OUTER SHELL ELECTRONS. “As given by Compton, the explanation of the Compton shift is that in the target material, graphite, valence electrons are loosely bound in the atoms and behave like free electrons.”
https://opentextbc.ca/universityphysicsv3openstax/chapter/the-compton-effect/#:~:text=The%20Compton%20effect%20is%20the,wavelength%20of%20the%20incident%20radiation