How Autoradiography Works Rays emitted by the isotope act on a photographic emulsion containing silver bromide crystals in a gelatin matrix.  Beta particles activate Ag+ ions when the particles interact with the silver ions. During development, the activated Ag+ ions are reduced to Ag(s).  These small grains of silver can be seen under a microscope as black grains. The unaffected Ag+ ions are removed by fixation, leaving grains of Ag(s) to mark the path of the beta particles.