Nuclear fusion. Magic neutron. (Basic)

[WackyRacer]

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.

 

[Astronuc]

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

http://csep10.phys.utk.edu/astr162/lect/energy/cno-pp.html
http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html

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
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/procyc.html#c4 (see this for a diagram of the weak interaction).

In the proton-proton fusion process, deuterium is produced by the weak interaction in a quark transformation which converts one of the protons to a neutron. The neutrinos quickly escape the sun, requiring only about 2 seconds to exit the sun compared to perhaps a million years for a photon to traverse from the center to the surface of the sun.

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).

 

[WackyRacer]

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 into 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?

 

[Astronuc]

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 into play to correct the situation.

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

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.

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.

On a side note, is the formation of deuterium possible outside of the sun?

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).

 

[WackyRacer]

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!

 

[Astronuc]

This diagram shows various weak interactions
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/qrkdec.html#c2

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.

 

[malawi_glenn]

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).

 

[WackyRacer]

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.

 

[malawi_glenn]

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.

 

[WackyRacer]

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.

 

[humanino]

As to exactly why this happens, I suppose I will have to accept that it's just the way things are.

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...

 

[malawi_glenn]

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.

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.