Question

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$s^{- 1}$

Answer 1

Answer: (B)

80$s^{- 1}$

Answer 2

$A = A_{0}\left( \frac{1}{2} \right)^{t/T_{1/2}}$⇒$5 = A_{0}\left( \frac{1}{2} \right)^{\frac{2 \times 60}{30}} = \frac{A_{0}}{16}$⇒$A_{0} = 80sec^{- 1}$