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I was wondering in a nuclear fission, why does the neutron get emitted but not the proton? Is there any case where in a nuclear fission, proton emission happens?

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I think it's based on which particles usually fission. As a rule, heavy nuclides undergo a fission. As the name suggests, these have a high mass number and we know that as the number of protons increases, the number of neutrons in the nuclide increases disproportionately aka if we have a heavy nuclide, there are more neutrons than protons in them.

[...] Among stable nuclei and naturally occurring nuclei, this ratio generally increases with increasing atomic number. This is because electrical repulsive forces between protons scale with distance differently than strong nuclear force attractions. In particular, most pairs of protons in large nuclei are not far enough apart, such that electrical repulsion dominates over the strong nuclear force, and thus proton density in stable larger nuclei must be lower than in stable smaller nuclei where more pairs of protons have appreciable short-range nuclear force attractions. [...]

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(picture and quote are from Wikipedia > Neutron-Proton Ratio)

So if a heavy nuclide were to split into two medium-heavy nuclides, the neutrons would be divided between the two nuclides, which means that both nuclides have an enormous neutron surplus and therefore emit a few neutrons (as we already know from radioactive decay). So there would be no reason for emiting a proton. At least that's how I always explained it to myself.

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Is there any case where in a nuclear fission, proton emission happens ?

There are radioactive decay reactions which result in the emission of a proton. However, the proton is positively charged, so it is repelled by other nuclei, which are also positively charged. It is therefore very unlikely to get close enough to another nucleus to initiate fission of that nucleus, and so it cannot initiate a chain reaction. The same goes for the emission of alpha particles, which are also positively charged.

So the fission reactions that we associate with nuclear reactors and nuclear bombs are ones that emit neutrons, which have no charge and so can trigger a chain reaction under the right conditions.

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  • $\begingroup$ That is true, but proton emission seems to be extremely rare. The OP does have a non-trivial point: who does nature not like to emit protons? I have to admit that I do not have a good explanation myself. I am sure it has something to do with the energetics and symmetry of nuclear states or even with the physics of the strong force, but I am not familiar enough with any of that to have a good idea what is going on. $\endgroup$ Commented Jun 4, 2023 at 21:10

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