The decay constant of a radioactive element is $1.5 \times {10^{ - 9}}$ per second. Its mean life in seconds will be

In a radioactive material, fraction of active material remaining after time $t$ is $\frac{9}{16}$ The fraction that was remaining after $\frac{t}{2}$ is 

In the uranium radioactive series, the initial nucleus is $_{92}{U^{238}}$ and the final nucleus is $_{82}P{b^{206}}$. When the uranium nucleus decays to lead, the number of $\alpha - $ particles emitted will be

At $t = 0$, number of active nuclei in a sample is $N_0$. How much no. of nuclei will decay in time between its first mean life and second half life?

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

A radioactive sample $\mathrm{S} 1$ having an activity $5 \mu \mathrm{Ci}$ has twice the number of nuclei as another sample $\mathrm{S} 2$ which has an activity of $10 \mu \mathrm{Ci}$. The half lives of $\mathrm{S} 1$ and $\mathrm{S} 2$ can be