1. 1
   A domestic smoke detector contains a small americium-241 source that ionises air between two electrodes so that a tiny current flows. If smoke enters the detector, it disrupts the ionisation and reduces the current, triggering the alarm. Americium-241 (atomic number 95, mass number 241) decays by emitting alpha particles.

   (a)

   What causes a nucleus of americium-241 to decay?

   [1 mark]

   • AThe nucleus is unstable and decay is a random process
   • BThe nucleus is heated above a critical temperature by the surrounding air
   • CAn engineer triggers the decay electrically when the detector is switched on
   • DThe nucleus absorbs light from the room and this forces it to split

   (b)

   Americium-241 is chosen for this smoke detector because it emits alpha radiation. Why is an alpha source suitable for use inside a sealed smoke detector, rather than a gamma source?

   [1 mark]

   • AAlpha radiation has no mass, so it cannot be absorbed by the plastic casing
   • BAlpha radiation travels further than gamma radiation, so it can detect smoke from another room
   • CAlpha radiation is not ionising, so it will not react with smoke particles
   • DAlpha radiation has a short range in air and is easily absorbed, so it stays inside the detector and does not pass through the casing

   (c)

   Americium-241 (atomic number 95, mass number 241) decays by emitting an alpha particle to form neptunium. State the mass number and atomic number of the neptunium nucleus formed, and explain how you worked out each value.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A hospital radiographer uses technetium-99m, a gamma-emitting isotope, as a tracer to image a patient's kidneys. The tracer is injected into the bloodstream and a gamma camera outside the body detects the emitted radiation.

   (a)

   Which type of nuclear radiation is technetium-99m emitting?

   [1 mark]

   • ABeta particles
   • BAlpha particles
   • CGamma rays
   • DNeutrons

   (b)

   Which property of gamma radiation makes it suitable for this medical tracer, allowing radiation from inside the body to be detected outside the body?

   [1 mark]

   • AGamma radiation has a very short range of only a few centimetres in air
   • BGamma radiation is highly penetrating and can pass through body tissue
   • CGamma radiation is strongly ionising and is stopped by a sheet of paper
   • DGamma radiation is a stream of helium nuclei

   (c)

   Technetium-99m eventually decays to technetium-99 by emitting a gamma ray. Explain why the emission of a gamma ray does not change the mass number or atomic number of the nucleus, and state what does change instead.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Following an accident at a nuclear power plant, inspectors found that soil in the surrounding fields contained caesium-137, a beta-emitting isotope, but the soil itself had not become part of the nuclear reaction.

   (a)

   Explain the difference between contamination and irradiation, and state which one describes the soil's situation.

   [3 marks]

   (b)

   Caesium-137 decays by beta emission. Write a word description of what happens inside the nucleus during beta decay, and explain the effect on the mass number and atomic number of the nucleus.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A museum wants to authenticate an artefact believed to date from ancient Egypt by measuring the proportion of a radioactive isotope remaining in an organic sample found with it. The conservation team must handle the sample safely, as it contains trace amounts of naturally occurring radioactive isotopes, and there is also general background radiation in the storage room.

   (a)

   Explain what is meant by radioactive decay being a random process, and discuss why this randomness does not prevent scientists from predicting how the activity of a large sample of atoms changes over time.

   [6 marks]

   (b)

   The museum's conservation team must handle the ancient organic sample safely, as trace amounts of naturally occurring radioactive isotopes are present in it, alongside general background radiation in the storage room. Explain what background radiation is, identify its sources, and discuss why suitable precautions are needed when handling any radioactive material regardless of its origin.

   [6 marks]

   Total for question 4: 12 marks

End of questions