Why does the mass attenuation coefficient have a negative gradient?

[epsilon]

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!

 

[PietKuip]

Everybody knows that x-rays penetrate matter, so you should not be surprised.

As to why, this is due to the matrix element of the electrical dipole operator between atomic wave functions. It goes down fast when the wavelength is smaller than the size of the orbitals.
Yes, the jagged parts are the absorption edges, when the photon energy is at the treshold for ionisation of an inner-shell electron.

 

[epsilon]

I also found that the MAC is easier to understand as the probability of a photoelectric absorption interaction per unit distance traveled into the material, for anyone else who is wondering!