1. 1
   A teacher shows a class a decay curve for a sample of a radioactive isotope, where the corrected count rate starts at 800 counts/minute and falls to 400 counts/minute after 10 days, and continues to be measured over several more weeks.

   (a)

   What term describes this time of 10 days?

   [1 mark]

   • AThe half-life of the isotope
   • BThe background radiation level
   • CThe proton number of the isotope
   • DThe corrected count rate

   (b)

   After a further 10 days (20 days in total from the start), what corrected count rate would you expect to measure for this sample?

   [1 mark]

   • A400 counts/minute
   • B200 counts/minute
   • C100 counts/minute
   • D0 counts/minute

   (c)

   State what is meant by the half-life of a radioactive isotope, and state the half-life of the isotope shown in this decay curve.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A hospital pharmacy stores a sample of technetium-99m, a radioactive isotope used in medical imaging, with a half-life of 6 hours, and monitors how its activity decreases over time before it is used in a patient scan.

   (a)

   If the sample has an initial activity of 640 MBq (megabecquerels), what will its activity be after 12 hours?

   [1 mark]

   • A80 MBq
   • B320 MBq
   • C160 MBq
   • D640 MBq

   (b)

   The pharmacy needs the sample's activity to have fallen to one eighth of its original value before it is disposed of as low-level waste. How many half-lives must have passed for the activity to fall to one eighth of its original value?

   [1 mark]

   • A4 half-lives
   • B8 half-lives
   • C2 half-lives
   • D3 half-lives

   (c)

   Calculate how many hours must pass before the sample's activity has fallen to one eighth of its original value, and explain why technetium-99m's relatively short half-life makes it suitable for use in patients, in terms of the radiation dose received.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An archaeologist finds an ancient wooden artefact and uses the radioactive decay of carbon-14 to estimate its age, comparing the activity remaining in the artefact with that expected in freshly cut wood of a similar size.

   (a)

   An archaeologist finds a wooden artefact and measures the activity of carbon-14 remaining in a sample of the wood, comparing it with the activity expected in a similar-sized sample of freshly cut wood. The measured activity in the artefact is one quarter of the activity in the fresh wood sample, and the half-life of carbon-14 is 5730 years. Calculate the estimated age of the artefact, showing your working, and explain, in terms of half-life, how this method of dating works.

   [3 marks]

   (b)

   A second, smaller wooden artefact is found nearby, and its measured carbon-14 activity is found to be 30% of the activity expected in an equivalent sample of fresh wood. Explain why it is not possible to give an exact whole number of half-lives for this artefact in the way that was possible for the first artefact, and describe, in general terms, how the age of this second artefact could still be estimated using a graph of activity against time.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A nuclear safety inspector is assessing a facility that stores several different sealed radioactive sources, each with a different half-life, ranging from a few hours to many thousands of years.

   (a)

   Explain what is meant by half-life, and explain, using the ideas of half-life and the random and spontaneous nature of radioactive decay, why it is impossible to predict when a particular unstable nucleus in a sample will decay, yet it is possible to state a definite half-life for a large sample of that isotope with confidence.

   [6 marks]

   (b)

   The inspector is reviewing storage plans for two of the facility's sources: source X, with a half-life of 8 hours and an initial activity of 2000 MBq, and source Y, with a half-life of 30 years and an initial activity of 500 MBq. The facility rule states that a source may be moved to lower-security general storage once its activity has fallen to below 5% of its initial value. Explain how the inspector could estimate the number of half-lives, and hence the approximate time, needed for each source to meet this rule, and discuss why source X and source Y require very different storage strategies despite this shared rule.

   [6 marks]

   Total for question 4: 12 marks

End of questions