Question

The half-life of a radioactive substance is 40 years. How long will it take to reduce to one fourth of its original amount and what is the value of decay constant.

• A. 40 year, 0.9173/year
• B. 90 year, 9.017/year
• C. 80 year, 0.0173 year
• D. None of these

Answer 1

Answer: (C)

80 year, 0.0173 year

Answer 2

To reduce one fourth it takes time $t = 2\left( T_{1/2} \right)$

= 2 × 40 = 80 years.

Decay constant $\lambda = \frac{0.693}{T_{1/2}} = \frac{0.693}{40} = 0.0173years$