Question

A freshly prepared radioactive source of half-life period 2 hour emits radiations of intensity which is 64 times of the permissible safe
level. The minimum time after which it would be possible to work with this source is:

• A. 6 hr
• B. 12 hr
• C. 24 hr
• D. 48 hr

Answer 1

The correct option is B 12 hr

$Permissiblelevel\left(N\right)=\frac{N_0}{64}=\frac{N_0}{2^6}$
$\therefore \frac{N_0}{N}=2^6$
$T=t_{1/2}\times n$
$\therefore T=2\times 6=12hr$