antineutron, antiparticle of the ordinary neutron, first produced in 1956 at the Bevatron particle accelerator at the University of California, Berkeley, by passing an antiproton beam through matter. Antineutrons were created when antiprotons in the beam exchanged their negative charge with nearby protons, which have a positive charge. The antineutrons were detected through their annihilation reactions with neutrons and protons.