Radiation Safety Practice Exam

How much of a parent isotope remains after two half-lives of decay?

1. 0.50 or 1/2

2. 0.75 or 3/4

3. 0.25 or 1/4

4. 0.10 or 1/10

The correct answer is: 0.25 or 1/4

After two half-lives of decay, one can determine the remaining amount of a parent isotope by understanding the concept of half-lives. A half-life is the time required for half of the parent isotope in a sample to decay into its daughter isotope. Initially, you start with 100% of the parent isotope. After one half-life, 50% remains, as half of it has decayed. After a second half-life, that remaining 50% is again reduced by half. Therefore, half of the 50% that was left after the first half-life decays as well, resulting in 25% of the original parent isotope remaining. This means that after two half-lives, 25% of the original parent isotope is left, which is represented as 0.25 or 1/4 of the initial amount. Understanding the decay process and how the amounts change over each half-life is crucial in radiation safety and helps in calculating the remaining isotopic composition after specific periods.