1. 1
   A smoke detector installed in a kitchen contains a small sealed source of americium-241, which emits alpha radiation to ionise the air inside a detection chamber. This forms part of the sensor circuit that triggers the alarm if smoke enters the chamber and disrupts the ionisation current.

   (a)

   Which statement correctly describes an alpha particle?

   [1 mark]

   • ATwo protons and two neutrons, with a strong ability to ionise but low penetrating power
   • BA high-energy electromagnetic wave that easily penetrates several centimetres of lead
   • CA fast-moving electron emitted from the nucleus
   • DA single uncharged particle found in the nucleus

   (b)

   Why is an alpha source safe to have sealed inside a smoke detector in normal use, even though alpha radiation is strongly ionising?

   [1 mark]

   • AAlpha radiation has no ionising effect at all
   • BAlpha radiation cannot travel more than a few centimetres through air and cannot penetrate the detector's casing
   • CAlpha sources do not emit any radiation unless disturbed
   • DAlpha radiation is not a form of ionising radiation

   (c)

   State two properties of gamma radiation that make it more penetrating than alpha radiation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A food processing company uses a gamma irradiation facility to sterilise packets of dried herbs and spices before export, killing bacteria without heating the product or opening the sealed packaging.

   (a)

   Why is gamma radiation, rather than alpha or beta radiation, used to sterilise the sealed packets of spices?

   [1 mark]

   • AGamma radiation has the shortest range of the three types, so it stays inside the packet
   • BGamma radiation is not a form of ionising radiation, so it cannot harm bacteria
   • CGamma radiation can penetrate through the packaging to reach the spices inside
   • DGamma radiation only works on materials that conduct electricity

   (b)

   Which of the following best explains why workers at the irradiation facility are not present in the same room as the gamma source during operation?

   [1 mark]

   • AGamma radiation would make the workers' clothing radioactive permanently
   • BThe gamma source needs complete darkness to work correctly
   • CGamma radiation is too weak to sterilise anything if people are present
   • DProlonged exposure to ionising radiation can damage living cells and cause mutations

   (c)

   Explain the difference between irradiation and contamination in the context of this process.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A school science club builds a simple demonstration to explain nuclear fission to younger students. In the model, a marble rolled towards a cluster of dominoes represents a neutron striking a nucleus of uranium-235, causing it to split and release further marbles that go on to strike other clusters of dominoes.

   (a)

   Using the ideas represented by the model, explain what happens when a nucleus of uranium-235 undergoes fission after being struck by a neutron, and explain how a chain reaction can be set up as a result.

   [3 marks]

   (b)

   In a real nuclear reactor, the process must be carefully controlled rather than allowed to run away as an uncontrolled chain reaction. Describe the roles of the control rods and the moderator in achieving this.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An industrial radiographer uses a small, sealed gamma source to check for hidden cracks in welded joints on a new gas pipeline. The source is placed on one side of the weld and a sheet of photographic film is placed on the other side; radiation that passes through the metal exposes the film, and any crack shows up as a darker area because more radiation reaches the film through the thinner, damaged metal.

   (a)

   Explain, in terms of the properties of gamma radiation, why gamma sources (rather than alpha or beta sources) are suitable for this radiography technique, and explain why the photographic film shows a darker area where there is a crack in the weld.

   [6 marks]

   (b)

   Explain why the radiographer must take precautions when using this sealed gamma source, and describe two precautions that could be used to reduce the risk to the radiographer and to other workers nearby.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A mountain research station operates a cloud chamber to detect ionising radiation from cosmic rays and from a small calibration source. When a particle of radiation passes through the chamber, it leaves a visible trail of condensed vapour droplets along its path.

   (a)

   A short, thick trail is observed in the cloud chamber, caused by a particle that strongly ionises the vapour along a very short path. Which type of radiation most likely produced this trail?

   [1 mark]

   • ABeta radiation
   • BGamma radiation
   • CAlpha radiation
   • DAn X-ray

   (b)

   A different trail is long, thin and faint, showing only weak ionisation along a much longer path. Which type of radiation most likely produced this trail?

   [1 mark]

   • AAlpha radiation
   • BBeta radiation
   • CA stationary particle
   • DNone of the above, as only alpha radiation can be detected in a cloud chamber

   (c)

   A nucleus of carbon-14 decays by emitting a beta particle to form a nucleus of nitrogen. State the mass number and atomic number of the nitrogen nucleus formed.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   An auction house asks a laboratory to check whether a wooden statue offered for sale is genuinely several thousand years old, using radiocarbon (carbon-14) dating on a small sample of the wood.

   (a)

   Why does the activity of the carbon-14 in the wood sample decrease over the thousands of years since the tree was cut down?

   [1 mark]

   • AThe number of undecayed carbon-14 nuclei in the sample decreases over time as they randomly decay
   • BCarbon-14 nuclei become less radioactive as they age without actually decaying
   • CThe wood loses carbon-14 nuclei only if it is physically damaged
   • DThe activity is constant and does not change over time

   (b)

   What is meant by the half-life of carbon-14?

   [1 mark]

   • AThe total time it takes for all the carbon-14 in a sample to decay completely
   • BThe time taken for a sample to become non-radioactive
   • CThe age of the wood when it was first measured
   • DThe time taken for the activity of a carbon-14 sample (or the number of undecayed nuclei) to fall to half its original value

   (c)

   The laboratory measures the activity of the carbon-14 in the sample and compares it with the activity expected in a similar sample of freshly cut wood. State what is measured and what this comparison, combined with the known half-life of carbon-14, allows the laboratory to estimate.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A physics teacher uses a classroom demonstration with two small models representing hydrogen nuclei to help students understand nuclear fusion. The teacher explains that fusing two light nuclei together to form a heavier nucleus, releasing energy, can only happen if the nuclei are brought close enough together, which requires extremely high temperatures and pressures.

   (a)

   Explain why nuclear fusion between two hydrogen nuclei does not happen under normal, everyday conditions of temperature and pressure.

   [3 marks]

   (b)

   Explain why extremely high temperatures and pressures make nuclear fusion possible, and describe what happens to the total mass of the nuclei involved when fusion takes place.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A deep-space probe, travelling too far from the Sun for solar panels to provide enough power, is instead powered by a radioisotope thermoelectric generator (RTG). Inside the RTG, a sealed pellet of a radioactive isotope steadily decays, and the heat released by this decay is converted into electricity to power the probe's instruments.

   (a)

   Explain, in terms of radioactive decay, how the pellet inside the RTG is able to continuously produce heat over many years without needing to be refuelled, and explain why the power output of the RTG is expected to gradually decrease over the probe's long mission.

   [6 marks]

   (b)

   Engineers considered whether a small nuclear fission reactor, rather than an RTG, could be used to power a future probe. Explain what would be needed to sustain a controlled chain reaction in such a reactor, and explain why simply carrying a lump of fissile material would be dangerous without these controls.

   [6 marks]

   Total for question 8: 12 marks

End of questions