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

$10\, gm$ of radioactive material of half-life $15$ year is kept in store for $20$ years. The disintegrated material is ............$gm$

Calculate the time (in $minutes$) interval between $33 \,\%$ decay and $67\, \%$ decay if half-life of a substance is $20\, minutes.$

During mean life of a radioactive element, the fraction that disintegrates is

A radioactive substance emits

The activity of a radioactive sample is measured as $9750$ counts per minute at $t = 0$ and as $975$ counts per minute at $t = 5$ minutes. The decay constant is approximately ............ per minute