \ minutes$?

After $2 \, \text{hours}$,the amount of a certain radioactive substance reduces to $1/16^{th}$ of the original amount (the decay process follows first-order kinetics). The half-life of the radioactive substance is $...... \, \text{min}$.

$A$ freshly prepared radioactive source of half-life $2 \ hours$ emits radiations of intensity which is $64$ times the permissible safe level. The minimum time after which it would be possible to work safely with this source is ......... $hours$.

The half-life period of a radioactive substance is $140 \ days$. After how much time will $15 \ g$ decay from a $16 \ g$ sample of it?