I'm reading Knoll's book "Radiation Detection and Measurement", and in the chapter discussing the interactions between gamma radiation and matter, he explains about the three main types of interaction: photo-electric absorption, Compton scattering and pair production. He proceeds to the relative strength of interactions (cross-sections) for different kind of materials and different photon energies and shows for example the following graph (originally from R. D. Evans in 1955): I understand that there is still a chance for Rayleigh scattering which is less dominant than the other three interactions I mentioned, but I was surprised that there is an interaction which is the strongest at the energy range of about 2-4 MeV and I never heard about it: Compton absorption. Notice it is not the same as Compton scattering - and this confuses me.
What is this interaction? If the photon is completely absorbed isn't it photo-electric absorption? (See the $\tau / \rho$ part of the picture, the strongest interaction in the low-energy range). If the photon isn't completely absorbed (meaning it got annihilated but another weaker photon was created) it is supposed to be Compton scattering!
I'm missing something here: what happens for photons with energy of about 2-4 MeV? What is this Compton-absorption?
Thank you very much in advance!