Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electron-Proton Fusion?

  1. Apr 17, 2008 #1
    1a. Is possible for an electron and a proton to fuse?

    1b. If yes, do you get a neutron?

    2. What PREVENTS protons and electrons fusing together in plasma [like in tokamak]?

    -Harry Wertmuller
  2. jcsd
  3. Apr 17, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    1.a Yes

    1.b Yes, we get a neutron and a neutrino. This is called electron capture

    2. I'll leave that til an expert in plasma physics (since I am not one of those) :-)
  4. Apr 17, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    A 'free' neutron is unstable with respect to beta decay, i.e. a neutron by itself will decay into a proton, beta and anti-neutrino, so the reverse action is unlikely. I suspect that electron capture may occur very rarely.

    EC does happen in certain proton-rich (neutron deficient) radionuclides, and typically it is a K-electron which is absorbed. In plasmas, the ions and electrons have kinetic energies much, much greater than the binding energy of electrons (in the light elements). At lower energies, electrons recombine with ions to form neutral atoms, which then leak out of plasmas.

    Protons (or more generally ions) and electrons interact in plasma, mostly by scattering or producing bremsstrahlung radiation, the latter of which results in energy loss from the plasma.

    Fusion plasmas are likely to be D+D or D+T. D+3He would be preferable since it's an aneutronic reaction, but it requires higher temperatures (with concommitant pressure for a given particle density) and 3He is extremely rare (and thus very expensive). He-3 can be obtained from the beta decay of T (H-3).

    Proton-proton fusion has a very low cross-section, and is the basis of one fusion process in stars - PP fusion or PP-chain.

    Otherwise, there are more exotic reaction like p + 11B -> 3 4He, but that requires significantly higher temperature than D+D or D+T.

    p + p -> d + e+ + 1.4 MeV, but
    p + d -> 3He + gamma + 5.5 MeV
  5. Apr 17, 2008 #4
    1a. Yes

    1b. Yes, and also a neutrino.

    2. The interaction cross-section is *very* small --- the reaction is mediated by the weak force, which is not called the weak force for nothing.
  6. Apr 17, 2008 #5
    Hmmm.. Astronuc wrote: "EC does happen in certain ... neutron deficient radionuclides and typically it is a K electron..."

    Q: Can you describe or cite experiment where EC happens or happened?

    Q: What is K electron? Lowest energy orbit in Hydrogen? So then does probability of EC INCREASE if they make one of those "Bose-Einstein" condensates using deuterium-free hydrogen?
    Last edited: Apr 17, 2008
  7. Apr 17, 2008 #6
    Are you questioning the possibility that it happens ?
    You can search Google for many peer-reviewed publications.
    A K-electron is indeed one in the lowest shell. But hydrogen is irrelevant for neutron capture. Here you want larger nuclei isotopes with too many protons, but such that beta+ decay (positron emission) is forbidden.

    Electron capture
  8. Apr 17, 2008 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    http://www.nndc.bnl.gov/chart/ has a listing of all radionuclides and decay modes. Click on a location on the chart and select Zoom 1 at the top right of the chart.

    The isotopes above or to the right of the black squares (stable elements) are proton-rich (neutron deficient) and preferentially decay by EC (electron capture, or K-capture), or in some cases positron emission. The further away the radionuclide is from the line of stability (black squares) the more rare it is, and usually it has a very short half-life.

    Electron capture is dependent on nuclear properties, and should not be dramatically affected by the chemical form (BEC), i.e. BEC would not increase probability of EC in H, or D, or whatever.
  9. Apr 17, 2008 #8
    Astronuc; Humanino; Thank you. Did not THINK of Googling. Did not know "ELECTRON CAPTURE" were the magic words.

    Google results yield another problem -- us civilians do not have access to and/or cannot afford sources like Physical Review Letters.

    Nuclides chart is really impressive. Thanks again.
    Last edited: Apr 17, 2008
  10. Apr 17, 2008 #9
    That's true unfortunately. However, most recent articles are available for free on arXiv. In addition, if you go to a university librairy for instance, you can find those articles printed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook