Pair Production: 2 Photos or Electron/Positron?

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Discussion Overview

The discussion revolves around the concept of pair production in the context of photon interactions, specifically whether one or two photons are required for the process. Participants explore the implications of momentum conservation in these interactions, considering scenarios involving high-energy photons and atomic nuclei.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions why pair production is typically described as involving a single photon producing an electron and a positron, suggesting that two photons might be necessary to conserve momentum.
  • Another participant clarifies that pair production in a vacuum does require two photons, but notes that a high-energy photon can produce a pair when interacting with a nucleus, which helps conserve momentum.
  • A participant inquires whether the recoil momentum of the nucleus is the mechanism that allows for single-photon pair production.
  • Another participant confirms the inquiry, explaining that heavier nuclei are more effective at absorbing recoil momentum, facilitating the process.

Areas of Agreement / Disagreement

Participants generally agree on the mechanics of pair production involving a nucleus but discuss differing views on the necessity of one versus two photons in various contexts. The discussion remains unresolved regarding the generalization of photon involvement in pair production.

Contextual Notes

The discussion highlights the dependence on specific conditions, such as the presence of a nucleus and the energy of the photon, which may affect the interpretation of pair production scenarios.

Claire84
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I'm pretty much okay with pair production but in the eqt we have that a photo gives an electron and a positron. If we do it the other way round we have an electron and a positron giving 2 photos in order to conserve momentum. Why do we not have 2 photos of energy for pair production ir order to conserve momentum?
 
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You are correct (up to a point). Pair production in a vacuum does require two photons to interact. However, when a high energy photon passes near a nucleus of an atom (the heavier the better), it can give rise to pair production alone, with the nucleus taking care of the momentum balance.
 
Is that to do with the recoil momentum of the nucleus? I've heard it mentioned before so just wondering if that's what is going on here.
 
Your description is correct. It also explains why heavier nuclei are better. It is easier for them to absorb the recoil momentum.
 

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