The mean lives of a radioactive sample are $30$ years and $60$ years for $\alpha$-emission and $\beta $ -emission respectively. If the sample decays both by $\alpha$- emission and $\beta $-emission simultaneously, the time after which, only one-fourth of the sample remain is :- ........... $years$
$10$
$20$
$40$
$45$
If a radioactive material remains $25 \%$ after $16$ days, then its half life will be ......... days
What is radioactivity ?
Half life of radium is $1620$ years. How many radium nuclei decay in $5$ hours in $5\, gm$ radium? ( Atomic weight of radium $= 223$)
Write the definition of half life of radioactive substance and obtain its relation to decay constant.
Which sample contains greater number of nuclei : a $5.00- \mu Ci$ sample of $_{240}Pu$ (half-life $6560\,y$) or a $4.45 - \mu Ci$ sample of $_{243}Am$ (half-life $7370\, y$)