Starting with a sample of pure ${}^{66}Cu$, $7/8$ of it decays into $Zn$ in $15 \ minutes$. The corresponding half-life is ................ $minutes$.

The decay constant of a radioactive element is $0.01 \ s^{-1}$. Its half-life period is ....... $s$.

The half-lives of two radioactive materials $A$ and $B$ are $T$ and $2T$ respectively. If the ratio of the initial masses of the materials $A$ and $B$ is $8:1$, then the time after which the ratio of the masses of the materials $A$ and $B$ becomes $4:1$ is

What mass in $\mu g$ of $Th^{227}$ is required to produce an activity of $1 \ mCi$ if its half-life is $1.9 \ \text{years}$?

The half-life period of a radioactive element $x$ is the same as the mean life time of another radioactive element $y$. Initially,they have the same number of atoms. Then:

The half-life period of a radioactive substance is $5\, min$. The amount of substance decayed in $20\, min$ will be..........$\%$