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What evidence is there that atomic nuclei contain neutrons and protons besides the fact that neutrons and protons appear when a nucleus is smashed?

This may seem like a silly question, but I believe this to be very fundamental because the Standard Model of particle physics seems based on the axiom or assumption that neutrons and protons exist “as-is” inside atomic nuclei. Edit: Why else would the standard model require a strong nuclear force to hold everything together?

Surely there must be more evidence that this is the case besides the fact that neutrons and protons appear when a nucleus is smashed ?