Q. A freshly prepared radioactive source of half-life 2 h emits radiation of intensity which is 64 times the permissible safe level. Calculate the minimum time after which it would be possible to work safely with this source.
A) 12 h
B) 24 h
C) 6 h
D) 130 h
Explanation
(a) For working safely, the activity must reduce to $\frac{1}{64}$
$\ \ \ \ \ \ \ \ \ \ \ \frac{N}{N_{0}} = \left( \frac{1}{2} \right)^{n} = \frac{1}{64}$
$\therefore\ \ \ \ \ \ \ \ \ n = 6$
Thus, $t = nT = 6\ \times 2\ = \ 12h$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.