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Pair Production and momentum

  1. Jul 19, 2008 #1


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    I read about "pair production" how a photon can create and electron and a positron. I read the calculations and I understand how this is impossible unless something else (a nucleus) is present to take away some of the initial photon momentum, however, the book doesn't say exactly 'how' the nucleus takes away the photons momentum. Any help?
  2. jcsd
  3. Jul 19, 2008 #2
    My best guess would be that momentum is transferred during a collision process- the nucleus gains momentum at the photon's expense. But that is only a semi-educated guess. Anyone?
  4. Jul 20, 2008 #3
    I think that's right muppet. But the photon momentum is so small compared to the nucleus mass that the nucleus barely moves in the collision - like an asteriod hitting the earth.
  5. Jul 20, 2008 #4


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    Several things happen, and are described by individual Feynman diagrams, four in fact to lowest order in the electron's charge, e.. They all involve photon absorption and emission by the nucleus. You can find these diagrams in many texts. There are two, which represent absorption of the photon by the nucleus followed by emission of another photon that turns into a pair. Also, there is the same diagram with absorption second and emission first.

    Then, the electron-nucleus and positron nucleus interactions involve single photon exchange between the leptons and the nucleus. The photon(s) interacting with the nucleus transmit the momentum and energy needed to make the production process go.- true for all diagrams.

    And yes, with a photon energy considerably less than the nuclear mass, the nuclear recoil is negligible.

    Reilly Atkinson
  6. Jul 20, 2008 #5


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    ok, so then this process can occur in empty space? However, they quickly annihilate one another again? But in order to prevent them from annihilating each other you must have a nearby nucleus?

    Is that all correct?
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