The activity of a freshly prepared radioactive sample is $10^{10}$ disintegrations per second, whose mean life is $10^9 s$. The mass of an atom of this radioisotope is $10^{-25} kg$. The mass (in $mg$ ) of the radioactive sample is

The half-life of radon is $3.8\, days$. Three forth of a radon sample decay in  …………$days$

The decay constant of a radioactive element is $0.01$ per second. Its half life period is …….$sec$

The half-life of $Au ^{198}$ is $2.7\, days.$ The activity of $1.50\, mg$ of $Au ^{198}$ if its atomic weight is $198\, g mol ^{-1}$ is …….$Ci$ $\left( N _{ A }=6 \times 10^{23}\, / mol \right)$

The half life of $^{131}I$ is $8\, days$. Given a sample of $^{131}I$ at time $t = 0,$ we can assert that