Question

The counting rate observed from a radioactive source at t = 0 seconds was 1600 counts per second and t = 8 seconds it was 100 count per second. The counting rate observed, as counts per second at t = 6 seconds will be :

• A. 400
• B. 300
• C. 200
• D. 150

Answer 1

Answer: (C)

200

Answer 2

After 8 seconds, the counting rate falls to (1 / 16)th = $\left( \frac{1}{2}\right)^4$th part. Hence time period is (8/4) s = 2s. Therefore after 6 second the counting rate should be $(1/2)^3$ th part of 1600. i.e. it should be $\frac{1}{8} \times$ 1600 = 200