• # question_answer A freshly prepared radioactive source of half- life 2 hours emits radiation of intensity which is 64 times the permissible safe level. The minimum time after which it would be possible to work safely with the source is: A) 6 hours             B) 12 hoursC) 24 hours                       D) 128 hours

$\frac{A}{{{A}_{0}}}=\frac{N}{{{N}_{0}}}$ Let safe-level activity be A. Initial activity = 64 A. Hence, $\frac{N}{{{N}_{0}}}=\frac{A}{{{A}_{0}}}=\frac{A}{64A}\,\,=\,\,\frac{1}{64}$ ${{\left( \frac{1}{2} \right)}^{n}}\,=\,\frac{1}{64}\,\,or\,\,n=6$ Hence, $\frac{t}{T}=n=6$ But $T\,\,=\,\,2$ hours,