 In February 1941 , Seaborg and his collaborators produced plutonium @-@ 239 through the bombardment of uranium . In their experiments bombarding uranium with deuterons , they observed the creation of neptunium , element 93 . But it then underwent beta @-@ decay , forming a new element , plutonium , with 94 protons . Plutonium is fairly stable , but undergoes alpha @-@ decay , which explained the presence of alpha particles coming from neptunium . Thus , on March 28 , 1941 , Seaborg , physicist Emilio Segrè and Berkeley chemist Joseph W. Kennedy were able to show that plutonium ( then known only as element 94 ) was fissile , an important distinction that was crucial to the decisions made in directing Manhattan Project research . In 1966 , Room 307 of Gilman Hall on the campus at the Berkeley , where Seaborg did his work , was declared a U.S. National Historic Landmark . 
