Which of the following is not a mode of radioactive decay

Two radioactive materials $A$ and $B$ have decay constants $10\,\lambda $ and $\lambda $, respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of a to that of $B$ will be $1/e$ after a time

Two radioactive nuclei $A$ and $B$ both convert into a stable nucleus $C$. At time $t = 0$ nuclei of $A$ are $4N_0$ and that of $B$ are $N_0$. Half life of $A$ is $1\, min$ and that of $B$ is $2\, min$. initially number of nuclei of $C$ are zero. At what time rate of disintegrations of $A$ and $B$ are equal .......... $min$

A freshly prepared radioactive source of half life $2$ hours $30$ minutes emits radiation which is $64$ times the permissible safe level. The minimum time, after which it would be possible to work safely with source, will be hours.

The decay constant of radium is $4.28 \times {10^{ - 4}}$ per year. Its half life will be ..........$years$

In a radioactive disintegration, the ratio of initial number of atoms to the number of atoms present at an instant of time equal to its mean life is