Question

From a newly formed radioactive substance (Half-life 2 hours), the intensity of radiation is 64 times the permissible safe level. The minimum time after which work can be done safely from this source is

• A. 6 hours
• B. 12 hours
• C. 24 hours
• D. 128 hours

Answer 1

Answer: (B)

12 hours

Answer 2

By using $A = A_{0}\left( \frac{1}{2} \right)^{n}$ ⇒ $\frac{A}{A_{0}} = \frac{1}{64} = \left( \frac{1}{2} \right)^{0} = \left( \frac{1}{2} \right)^{n}$

⇒ n = 6

⇒ $\frac{t}{T_{1/2}} = 6$ ⇒ $t = 6 \times 2 = 12$ hours.