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How are neutrons made?

Perhaps no-one really knows, but I'll ask anyway. Are they still being made? or were they all made at some early instant of the universe?

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    $\begingroup$ en.wikipedia.org/wiki/Neutron#Sources_and_production $\endgroup$ – Mike Apr 14 '16 at 21:20
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    $\begingroup$ Sources and production sounds a likely place to look, but its more about how neutrons are liberated from a nucleus, than about how they are made in the first place. $\endgroup$ – user2800708 Apr 15 '16 at 16:07
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We think we understand fairly well how the universe makes neutrons: initially by baryogenesis and nucleosynthesis (see https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis#Neutron.E2.80.93proton_ratio and https://en.wikipedia.org/wiki/Nucleosynthesis) and later, as soon as the stars start to shine mostly by fusion (see https://en.wikipedia.org/wiki/Stellar_nucleosynthesis). Unfortunately, the details are complicated and, to some extent, the calculations have the charm of a major accounting exercise, so you have to decide for yourself if you really want to know the details... The data to back them up comes from detailed studies of the early universe, star formation, stellar development, nuclear physics and have to be calibrated against isotope ratio measurement on Earth and from material in the solar system. We are also very interested in the composition of cosmic rays, which gives us information about processes happening in far away regions of space. Since the total amount of accessible matter from outside of Earth has been limited, there are certainly a few surprises still waiting to be discovered, but the big picture is emerging.

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Neutrons are definitely still being made.

Most of the visible matter created during Big Bang nucleosynthesis was in the form of hydrogen, helium and lithium. Hydrogen sometimes contains a neutron (forming the stable deuterium), and all stable isotopes of helium of lithium do. So assuming the model is correct, neutrons must have been formed during this event.

Keep in mind that free neutrons are not stable particles, and decay with a half-life of about 10 minutes. So we wouldn't expect to find neutrons that aren't confined to atomic nuclei; it's not as if there's some soup of stable protons and neutrons that sometimes creates atoms.

But we also know that this isn't the only source of neutrons. For one, we know of unstable nuclei that spontaneously change a proton to a neutron or vice versa. The scenario you care about is called beta+ decay. A proton in a nucleus is converted into a neutron, a positron and a neutrino. An example would be the nucleus of magnesium-23 transforming into sodium-23 (note that the nucleon number stays the same, but the element changes).

The Sun relies on neutron production too. The bulk of the Sun's mass is simple hydrogen, H-1. It doesn't have neutrons. When you smash two hydrogen nuclei together, you get He-2 (also known as a diproton), which is extremely unstable and very quickly decays back into two separate hydrogen nuclei. Needless to say, you haven't released any energy in such a reaction!

What needs to happen is that one of the protons in the diproton changes into a neutron. Free protons do not spontaneously change into neutrons, because neutrons are actually very slightly more massive than protons (and for the same reason, free neutrons do spontaneously decay into protons). However, a diproton has a higher binding energy than a deuteron (H-2), so one of the protons can in fact change into a neutron, while releasing a positron, a neutrino and the excess binding energy.

This reaction is mediated through the weak nuclear force, which is relatively weak on large distances ("large" here meaning "distances comparable to the size of a nucleus"). This, combined with the instability of the diproton, means that only a very small amount of diprotons ever transmute to a deuteron. But without this rare event, the Sun wouldn't have its fusion furnace. Interestingly enough, it's also a major rate-limiter for the Sun's fusion - if the weak force were stronger, the Sun would burn through its fuel supply much faster. So far, it seems that the weak force is the only force that can change the flavour of quarks, thus changing the baryon number.

For completion sake, we should also say that neutrons are created in sufficiently energetic collision, in pairs with anti-neutrons.

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  • $\begingroup$ We used a T(p,n) reaction to produce neutrons and most of the neutrons were not annihilated. $\endgroup$ – jmh Sep 26 '19 at 19:01
  • $\begingroup$ @jmh Hmm, I didn't think much about that last sentence; I suppose the annihilation is a lot less likely than with electrons, given the lack of electromagnetic attraction? It probably doesn't hurt that there's higher velocities involved. $\endgroup$ – Luaan Sep 27 '19 at 6:56
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    $\begingroup$ Diprotium decay to deuterium is certainly the main way neutrons are created in the post Big Bang era (although the neutrons created during baryogenesis and BB nucleosynthesis still dominate the numbers). But I think you should also mention the neutrons created when a supernova leaves a neutron star remnant. $\endgroup$ – PM 2Ring Sep 27 '19 at 11:02
  • $\begingroup$ @PM2Ring I'm always a bit worried about mentioning supernovae and neutron stars, given how many awesome new things we've learned about them :) $\endgroup$ – Luaan Sep 27 '19 at 12:41

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