The half-life of a radioactive nucleus is $5$ years. The fraction of the original sample that would decay in $15$ years is

After $1$ hour,$1/16$ of the radioactive substance remains. Its half-life is ....... minutes.

The half-life of $Ra^{226}$ is $1620$ years. Calculate the number of atoms that decay in one second in $1 \ g$ of radium (Avogadro number $= 6.023 \times 10^{23}$).

Write down the definition and formula of half-life of a radioactive substance.

For a substance,the average life for $\alpha$-emission is $1620$ years and for $\beta$-emission is $405$ years. After how much time will $1/4$ of the material remain after both $\alpha$ and $\beta$ emissions?

$A$ radioactive element having a half-life of $30 \ min$ is undergoing beta decay. The fraction of the radioactive element that remains undecayed after $90 \ min$ will be: