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Can a SINGLE photon produce an electron-positron pair?

  1. Feb 26, 2014 #1
    A homework question is asking for a diagram illustrating a photon becoming a electron-positron pair (please don't post an image as it's homework).

    Looking through the internet there seems to be some disagreement as to whether a single photon can create an electron-positron pair:

    "Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta."

    "For example you've got a 5MeV photon, so you think that there is plenty of energy to make e − e + pair. Now you make a boost along the direction of the photon momentum with v=0.99c and you get a 0.35MeV photon. That is not enough even for one electron. "

    so a photon would have to have a massive amount of energy, but is it even possible?

    if not why is this diagram on wiki?

    http://upload.wikimedia.org/wikipedia/commons/a/a5/Pair_Production.png

    i could just draw that diagram but if it's not a realistic possibility then would like to put in a comment as such.

    Thanks for any comments / ideas on this.
     
  2. jcsd
  3. Feb 26, 2014 #2

    vela

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    You seemed to have missed the point of the example. Say the photon did have even more energy in your frame. Can't you always boost to a frame where the energy of the photon is too low for pair production?
     
  4. Feb 27, 2014 #3
    Thanks for the reply.

    are you saying you could be in a frame where the energy required would be LESS to produce a positron electron pair? therefore it is possible?
     
  5. Feb 27, 2014 #4

    vela

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    How did you get that interpretation from the phrase "too low for pair production"?

    Try writing down the equations using conservation of momentum and conservation of energy for a photon decaying into an electron-position pair. You should be able to show there's no solution to those equations. Think about what happens in the center-of-mass frame of the electron and positron.
     
    Last edited: Feb 27, 2014
  6. Feb 27, 2014 #5
    hmm not sure, went a little abstract.

    Ahh, thanks I see now!

    Also asked my tutor who says there would have to be nuclear recoil, i.e. it would have to interact with something to produce a electron positron pair, but that it is often ignored in a simple diagram.

    Problem solved, thanks again!
     
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