1. 1
   A physics teacher demonstrates radioactive decay using a Geiger-Muller tube to measure the count rate from a sealed sample of protactinium-234m, which has a very short half-life. The initial count rate above background is 800 counts per minute, and after 70 seconds it has fallen to 400 counts per minute.

   (a)

   What is the half-life of this isotope, based on this data?

   [1 mark]

   • A70 seconds
   • B35 seconds
   • C140 seconds
   • D400 seconds

   (b)

   Using the half-life found from the count rate falling from 800 to 400 counts per minute in 70 seconds, what would the count rate above background be after a further 70 seconds (140 seconds after the start)?

   [1 mark]

   • A0 counts per minute
   • B200 counts per minute
   • C300 counts per minute
   • D400 counts per minute

   (c)

   Explain what is meant by the half-life of a radioactive isotope, and explain why the teacher needs to first measure and subtract the background count rate before calculating the half-life from the Geiger-Muller tube readings.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A hospital stores a supply of iodine-131, used to treat thyroid conditions, which has a half-life of 8 days. A fresh batch arrives with an activity of 640 MBq.

   (a)

   What was the original activity of the batch 24 days before this delivery, assuming it came from the same original stock?

   [1 mark]

   • A1920 MBq
   • B1280 MBq
   • C320 MBq
   • D5120 MBq

   (b)

   Iodine-131 is described as having an activity of 640 MBq. What does the becquerel (Bq) measure?

   [1 mark]

   • AThe dose of radiation absorbed by a patient's tissue
   • BThe total energy released by the source per second
   • CThe number of nuclear decays occurring per second
   • DThe distance the radiation travels before being absorbed

   (c)

   After treatment, hospital staff must store used iodine-131 waste until its activity has fallen to a safe level before disposal. Explain, in terms of half-life, why waste containing an isotope with a short half-life like iodine-131 becomes safe to dispose of sooner than waste containing an isotope with a very long half-life.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An archaeologist analyses a wooden artefact and finds that the activity of carbon-14 remaining in the sample is one eighth of the activity expected in a similar piece of freshly cut wood. The half-life of carbon-14 is 5730 years.

   (a)

   Calculate how many half-lives have passed, and hence estimate the age of the artefact.

   [3 marks]

   (b)

   Explain why the archaeologist can only give an estimate of the artefact's age, rather than an exact value, even though the half-life of carbon-14 is known precisely. Refer to the random nature of radioactive decay in your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A nuclear power company must decide how to safely store spent fuel rods for the coming decades, and is considering two storage sites: one near a populated town and one in a remote desert region.

   (a)

   Explain the concept of half-life and discuss, using the idea of half-life, why some radioactive waste products must be stored for hundreds or thousands of years while others become safe within days or weeks.

   [6 marks]

   (b)

   The power company is considering two storage sites: one near a populated town and one in a remote desert region. Discuss, using ideas about activity, half-life and radiation hazards, the factors the company should evaluate when choosing where to store long-lived radioactive waste, and explain why perceived risk to the public might differ from the actual physical risk.

   [6 marks]

   Total for question 4: 12 marks

End of questions