Question

The half-life of $215At$ is 100 μs. The time taken for the radioactivity of a sample of $215At$ to decay to 1/16^th of its initial value is

• A. 400 μs
• B. 6.3 μs
• C. 40 μs
• D. 300 μs

Answer 1

Answer: (A)

400 μs

Answer 2

By using $N = N_{0}\left( \frac{1}{2} \right)^{n}$ ⇒ $\frac{N}{N_{0}} = \left( \frac{1}{2} \right)^{t/T_{1/2}}$

⇒ $\frac{1}{16} = \left( \frac{1}{2} \right)^{t/100}$ ⇒ t = 400 μ sec.