At these energies, the energy to the photon is used to generate a photoelectron. This causes discontinuities in the absorption spectra, and are referred to as absorption edges. The presence of these photoelectrons in the material has an effect on the overall absorption of x-rays. As the photoelectron waves propagate through the material, the various waves will interfere with one another.
Positive interference causes an increase in the overall wavefunction.
Negative interference causes a decrease in the overall wavefunction. What type of interference is determined by the distance between nearest neighbors. As the wavefunction of the photoelectron interacts with the atoms in its neighborhood, it causes changes in the core state of the atoms, and thus changes the absorption. By changing the x-ray energy, the wavelength of the wavefunction is modified. The net effect is to cause variations, positive and negative, in the absorption in the region.