Dear Clombard,
thank you for your detailed answer. However I have a doubt. Can photons be emitted say by nuclear reactions rather than by an electron changing its orbital level? What about the electron-positron annihilation? I think it emits a photon, but I'm unsure we can talk about the electron orbit level in this case. What do you think?

__________________
Isaac
If the problem is too hard just let the Universe solve it.