The activity of a radioactive substance can be represented by various units. Select the correct option.

The number of undecayed nuclei $N$ in a sample of radioactive material as a function of time $t$ is shown in the figure. Which of the following graphs correctly shows the relationship between $N$ and the activity $A$?

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

Explain the decay constant and write down its definition.

For a substance,the average life for $\alpha$-emission is $1620 \ years$ and for $\beta$-emission is $405 \ years$. After how much time will $\frac{1}{4}$ of the material remain due to simultaneous emission?

In a radioactive disintegration,the ratio of the initial number of atoms to the number of atoms present at an instant of time equal to its mean life is: