MYP Physics sample question: Half-life and radioactive decay

A radioactive isotope has a half-life of 6 hours. A sample has an initial activity of 800 counts per minute.

1. (a)State what is meant by half-life.[1]
   Show the mark scheme for (a)

   • • The time taken for the activity (or the number of undecayed nuclei) to fall to half of its original value (1)
2. (b)Calculate the activity after 18 hours.[2]
   Show the mark scheme for (b)

   • • 18 hours is 3 half-lives (1)
   • • 800 → 400 → 200 → 100 counts per minute (1)
3. (c)Explain why an isotope with a half-life of 6 hours is more suitable as a medical tracer than one with a half-life of 6 years.[2]
   Show the mark scheme for (c)

   • • It decays quickly, so the patient is exposed to radiation for a short time (1)
   • • It does not stay radioactive in the body or the environment for years (1)

Count the number of half-lives first, then halve the activity that many times. A quick table of time and activity avoids slips.