Saxandthecity
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Hi guys,

I just did a question on a Unit 5A AQA physics mock where it required you to explain that a moderator nucleus that collides with a high energy neutron in a nuclear reactor emits a gamma photon. It says in the mark scheme 'because the nucleus de-excites to it's ground state'. I wasn't aware that nuclei could move between energy levels and was under the impression it was only electrons? Is that what the mark scheme is trying to say or am I mistaken?

Thanks in advance
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mik1a
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That's a bit zany to put on a 6th form mark scheme, but it's true. Electrons configurations have discrete energy states because there are finitely many solutions to the Schrödinger equation. Since they are fermions, electrons cannot all occupy the ground state so they queue up to occupy the states with the lowest energy. Well, protons and neutrons are fermions as well and they are also governed by the Schrödinger equation, only this time their binding force is the strong force rather than the electric force. In practice, that means you have nuclear energy states just like you have electron (or "atomic" states. Since the strong force is so strong, the energy between these states tends to be in the MeV rather than the eV, so the radiation is in the gamma part of the spectrum rather than the visible/UV.

I have to edit this; electrons don't really queue up, as you may know from chemistry they have some strange rules when populating different shells. A better description is "the electrons will, as a group, arrange themselves into a configuration state with minimum energy".
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