The activity of a radioactive material is $6.4 \times 10^{-4}$ curie. Its half life is $5\; days$. The activity will become $5 \times 10^{-6}$ curie after $.......day$
$7$
$15$
$25$
$35$
The half life period of a radioactive substance is 60 days. The time taken for $\frac{7}{8}$ th of its original mass to disintegrate will be $......days$
Decay constant of radium is $\lambda $. By a suitable process its compound radium bromide is obtained. The decay constant of radium bromide will be
$A$ and $B$ are two radioactive substances whose half lives are $1$ and $2$ years respectively. Initially $10\, g$ of $A$ and $1\,g$ of $B$ is taken. The time (approximate) after which they will have same quantity remaining is ........... $years$
A radioactive material has a half life of $10$ days. What fraction of the material would remain after $30$ days
$N$ atoms of a radioactive element emit $n$ alpha particles per second. The half life of the element is