Question

The counting rate observed from a radioactive source at t = 0 second was 1600 counts per second and at t = 8 seconds it was 100 counts 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

By using $A = A_{0}\left( \frac{1}{2} \right)^{n}$ ⇒ $100 = 1600\left( \frac{1}{2} \right)^{8/T_{1/2}}$

⇒ $\frac{1}{16} = \left( \frac{1}{2} \right)^{8/T_{1/2}}$

⇒ $T_{1/2} = 2sec$

Again by using the same relation the count rate at t = 6 sec will be $A = 1600\left( \frac{1}{2} \right)^{6/2} = 200$.