Where do neutrons come from?

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Offline bizerl

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Where do neutrons come from?
« on: 21/09/2012 01:51:28 »
Not sure if this is physics or chemistry - the moderators can decide that.

I might be missing something here, but my understanding is that if an hydrogen atom (1 proton and 1 electron) gets squashed together with another hydrogen atom (again, 1 proton and 1 electron) it makes an helium atom and some energy, hence the Sun!

If an helium atom is made of 2 protons, 2 electrons and at least 1 neutron, where did the neutron come from?

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Offline imatfaal

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Re: Where do neutrons come from?
« Reply #1 on: 21/09/2012 10:41:29 »
You are dead right the numbers seem to be squiffy - the most important of the fusion reactions is the proton proton chain
1. 2 Hydrogen make a weird Isotope of He called a  Diproton (ie two protons)
2. This Diproton decays to Deuterium (one proton and one neutron) and a positron and a neutrino
3. This fuses with another Hydrogen to create a more normal (but not standard) Helium (2 protons 1 neutron)

Then you get a lot of ways to get from this Helium to normal Helium . 
a .  Two of these helium can fuse to give a normal helium (2 protons 2 neutrons) and two normal hydrogen (1 proton)
b   You can create isotopes (with too few neutrons) of element Berylium which decays to Lithium which fuses with Hydrogen and splits  to form two Helium
c  An even more complex route via Boron
d.  Technically you can go from 1 neutron form of Helium plus Hydrogen straight to 2 neutron form of Helium if you can convert the proton into a neutron by positron emission beta decay - this doesn't really happen much tho

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Offline evan_au

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Re: Where do neutrons come from?
« Reply #2 on: 25/09/2012 09:23:37 »
There are some diagrams here: http://en.wikipedia.org/wiki/Hydrogen_fusion#Astrophysical_reaction_chains

There are some neutrinos emitted in the process (there is some debate about whether a neutrino is the same as an antineutrino...).