Nuclear Shell Model - pp bound states?

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Discussion Overview

The discussion revolves around the nuclear shell model, specifically addressing the concept of proton-proton (pp) bound states and the implications of filling nuclear shells with protons and neutrons. Participants explore the theoretical aspects of these states and their stability in relation to observed phenomena.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that the nuclear shell model allows for the filling of shells with protons and neutrons similarly to electrons, questioning the possibility of a bound state of two protons in the 1s shell.
  • Another participant counters that no theory would predict a bound state for the diproton, citing experimental evidence that indicates it is unbound due to the presence of repulsive forces.
  • A further contribution notes that attempting to fill the proton shell results in a total energy that is above that of two free protons, indicating extreme instability of such a state.
  • A later reply acknowledges a misunderstanding of the model and expresses appreciation for the clarifications provided by other participants.

Areas of Agreement / Disagreement

Participants generally disagree on the possibility of pp bound states, with one participant hypothesizing their existence while others assert that experimental evidence shows they cannot exist.

Contextual Notes

The discussion highlights the dependence on experimental observations and theoretical frameworks, with unresolved questions regarding the stability of proposed states and the implications of the nuclear shell model.

sk1105
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I have looked around for help with this, including on existing threads, but I can't quite find what I'm looking for.

I know that in the nuclear shell model we fill the shells in the same way as with electrons, so 2 protons in the first and 6 in the second etc, with the same being true for neutrons. I take this to mean that the 1s shell, for example, can contain 2 protons and 2 neutrons. Do I have the right idea? If so, does this not contradict with the fact that proton-proton bound states are not observed in nature? Theoretically could we not have Helium-2, with pp in the 1s shell?
 
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What do you mean by "theoretically". No theory that attempted to explain nature would predict this, because it is known from experiment that the diproton is unbound. Indeed, since the dineutron is unbound, and since the diproton adds the repulsive electrostatic force on top of that, it's guaranteed to be unbound.
 
If you try to fill the proton shell only, you'll get a total energy above the energy of two free protons - the state is extremely unstable.
 
Sorry, I was unscientific with my words. I was hypothesising based on my incorrect understanding of the model. But both answers were helpful, thanks! :)
 

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