SAT Subject Physics Practice Question 515: Answer and Explanation
1. A radioactive substance is observed to have a count rate of 500 counts/sec. Two hours later the count rate is 62.5 counts/sec. What is the half life of the substance?
A. 10 min
B. 20 min
C. 30 min
D. 40 min
E. 50 min
Correct Answer: D
The correct answer is (D). Take the count rate of 62.5 counts/ sec and divide by 500 counts/sec. The result is the decimal number of .125, which is (1/2)3. The 3 half-lives occur during the two-hour period. We have 120 min ÷ 3 = 40 min per half-life.