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The count rate of a Geiger Muller counter for the radiation of a radioactive material of half-life 30 minutes decreases to 5 sec–1 after 2 hours. The initial count rate was
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- 20 sec–1
- 25 sec–1
- 80 sec–1
- 625 sec–1
Correct Option: C
Half-life = 30 minutes; Rate of decrease (N) = 5 per second and total time = 2 hours = 120 minutes. Relation for initial and final count rate
| = |  |  | time/half-life | |||
| N0 | 2 |
| = | | | 120/30 | 2 |
| = | | | 4 | = | |||
| 2 | 16 |
Therefore, N0 = 16 × N = 16 × 5 = 80 s–1.
