Question

From a newly formed radioactive substance (Half-life 2 hours), the intensity of radiation is 64 times the permissible safe level. The minimum time after which work can be done safely from this source is

• A. 6 hours
• B. 12 hours
• C. 24 hours
• D. 128 hours

Answer 1

The correct option is B 12 hours

By using $A=A_0{\left(\frac{1}{2}\right)}^n\Rightarrow \frac{A}{A_0}=\frac{1}{64}={\left(\frac{1}{2}\right)}^6={\left(\frac{1}{2}\right)}^n\Rightarrow n=6$
$\Rightarrow \frac{t}{T_{\frac{1}{2}}}=6\Rightarrow t=6\times 2=12hours$.