Define the $SI$ unit of radioactivity.

$A$ radioactive substance has a half-life of $1$ year. The fraction of this material that would remain after $5$ years will be

The graph between the instantaneous concentration $(N)$ of a radioactive element and time $(t)$ is:

If the decay or disintegration constant of a radioactive substance is $\lambda$,then its half-life and mean life are respectively:

Two radioactive elements $A$ and $B$ initially have the same number of atoms. The half-life of $A$ is equal to the mean life of $B$. If $\lambda_A$ and $\lambda_B$ are the decay constants of $A$ and $B$ respectively,then choose the correct relation from the given options.

The activity of a radioactive element decreases to one-third of its original activity $R_0$ in $9$ years. After a further $9$ years,its activity will be