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Where do positrons come from?

  1. Jul 25, 2012 #1
    I recently read that a photon can decay into an electron and a positron under certain conditions, like when one passes near a heavy nucleus. I think I understand where the electrons come from, but where do the positrons come from? I thought that the charge of an atom is from the electrons and the protons in the nucleus which generally balance. I was just curious about the source of the positrons.


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  3. Jul 25, 2012 #2


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    Both are created in the reaction. They do not "come from something", and they are not there before the pair production occurs. The charge of electrons and nucleons in an atom are not directly related to this. You need the nucleus (or some other charged object) for energy/momentum-conservation, but that is a technical detail here.
  4. Jul 25, 2012 #3


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    A positron is an electron's antimatter counterpart. It has the same mass as an electron but a positive charge. As you are no doubt aware, when you combine matter and antimatter in equal amounts it results in complete annihilation. The only product of such a reaction is a high energy photon. (Actually, I think there's more to it than this, but let's leave it simple.)

    If you filmed it, you'd see two particles e- and e+ come together, and one photon leave.

    Now, simply play the film backwards.

    You'll see a high energy photon enter and spontaneously split into an electron and a positron.
  5. Jul 25, 2012 #4
    Perhaps I'm not asking the question correctly. Consider an electrical circuit that produces an electromagnetic wave/particle stream. The electrons are contributed by the current source for the circuit, right? My question is if an equal number of positrons are required to produce the EM field which consists of photons, comprised of an electron and a positron, from where in the circuit do the positrons come? All I see are atoms, electrons, protons neutrons. Conservation of charge seems to indicate that the protons balance the charge of the electrons. So I see an injection of positively charged particles coming from somewhere, but I just don't understand where.
  6. Jul 25, 2012 #5
    Electrons, neutrons, and protons are far from being the whole story. The Standard Model is filled with a whole zoo of dozens and dozens of different particles.
  7. Jul 25, 2012 #6
    I realize that, I'm really interested in an answer to my question.
  8. Jul 25, 2012 #7
    What exactly is your question? Positrons don't 'come' from anywhere. They aren't a part of atoms, either. Every particle has an opposing anti-particle, hence the positron must exist as a anti-particle to the electron.

    Also, you don't need positrons for EM fields to be produced. Electrons, or any charged particle, can induce a field just fine.
  9. Jul 25, 2012 #8
    Is he asking about 'holes' in a semiconductor?

    I'm not really fathoming the question . . .
  10. Jul 25, 2012 #9


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    They don't come from the circuit. They come from the energy of the photon. The electron and the positron must be produced together to satisfy all of the conservation laws.
  11. Jul 25, 2012 #10


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    At first we were talking about positrons, real particles with the mass of an electron and a positive charge. Now it seems the question is about electrical circuits and the movement of charges, which has nothing to do with positrons.

    I think tasp77 is right, the OP is conflating positive charge "holes" moving through a circuit with positrons.
  12. Jul 25, 2012 #11
    There are no positrons involved with the creation of the vast majority of photons. Your EM wave/photon stream is created by electron motion alone. The electron-positron creation-annihilation involve only high energy gamma ray photons of a specific energy (511 keV) and are quite rare as photons go.
  13. Jul 25, 2012 #12
    I guess I'm misinformed. I was under the impression that EM radiation is made up of photons, not simply electrons but an electron and a positron (which confusingly to me, the photon has no net electrical charge). I don't think you can create a EM field with only electrons. I thought this was a well understood phenomenon with a straignt-forward answer which physics guys would have off the cuff. I'm not a physicist just a curious engineer, I apologize for asking a question which I would appearently not understand the answer to.
  14. Jul 25, 2012 #13
    You can create an EM field with only electrons. All you need to create an EM field is a charge. There is a field that surrounds an electron that will attract positively charged particles.
  15. Jul 25, 2012 #14


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    Just to make sure we get the semantics right: EM radiation is "made up of" photons, but photons are NOT made up of electrons and positrons (not only are photons neutral (no electric charge), they also have zero rest mass). A charged particle can generate EM radiation, but the charged particle is not part of the radiation itself.
  16. Jul 25, 2012 #15


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    There is misinformation, and there is misinterpretation.
    That's what I've heard.
    Don't know where you heard that.
    As a wanna-be electrical engineer, that's the only way I know how to make one.
    The universe is complex.
    Keep asking. I asked this question 30 years ago. No one has yet answered it.
  17. Jul 25, 2012 #16
    A photon is NOT made up of an electron and a positron. Really, drop that idea, it is leading you to wrong headed problems. 511keV photons are a very special case.
  18. Jul 25, 2012 #17
    Isn't it two photons, not one?
    Pair production: two photons are transformed into an electron and a positron.
    Annihilation: An electron and a positron are transformed into two photons.
    I seem to remember being taught that, with only one photon, momentum would not be conserved.
    I could be wrong about this. Did I imagine having learned that?
  19. Jul 25, 2012 #18


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    I think this got confusing because of a mix regarding the production of EM radiation/photons. There are a number of different processes that generate such, e.g.:

    1) Annihilation; a particle/antiparticle annihilate and two photons are emitted.
    2) An electron in an excited atom returns to a lower state and emits a photon
    3) Moving charges generating EM radiation (e.g. radio)

    pvinventor, regarding 3), the simplest analogy I can think of would be something like this; fasten one end of a string to a wall. Grab the other end with your hand. Move your hand up and down. Your hand will produce waves travelling along the string.

    Your hand = an electrical charge (+ or -, doesn't matter) moving up and down
    The string = the electromagnetic field
    The waves = EM radiation (made up of "packets" called photons)

    This is basically how a radio antenna works (but with no strings attached :smile:).
    Pair production; needs only one photon (near a nucleus)
    Annihilation; creates at least two photons.
    Last edited: Jul 25, 2012
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