The half-life of radium is $1600$ years. Its mean life will be ....... years.

State and derive the law of radioactive decay.

$A$ nucleus has a half-life of $30 \; min$. At $3 \; PM$, its decay rate was measured as $120000 \; cps$. What is the decay rate in $cps$ at $5 \; PM$?

The decay constant of a radioisotope is $\lambda$. If its activity at times $t_1$ and $t_2$ are $A_1$ and $A_2$ respectively,then the number of nuclei that have decayed during the time interval $(t_1 - t_2)$ is:

The mean life time of a radionuclide,if its activity decreases by $4\%$ for every $1 \ h$,would be .......... $h$ (product is non-radioactive,i.e.,stable).

The half-life of a radioactive substance is $10 \, \text{minutes}$. If $n_1$ and $n_2$ are the number of atoms decayed in $20 \, \text{minutes}$ and $30 \, \text{minutes}$ respectively, then $n_1 : n_2 =$