In 1932 , soon after the prediction of positrons by Paul Dirac , Carl D. Anderson found that cosmic-ray collisions produced these particles in a cloud chamberâ '' a particle detector in which moving electrons ( or positrons ) leave behind trails as they move through the gas . In 1932 , soon after the prediction of positrons by Paul Dirac , Carl D. Anderson found that cosmic-ray collisions produced these particles in a cloud chamber -- a particle detector in which moving electrons ( or positrons ) leave behind trails as they move through the gas .