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Author Topic: Form where do neutrons come from?  (Read 9974 times)

Offline Aman Sharma

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Form where do neutrons come from?
« on: 01/07/2012 11:00:32 »
Hydrogen atoms which have one proton and one electron fuse to form a deuterium atom which has one neutron, one proton and one electron. From where do the neutron come from?


 

Offline Soul Surfer

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Re: Form where do neutrons come from?
« Reply #1 on: 04/07/2012 11:43:30 »
The simple image is that a neutron is a proton and an electron fused together to produce a neutral particle but that is not really true because electrons and protons attract each other strongly by the electrostatic force and if they combined that easily all the protons and electrons would have fused together to form neutrons.

The real reason is that protons ann neutrons (nucleons)  are composite and both formed from three quarks so the electron has to interact with one of the quarks to change it.  These quarks are called up (positive charge 2/3) and down (negative charge -1/3)  for no real reason.  These quarks are batting around with much more energy than their rest mass inside the nucleon like balls on an elastic string held together by their colour charge which is stronger than any electric charge repulsion the have.  They are also much "smaller" are and more difficult to interact with.  Another important fact is that a neutron on its own is not stable and falls apart after about 15 minutes to release an electron and a neutrino (strictly an ANTIneutrino)

The quarks can never be seen on their own because the force between them gets stronger (like an elastic band) as they are separated.  Removing one always creates more quarks to result in more neucleons or mesons (pairs of quarks bound together)
 

Online evan_au

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Re: Form where do neutrons come from?
« Reply #2 on: 04/07/2012 14:12:12 »
Fusion in stars today does not directly fuse hydrogen nuclei into deuterium or helium, as the two hydrogen nuclei have positive charges, and repel each other strongly. This would require extremely high temperatures and pressures that don't exist in the Sun.
Instead, it is catalysed by Carbon, Nitrogen and Oxygen, where the neutrons in these heavier nuclei provide some of the strong nuclear force, which helps overcome the electrostatic repulsion.
This CNO cycle manufactures neutrons which eventually get emitted as an alpha particle (helium nucleus).
See http://en.wikipedia.org/wiki/CNO_cycle
 

Offline Soul Surfer

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Re: Form where do neutrons come from?
« Reply #3 on: 04/07/2012 23:21:03 »
I agree evan that the road to fusion cannot be achieved directly but I believe that our questioner was asking a more fundamental question which I answered rather than go into the details of the process.
 

Online evan_au

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Re: Form where do neutrons come from?
« Reply #4 on: 04/07/2012 23:39:33 »
Correction: proton proton fusion is thought to be the dominant process in the sun. Wikipedia has some nice diagrams on this.
 

Offline CliffordK

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Re: Form where do neutrons come from?
« Reply #5 on: 05/07/2012 09:08:17 »
According to Wikipedia:

The first step involves the fusion of two 1H nuclei (protons) into deuterium, releasing a positron and a neutrino as one proton changes into a neutron. It is a two-stage process; first, two protons fuse to form a diproton


0cbafce45f18dd5efe8e29a1837e7bef.gifH + 0cbafce45f18dd5efe8e29a1837e7bef.gifH → d893654f5f24409552aea32998f3771e.gifHe

followed by the beta-plus decay of the diproton to deuterium:

d893654f5f24409552aea32998f3771e.gifHe7ef06d7f357c993487c0f1cec7558a8a.gifD + e+ + νe

with the overall formula:

0cbafce45f18dd5efe8e29a1837e7bef.gifH + 0cbafce45f18dd5efe8e29a1837e7bef.gifH → 7ef06d7f357c993487c0f1cec7558a8a.gifD + e+ + νe +  0.42 MeV

This first step is extremely slow, because the beta-plus decay of the diproton to deuterium is extremely rare (the vast majority of the time, it decays back into hydrogen-1 through proton emission).

The positron immediately annihilates with an electron, and their mass energy, as well as their kinetic energy, is carried off by two gamma ray photons.

e + e+ → 2γ + 1.02 MeV

So, while it is easy to think of a proton and electron fusing, according to Wikipedia, the proton actually releases a positron, which then annihilates an electron.

This makes sense because it would be far more difficult for the proton and electrons to meet and fuse.  But, the charge must always be balanced, so if a proton is converted to a neutron, then an electron must be lost.
 

Offline Soul Surfer

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Re: Form where do neutrons come from?
« Reply #6 on: 05/07/2012 09:45:26 »
Interesting Evan.  I always thought that the CNO process was the main process in the main sequence around the sun's mass although there were always worries about this because of its extreme core temperature sensitivity.  However I have not revised opinions on this for many years when you consider the lifetime of the diproton (resonance) and the time scales usually involved in weak interactions it must be a very rare event but the vast numbers of proton proton collisions at the critical energy make up for this.  I have also confirmed the Wikipedia result with my current physical astronomy text book  the CNO process is now thought to cut in somewhat above the sun's mass in the main sequence.
 

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Re: Form where do neutrons come from?
« Reply #6 on: 05/07/2012 09:45:26 »

 

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