What is potential barrier against the photon penetration

[xhtp2000]

It is known that the penetration depth of photon into a material depends on the energy of the photon, density and atomic number of the material; because the photon energy is normally smaller than the potential against the penetration of photon, which is the potential barrier, so the incident photon attenuates. My question is, what is this potential, and how is it related to other parameters in quantum mechanics, e.g. atomic number, density, temperature etc.. Could someone recommend a book or show some curves of this potential over the surface of some material?

 

[Simon Bridge]

Photons travel through matter until they get absorbed.