Half-lifeIB MYP Physics: Subtopic test

1. 1
   A hospital in Singapore uses technetium-99m as a medical tracer. Its half-life is 6 hours. A sample is delivered at 08:00 with an activity of 400 MBq (megabecquerels).

   (a)

   What is meant by the half-life of a radioactive isotope?

   [1 mark]

   • AThe time taken for all of the nuclei in a sample to decay
   • BHalf of the time that a sample stays dangerous
   • CThe time taken for each nucleus to lose half of its mass
   • DThe time taken for the activity of a sample to fall to half of its original value

   (b)

   What is the activity of the sample at 20:00 on the same day?

   [1 mark]

   • A50 MBq
   • B100 MBq
   • C200 MBq
   • D300 MBq

   (c)

   Calculate the activity of the sample at 14:00 on the following day.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student measures the corrected count rate (background already subtracted) from a radioactive sample over 40 minutes. At 0 minutes the count rate is 160 counts per second. At 10 minutes it is 80 counts per second, at 20 minutes it is 40 counts per second, at 30 minutes it is 20 counts per second and at 40 minutes it is 10 counts per second.

   (a)

   What is the half-life of the sample?

   [1 mark]

   • A5 minutes
   • B20 minutes
   • C10 minutes
   • D40 minutes

   (b)

   What count rate would you expect at 60 minutes?

   [1 mark]

   • A2.5 counts per second
   • B5 counts per second
   • C0 counts per second
   • D1.25 counts per second

   (c)

   Describe the pattern shown by the count rate, using the data to support your answer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A company is choosing a radioactive isotope for two jobs: as a medical tracer injected into a patient's blood, and as the radiation source in a smoke alarm. Isotope P has a half-life of 6 hours and emits gamma rays. Isotope R has a half-life of 432 years and emits alpha particles.

   (a)

   Identify which isotope should be used as the medical tracer and explain your choice.

   [3 marks]

   (b)

   Explain why isotope R is suitable for a smoke alarm but not as a medical tracer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Archaeologists in Peru find a wooden tool. A living tree contains a fixed small fraction of radioactive carbon-14, which has a half-life of 5730 years. When a tree is cut down it takes in no more carbon-14, and the carbon-14 already in the wood decays. The wood in the tool contains one quarter of the fraction of carbon-14 found in living trees.

   (a)

   Calculate the age of the wooden tool and explain how the method works.

   [6 marks]

   (b)

   Discuss why carbon-14 is suitable for dating a wooden tool that is about 11 000 years old but not a dinosaur bone that is 70 million years old, and evaluate the benefit of carbon dating to society.

   [6 marks]

   Total for question 4: 12 marks

End of questions