Creation

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Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes. The neutron is essential to the production of nuclear power.

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Since isolated neutrons decay into protons, electrons and electron antineutrinos, they must be getting produced somewhere in nuclei, where they can be stabilized by the strong potential well.

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One obvious place is in deuterons, a stable bound state of a proton and a neutron, formed mainly by p+p→π+ +d reactions in the early universe.

The deuterons then fuse into alpha particles (helium-4 nuclei: two of each) and so on for a while.

But beyond oxygen-16 the elements start needing extra neutrons to be stable. You can turn a proton into a neutron by positron emission or electron K-capture (emitting an electron neutrino in both cases), but I wonder if that’s common enough to account for all the extra neutrons in the stable isotopes up to iron — after which you have to add energy to get further fusion, so that stuff mostly requires supernovae for the required energy — and supernovae are themselves the result of electrons getting shoved back into protons by intense gravitational pressure, emitting electron neutrinos in the process and blowing off a large fraction of the star into a nebula while the rest collapses (usually[?]) into a neutron star.

Certainly a lot of “extra” neutrons are available in a supernova for making those heavy elements, but is that how the “middle elements” get their extra neutrons? I don’t know! Hopefully we will hear from someone who does.

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Decay  (β Radiation)

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The neutron is an unstable particle that on the average lives for about 10 minutes. It decays into a proton, and electron and and antineutrino. [3]

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However, that does not mean that a neutron will always live 10 minutes.  Sometimes it will live 5 minutes, sometimes 10 seconds, sometimes 30 minutes  Only by observing many neutron decay events can one make up an average and determine what physicists call the lifetime of the neutron. [3]

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or, other way of presenting :

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Neutrons do usually not decay within nuclei because it is energetically too difficult.  While the mass of the neutron is larger than the sum of the masses of a proton and an electron, the margin is small (about 0.78 Mev).  The difference in binding energies of neutron and proton may (and often will) be more than this small margin and in those cases neutrons in a nucleus cannot decay.

There are nuclei for which the energy balance leaves a margin, and then a neutron in such a nucleus can and will decay.  that decay is called β-radioactivity (β-decay). [3]

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Crossing

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