text: Although the process is still not fully understood, diffraction patterns of crystals typically decay with X-ray exposure due to a number of processes which non-uniformly and irreversibly modify molecules that compose the crystal.  These modifications induce disorder and thus decrease the intensity of Bragg diffraction.  The processes behind these modifications include primary damage via the photo electric effect, covalent modification by free radicals, oxidation (methionine residues), reduction (disulfide bonds) and decarboxylation (glutamate, aspartate residues).