The activity of a radioactive sample falls from $700 \;\mathrm{s}^{-1}$ to $500\; \mathrm{s}^{-1}$ in $30\;min$. Its half life is close to.........$min$
$66$
$52$
$72$
$62$
A radioactive substance decays to $\left(\frac{1}{16}\right)^{t h}$ of its initial activity in $80\, days.$ The half life of the radioactive substance expressed in days is ... .
The decay constant of a radio active substance is $0.173\, (years)^{-1}.$ Therefore :
A radioactive isotope $X$ with a half-life of $1.37 \times {10^9}$ years decays to $Y$ which is stable. A sample of rock from the moon was found to contain both the elements $X$ and $Y$ which were in the ratio of $1 : 7$. The age of the rock is
Explain decay constant and write down its definition.
$N$ atoms of a radioactive element emit $n$ number of $\alpha$-particles per second. Mean life of the element in seconds, is