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

Nuclear fusion. Magic neutron. (Basic)

  1. Jul 25, 2008 #1
    This question is probably very basic for those who understand nuclear fusion but it has been annoying me for a while now and I would like an answer. I have tried searching these forums but I can't find what I'm looking for.
    The sun is constantly fusing hydrogen atoms to form helium atoms, correct? Well, if hydrogen nuclei only contain a single proton, where do the neutrons for the helium atom come from? An answer would be much appreciated, try not to make it too complicated, thanks.
  2. jcsd
  3. Jul 25, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Well there are two possibile routes for fusion in the sun - pp-chain and the CNO-cycle - but the pp-chain is predominant (~98%).


    In the p-p chain, the first step is the collision of two protons, up which on proton is transformed by the weak interaction into a neutron with the resulting emission of a positron and neutrino.

    http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/procyc.html#c4 (see this for a diagram of the weak interaction).
    The next step would be p + d -> 3He + gamma, where the deuteron d = (pn)

    Now with two reactions may occur with the 3He ion:

    p + 3He -> 4He + e+ + [itex]\nu[/itex], i.e. another weak intereaction or,

    3He + 3He -> 4He + 2 p

    and the 2 p are free to continue the pp-cycle.

    It should be obvious that 3He = (pnp), and a triton (nucleus of tritium) = (pnn).
    Last edited: Jul 25, 2008
  4. Jul 25, 2008 #3
    Thanks for the reply and sources. Basic as this may be to you, I am struggling.
    The only part that I am having trouble with is the neutrino part.
    The proton transmutes to a neutron because a 2 proton, 1 electron atom would fall apart, right? So the weak force comes in to play to correct the situation. Switching an up quark to a down quark. A W* decays into an electron neutrino and a positron, during the transmutation, these are then ejected from the sun? We now have a deuterium atom.
    On a side note, is the formation of deuterium possible outside of the sun?
  5. Jul 25, 2008 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Well it's more complicated than that. I'll defer to a particle physicist on the initiation of the quark transformation.

    More or less, yes on the quark transformation. The neutrino would leave the sun, but the positron would pretty quickly find an electron, and they would annihilate to form two 0.511 MeV gamma rays.

    Yes. A proton will absorb (combine) with a neutron and form deuterium with the emission of gamma ray. Deuterium naturally occurs on earth, but was can produced it artificially with a source of neutrons and hydrogen (protons), e.g. in a light water reactor (LWR).
  6. Jul 25, 2008 #5
    Thanks for the reply, once again. Hopefully a particle physicist comes along soon to answer this question. There's know point knowing what happens without understanding why. I don't like it when small problems get in the way of my thirst for knowledge!
  7. Jul 25, 2008 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    This diagram shows various weak interactions

    The two protons have to have enough kinetic energy to overcome the coulomb repulsion of the two electric fields, then nuclear fields or quarks interact. The sun is actually a low energy process. The core temperature of 15 million K is equivalent to only ~1.3 keV, which is low energy considering the rest mass of a proton is ~0.938 GeV, and particle accelerators are pushing TeV (1000 GeV) levels these days.

    Ultimately one gets into the different flavors of quarks and why is Nature that way. All we can really say is - that's the way it is.
  8. Jul 26, 2008 #7


    User Avatar
    Science Advisor
    Homework Helper

    p + 3He -> 4He + e+ + [itex]\nu[/itex]

    Is that reaction you have a question about?

    Or is it the p-p reaction?

    Anyway, we don't have atoms in the sun, we have plasma (free nuclei and electrons).
    Last edited by a moderator: Jul 26, 2008
  9. Jul 26, 2008 #8
    I am asking about the p-p reaction. I know that the weak force is involved in changing the flavour of the quark from up to down. I am still confused as to why this happens though, maybe a better understanding of the weak force could help me? I shall read up on the weak force some more to try and understand why the flavour of the quark is changed.
  10. Jul 27, 2008 #9


    User Avatar
    Science Advisor
    Homework Helper

    Well the first thing is that it CAN change the flavour, chaning the proton to a neutron in the p-p 'system' makes the system loose energy and become bound (pp is not bound, but np is bound), nature always strive to minimize the energy - and bound systems have lower energy then unbound.

    Secondly, if you study the weak interactions you'll see that since there is 2 charged force mediators (gauge bosons) of the weak interaction, and that electric charge must be conserved, the u quark must change into a down quark or opposite.
  11. Jul 29, 2008 #10
    Okay, I've done some more research.
    An up-quark decays to a down-quark producing a virtual force carrier, the W+, which leaves behind an electron neutrino and a positron. The virtual force carrier does not break the conservation laws due to the uncertainty principal, the positron will find an electron and annihilate, while the neutrino will escape from the sun and drift. As to exactly why this happens, I suppose I will have to accept that it's just the way things are.
  12. Jul 29, 2008 #11
    I must say, I've been reading this thread, trying to find something more convincing, hoping somebody came up with one, but eventually, I guess indeed, you will not find much more answers at this point.

    The up and down are really part of the same multiplet connected by this W boson. It's the way Nature was assigned, of has chosen, Her symmetries...
  13. Jul 30, 2008 #12


    User Avatar
    Science Advisor
    Homework Helper

    Well as I said in an earlier post, this happens because it can happen. That is the first 'why'. But 'why' it goes just this way, that nature follow this scheme (why the quark has their electric charges, why the W's have charge 1 etc.) is just what nature is. No matter how far we go in our quest for finding the most fundamental parts of nature, we must eventually accept things as they are. It's just plain philosophy.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Nuclear fusion. Magic neutron. (Basic)
  1. Nuclear fusion (Replies: 7)

  2. Nuclear fusion (Replies: 1)

  3. Neutron Fusion (Replies: 5)