Q. The count rate of a Geiger-Muller counter for the radiation of a radioactive material of half life of $30\ minutes$ decreases to $5\ s^{- 1}$ after 2 hours. The initial count rate was
A) $25\ s^{- 1}$
B) $80\ s^{- 1}$
C) $625\ s^{- 1}$
D) $20\ s^{- 1}$
Explanation
(b) $A = A_{0}\left( \frac{1}{2} \right)^{t/T_{1/2}} \Rightarrow 5 = A_{0}\left( \frac{1}{2} \right)^{\frac{2 \times 60}{30}} = \frac{A_{0}}{16} \Rightarrow A_{0} = 80s^{- 1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.