The half-life of $_{38}^{90}Sr$ is $20 \text{ years}$. If a sample having an initial activity of $8000 \text{ dis/min}$ is taken,what would be its activity after $80 \text{ years}$?

How many alpha particles are emitted per second by $1 \ \mu g$ of radium?

If $n_t$ number of radioatoms are present at time $t$,the following expression will be a constant:

The half-life of a radioactive disintegration $(A \to B)$ having a rate constant of $231 \ sec^{-1}$ is:

$A$ sample is obtained from sea water. $11.9$ $C^{14}$ atoms dissociate from $1 \ g$ of carbon. $15.3$ $C^{14}$ atoms dissociate from a live sample. How many years old is the sample? $(t_{1/2} = 5730 \ \text{years})$

If the half-life of an isotope $X$ is $10 \ yr$,its decay constant is (in $yr^{-1}$)