OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) creation and annihilation of particle–antiparticle pairs
Full specification wording
Nuclear fission and fusion: (c) creation and annihilation of particle–antiparticle pairs
Pair creation is the conversion of energy into a particle and its antiparticle, for example a photon producing an electron and a positron.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 18
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Pair creation is the conversion of energy into a particle and its antiparticle, for example a photon producing an electron and a positron. It can occur only when the photon energy is at least the total rest energy of the pair, 2m₀c², and in the presence of a nucleus to conserve momentum. Pair annihilation is the reverse: a particle and its antiparticle combine and their mass is converted into photons, commonly two gamma-ray photons each of energy m₀c² emitted in opposite directions to conserve momentum. For an electron–positron pair the minimum photon energy is about 1.02 MeV. Charge, lepton number and momentum are conserved in both processes.
What earns the marks
- Define pair creation as energy converting into a particle and its antiparticle, and annihilation as a particle and antiparticle converting into photons.
- State that the minimum photon energy for creation of a pair of rest mass m₀ each is 2m₀c².
- Explain that a nearby nucleus or other body is needed in pair creation to conserve momentum.
- State that annihilation typically produces two photons of energy m₀c² each, emitted in opposite directions to conserve momentum.
- Check conservation of charge, lepton number and momentum in a given particle reaction.
Common mistakes and how to put them right
Thinking a single photon can create a particle–antiparticle pair in empty space.
Put it right: A nucleus or other mass must be present to conserve momentum.
Using m₀c² rather than 2m₀c² as the minimum energy for pair creation.
Put it right: Two particles of rest mass m₀ are produced, so the threshold is 2m₀c².
Believing annihilation produces a single photon.
Put it right: Two photons are emitted in opposite directions so that momentum is conserved.
Exam tips
- Write the particle equation with charges and lepton numbers shown above each symbol before deciding whether it is allowed.
- For threshold questions, add the rest energies of both particles produced.
- For annihilation, state the direction and equal energy of the two photons, not just their existence.
Try the lesson
The start of the Nuclear fission and fusion: (c) creation and annihilation of particle–antiparticle pairs lesson, made from checked teaching notes for this specification point.
Listen
Pair creation is the conversion of energy into a particle and its antiparticle, for example a photon producing an electron and a positron. It can occur only when the photon energy is at least the total rest energy of the pair, 2m₀c², and in the presence of a nucleus to conserve momentum. Pair annihilation is the reverse: a particle and its antiparticle combine and their mass is converted into photons, commonly two gamma-ray photons each of energy m₀c² emitted in opposite directions to conserve momentum. For an electron–positron pair the minimum photon energy is about 1.02 MeV. Charge, lepton number and momentum are conserved in both processes. A strong answer earns marks for these points. Define pair creation as energy converting into a particle and its antiparticle, and annihilation as a particle and antiparticle converting into photons. State that the minimum photon energy for creation of a pair of rest mass m₀ each is 2m₀c². Explain that a nearby nucleus or other body is needed in pair creation to conserve momentum. State that annihilation typically produces two photons of energy m₀c² each, emitted in opposite directions to conserve momentum. A common slip: Thinking a single photon can create a particle–antiparticle pair in empty space. Instead, a nucleus or other mass must be present to conserve momentum. Exam tip: Write the particle equation with charges and lepton numbers shown above each symbol before deciding whether it is allowed.
Key words
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Pair creation
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Pair annihilation
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Conservation laws in particle interactions
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See every definition
- Pair creation
- The conversion of energy into a particle and its antiparticle, for example a photon producing an electron and a positron.
- Pair annihilation
- A particle and its antiparticle combine and their mass is converted into photons, commonly two gamma-ray photons each of energy m₀c² emitted in opposite directions to conserve momentum.
- Conservation laws in particle interactions
- Charge, lepton number and momentum are conserved in both pair creation and pair annihilation.
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