Bevatron

…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…

…high-energy protons produced by the Bevatron, an accelerator at what is now the Lawrence Berkeley National Laboratory in California. Shortly afterward, a different team working on the same accelerator discovered the antineutron.