The riddle of the neutron

  1. bobie

    bobie 682
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    What is the force that makes a neutron eject an electron, and how strong is it?
     
  2. jcsd
  3. tiny-tim

    tiny-tim 26,041
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    hi bobie! welcome to pf! :smile:
    a neutron can't eject an electron :confused:

    if you mean the force when a neutron "decays" to a proton an electron and a neutrino …

    that would be the weak force :smile:
     
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  4. bobie

    bobie 682
    Gold Member

    Thanks, tiny-tim, for correcting my wording,
    I just wanted to stress the fact that in order to "repel " an electron you must overcome the electric attraction, which at that distance, is extremely strong.
    I couldn't find details, but wiki says the process requires less than 1 MeV, and that weak force acts between electrons and neutrinos.

    Could you kindly explain exactly what happens when an electron is "captured" by a proton and when it is "freed"?
    Is the neutrino formed inside the neutron when the electron is captured? and how?
    Is 0.8 MeV enough to compensate the Coulomb force?
    What is the speed of the extranged electron and neutrino?
    Why does it take so long (15 minutes) for the weak force to act?

    I am much obliged
     
    Last edited: Sep 25, 2013
  5. tiny-tim

    tiny-tim 26,041
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    which wikipedia page are you referrfing to?
     
  6. bobie

    bobie 682
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  7. tiny-tim

    tiny-tim 26,041
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    i don't see anything there about electrons being "captured" :confused:

    can you please quote the passage you don't understand, and why? :smile:
     
  8. mfb

    Staff: Mentor

    There are no captured electrons.
    In a beta decay of a neutron, the electron is created in the decay process. It has enough energy to get separated from the newly formed proton (that's why the decay happens at all). The sum of the kinetic energies of all 3 particles (electron, neutrino, proton) is ~782 keV - the coulomb interaction is taken into account in that value. The process does not need any energy source.

    The weak interaction is not like a classical force - it can create and destroy particles, that's why the term "weak interaction is better. The neutron has a long lifetime as the interaction is weak (literally).

    The neutrino is (nearly always) very close to the speed of light, the electron can be everything from "very slow" to a significant fraction of the speed of light.
     
  9. bobie

    bobie 682
    Gold Member

    Thanks,
    when I said captured i was referring to the formation of a neutron: in a nucleus a proton captures an electron forming a neutron , is that wrong?
    If a neutron is freed from the nucleus it decays into a proton emitting the electron plus a neutrino.

    You say the electron is created in the process, but an electron ought to be already in a proton so that it can have no charge, is that wrong too?
     
  10. mfb

    Staff: Mentor

    That can happen in some isotopes (it is a rare process, however). The electron is destroyed in the process.

    No, there is no "electron in a proton".
     
  11. Dadface

    Dadface 2,084
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    the capturing of an electron that you are referring to is often known as K(or L) capture
     
  12. tiny-tim

    tiny-tim 26,041
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    yes (wrong): the electrons round a nucleus have nothing to do with it

    a neutron will "decay" (with a 15-minute half-life) if there are no electrons around it

    (inside a nucleus, a neutron is much more stable)

    when a neutron "decays", the energy produced is sufficient to create a proton an electron and a neutrino, all at the same time

    (when a proton inside a nucleus "decays" into a neutron, it can be either by "electron capture" or by "positron emission", see http://en.wikipedia.org/wiki/Electron_capture and http://en.wikipedia.org/wiki/Positron_emission)​
     
    Last edited: Sep 25, 2013
  13. mfb

    Staff: Mentor

    That depends on the nucleus. Helium-6, for example, decays to Li-6 with a half-life of less than a second.
     
  14. bobie

    bobie 682
    Gold Member

    If there is no negative particle insaide a neutron, how come it has no positive charge?

    If a neutron decays into a proton and an electron , what prevents you from saying that a neutron is made up by a proton and an electron?
    How can an electron be created in the process without a positron being created too?

    Thanks for your patience!
     
  15. Dadface

    Dadface 2,084
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    It's tempting to think of a neutron being composed of a proton and an electron somehow bound together but its more involved than that. During the capture reaction the quark structure is changed from that of the proton(up,up,down)to that of the neutron(up,down,down) .All conservation rules, including that of charge, apply
     
  16. jtbell

    Staff: Mentor

    There are no electrons inside a neutron. It's well established that a neutron is made up of an up quark (charge +2/3e) and two down quarks (each with charge -1/3e), which have a total charge of zero. In beta decay, a down quark converts to an up quark and an electron and antineutrino are created.
     
  17. bobie

    bobie 682
    Gold Member

    Even if the electron is created in the process, once it comes to exist it is attracted by the proton by the Coulombs law, and there is no other interaction between them, right?...now, how come they acquire different speeds?
    Again, could you expand on the process?...
    .. as soon as a neutron is free the process of decay starts creating an electron and a neutrino, these two interact and the neutrino expels the electron? ...and who expels the neutrino at the speed of light? the electron?
     
  18. bobie

    bobie 682
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    And so there is no positron in a proton an up-quark is equivalent to a positron?
     
  19. ZapperZ

    ZapperZ 30,307
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    Please look at the section on neutron decay (who knows, maybe a diagram makes it easier to understand).

    http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html

    The down quark changes flavor via the weak interaction to an up quark. There is no electron and there is no proton bound together to form a neutron.

    Zz.
     
  20. tiny-tim

    tiny-tim 26,041
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    and a proton, yes :smile:

    except it doesn't "start to" create it, it isn't a gradual process, it's a very sudden one, that could happen at any time, but tends to take 15 minutes …

    it isn't decay like wood decays, it's more like someone running about in a mine-field :redface:
    the electron and the neutrino do not interact

    after they are both created together, they have nothing to do wiht each other (the neutrino goes away at almost the speed of light anyway)

    (and the electron manages to escape from the proton because the electron is created with a lot of energy and momentum)
     
  21. bobie

    bobie 682
    Gold Member

    That is what I do not understand: wiki says that weak force is an interactions between leptons, and here we have only one electron and a neutrino, if they do not interact where is the weak force? who starts the process of decay?.

    Zapper says the down quark is changed to an up quark via the weak force, is it an interaction between quarks?.. and the neutrino zips off instantly via interaction with a quark? ... and what gives the neutrino the huge KE and direction?
     
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