I am currently trying to understand the mass attenuation coefficient (MAC) with regards to photoelectric absorption. Consider this graph: Why does this have a negative gradient? Why does the MAC decrease as the photon energy increases? From what I understand, the MAC is the rate of energy loss of a photon as it traverses a medium, independent of the density of the medium and is therefore solely a property of the atomic arrangement, etc. Hence why would a photon be losing energy slower if it is initially higher energy? Also, I believe the jagged parts of the line are the electron excitation energies of atoms, but how does this tie into the negative gradient? Many thanks in advance!