In a $RA$ element the fraction of initiated amount remaining after its mean life time is 

Radioactive substances do not emit

Radioactivity was discovered by

Activity of radioactive element decreased to one third of original activity ${R_0}$ in $9$ years. After further $9$ years, its activity will be

Two radioactive nuclei $P$ and $Q$ in a given sample decay into a stable nucleus $R$. At time $t = 0$, number of $P$ species are $4\, N_0$ and that of $Q$ are $N_0$. Half-life of $P$ (for conversion to $R$) is $1$ minute where as that of $Q$ is $2$ minutes. Initially  there are no nuclei of $R$ present in the sample. When number of nuclei of $P$ and $Q$ are equal, the number of nuclei of $R$ present in the sample would be

Two radioactive materials $A$ and $B$ have decay constant $5\lambda$ and $\lambda$ respectively.At $t=0$  they have the same number of nuclei, then the ratio of the number of nuclei of $A$ to that $B$ will be $(1/e)^2$ after a time interval