Antiparticles, photons and annihilationAQA A-Level Physics: Subtopic test

1. 1
   A positron emission tomography (PET) scanner uses a tracer that emits positrons inside a patient. Each positron quickly meets an electron in the tissue and the pair annihilate, producing two gamma-ray photons that are detected by a ring of sensors around the patient. The rest energy of an electron is 0.511 MeV.

   (a)

   Which row correctly compares a positron with an electron?

   [1 mark]

   • ASame mass, same charge
   • BSame mass, opposite charge
   • CSmaller mass, opposite charge
   • DGreater mass, same charge

   (b)

   A positron and an electron, both essentially at rest, annihilate to produce two photons of equal energy. What is the energy of each photon?

   [1 mark]

   • A0.256 MeV
   • B0.722 MeV
   • C0.511 MeV
   • D1.022 MeV

   (c)

   Explain why the annihilation of an electron and a positron at rest produces two photons rather than one, and why the photons travel in opposite directions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a bubble chamber experiment, a gamma-ray photon of energy 1.50 MeV travels through liquid hydrogen. Close to a nucleus it disappears and a pair of particles is produced. The rest energy of an electron is 0.511 MeV.

   (a)

   Which pair of particles could be produced?

   [1 mark]

   • AAn electron and a positron
   • BA proton and an antiproton
   • CTwo electrons
   • DA positron and an antiproton

   (b)

   Assuming that the pair produced is an electron and a positron, what is their total kinetic energy?

   [1 mark]

   • A0.989 MeV
   • B0.239 MeV
   • C1.50 MeV
   • D0.478 MeV

   (c)

   Calculate the longest wavelength of photon that could produce an electron–positron pair. Give your answer in metres.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Physicists at a research laboratory study the production and annihilation of proton–antiproton pairs. The rest energy of a proton is 938 MeV, and an antiproton has the same rest energy.

   (a)

   Calculate the minimum frequency of a photon that can produce a proton–antiproton pair.

   [3 marks]

   (b)

   A proton and an antiproton, each with kinetic energy 120 MeV, collide and annihilate to produce two photons of equal energy. Calculate the frequency of each photon.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A gamma-ray observatory detects photons of energy 0.511 MeV coming from the centre of the galaxy, which astronomers interpret as evidence of electron–positron annihilation. In the laboratory the reverse process is also observed, in which a gamma-ray photon of sufficient energy disappears and produces a pair of particles. The rest energy of an electron is 0.511 MeV and the rest energy of a proton is 938 MeV.

   (a)

   Explain how the detection of photons of energy 0.511 MeV is evidence for the annihilation of electrons and positrons. Your answer should include the origin of this photon energy, why at least two photons are produced and the frequency of each photon.

   [6 marks]

   (b)

   Compare pair production with annihilation. A photon of energy 3.00 MeV passes through matter. Evaluate whether it could produce an electron–positron pair and whether it could produce a proton–antiproton pair.

   [6 marks]

   Total for question 4: 12 marks

End of questions