 In 1932 and 1933 , the scientists at the Cavendish Laboratory made a series of ground @-@ breaking discoveries . Cockroft and Walton bombarded lithium with high energy protons and succeeded in transmuting it into energetic nuclei of helium . This was one of the earliest experiments to change the atomic nucleus of one element to another by artificial means . Then Chadwick devised an experiment that discovered a new , uncharged particle with roughly the same mass as the proton : the neutron . In 1933 , Blackett discovered tracks in his cloud chamber that confirmed the existence of the positron and revealed the opposing spiral traces of positron – electron pair production . 
