The count rate of radioactive material is 800 count/min. If the half-life of the material is 4 days, what would the count rate be in 16 days later?

A. 200 count / min
B. 100 count / min
C. 50 count / min
D. 25 count / min

Correct Answer:

Option C – 50 count / min

Explanation

116 days = 4 half-lives
Thus, after 4 half lives the fraction left of any radioactive materials is given by (¹/₂)⁴ = ¹/₁₆
∴ Count rate / amount left = ¹/₁₆ x ⁸⁰⁰/₁
= 50 count / min