McMillan heard disturbing news in April 1944 , and drove out to Pajarito Canyon to confer with Segrè . Segrè 's group had tested samples of plutonium bred in the Manhattan Project 's nuclear reactors and found that it contained quantities of plutonium-240 , an isotope that caused spontaneous fission , making Thin Man impractical . In July 1944 , Oppenheimer reorganised the laboratory to make an all-out effort on implosion . McMillan remained in charge of the gun-type weapon , which would now be used only with uranium-235 . This being the case , Thin Man was replaced by a new , scaled-back design called Little Boy . McMillan was also involved with the implosion as the head of the G-3 Group within the G ( Gadget ) Division , which was responsible for obtaining measurements and timings on implosion , and served as the laboratory 's liaison with Project Camel , the aerial test program being carried out by Caltech . On July 16 , 1945 , he was present at the Trinity nuclear test , when the first implosion bomb was successfully detonated
Trinity nuclear test
Trinity (nuclear test)
Edwin McMillan