The Manhattan Project: Making the Atomic Bomb
Part IV: The Manhattan Engineer District in Operation
Elimination of Thin Man
Thin Man was eliminated four months later because of the plutonium-240 contamination problem. Seaborg had warned that when plutonium- 239 was irradiated for a length of time it was likely to pick up an additional neutron, transforming it into plutonium-240 and increasing the danger of predetonation (the bullet and target in the plutonium weapon would melt before coming together). Measurements taken at Clinton confined the presence of plutonium-240 in the plutonium produced in the experimental pile. On July 17 the difficult decision was made to cease work on the plutonium gun method. Plutonium could be used only in an implosion device, but in summer 1944 an implosion weapon looked like a long shot.
Abandonment of the plutonium gun project eliminated a shortcut to the bomb. This necessitated a revision of the estimates of weapon delivery Bush had given the President in 1943. The new timetable, presented to General Marshall by Groves on August 7, 1944 two months after the Allied invasion of France began at Normandy on June 6 - promised small implosion weapons of uranium or plutonium in the second quarter of 1945 if experiments proved satisfactory. More certain was the delivery of a uranium gun bomb by August 1, 1945, and the delivery of one or two more by the end of that year. Marshall and Groves acknowledged that German surrender might take place by summer 1945, thus making it probable that Japan would be the target of any atomic bombs ready at that time.