IMPLOSION BECOMES
A NECESSITY
Los Alamos: Laboratory (1944)
Events: Bringing It All
Together, 1942-1945

Because the gun-type bomb design seemed so simple and practical, Deke Parsons had assigned implosion studies a low priority and placed the emphasis on the more familiar artillery method.  Consequently, Seth H. Neddermeyer performed his early implosion tests in relative obscurity.  Neddermeyer found it difficult to achieve symmetrical implosions at the low velocities he had achieved.  When the Princeton mathematician John von Neumann, a Hungarian refugee, visited Los Alamos late in 1943, he suggested that high-speed assembly and high velocities would prevent predetonation and achieve more symmetrical explosions.  A relatively small, subcritical mass could be placed under so much pressure by a symmetrical implosion that an efficient detonation would occur.  Less fissionable material would be required, bombs could be ready earlier, and extreme purification of plutonium would be unnecessary.  Von Neumann's theories excited Robert Oppenheimer, who assigned Parsons's deputy, George B. Kistiakowsky, the task of perfecting implosion techniques.  (Kistiakowsky would later become President Dwight D. Eisenhower's science adviser.)  Because Parsons and Neddermeyer did not get along, it was Kistiakowsky who worked with the scientists on the implosion project.  

While experiments on the gun and implosion methods continued, Parsons directed much of his effort toward developing bomb hardware, including arming and wiring mechanisms and fusing devices.  Working with the Army Air Force, Parsons's group developed two bomb models by March 1944 and began testing them with B-29s.  "Thin Man," named for President Roosevelt, utilized the plutonium gun design, while "Fat Man" (above), named after Winston Churchill, was an implosion prototype.  (Emilio Segrè's lighter, smaller uranium design became "Little Boy," Thin Man's brother).  

In the summer of 1944, however, it became clear that, because of the plutonium-240 problem, a gun-type design would not work for the plutonium bomb.  The implosion method was now transformed from an intriguing possibility into a difficult necessity.  Glenn 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, i.e., the bullet and target in the plutonium weapon would melt before coming together.  Measurements taken at Oak Ridge confirmed the presence of plutonium-240 in the plutonium produced in their experimental pile (X-10).  On July 17, the difficult decision was made to cease work on the plutonium gun method -- there would be no "Thin Man."  Plutonium could be used only in an implosion device, but in the summer of 1944 an implosion weapon looked like a long shot.  

Abandonment of the plutonium gun project eliminated a shortcut to the bomb.  This necessitated revision of the estimates of weapon delivery Vannevar Bush had given the President in 1943.  The new timetable, presented to General George Marshall by Leslie Groves on August 7, 1944 -- two months after "D-Day," the Allied invasion of France -- 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-type bomb by August 1, 1945, and the delivery of one or two more by the end of that year.  Marshall and Groves agreed that Germany might well surrender by the summer of 1945, thus making it probable that Japan would be the target of any atomic bombs ready by that time.  

Oppenheimer acted quickly to maximize the laboratory's efforts to master implosion.  Only if the implosion method could be perfected would the plutonium produced at Hanford come into play.  Without either a plutonium gun bomb or implosion weapon, the burden would fall entirely on uranium and the less efficient gun method.  Oppenheimer directed a major reorganization of Los Alamos in July 1944 that prepared the way for the final development of an implosion bomb.  Robert Bacher took over G Division (for "Gadget") to experiment with implosion and design a bomb; Kistiakowsky led X Division (for "explosives") in work on the explosive components; Hans Bethe continued to head up theoretical studies; and Parsons now focused on overall bomb construction and delivery.  

Field tests performed with uranium-235 prototypes in late 1944 eased doubts about the gun-type method to be employed in the uranium bomb.  It was clear that the uranium-235 from Oak Ridge could be used in a gun-type nuclear device to meet the August 1 deadline Groves had given General Marshall and the Joint Chiefs of Staff.  The plutonium produced at such expense and effort at Hanford (right), however, would not fit into wartime planning unless a breakthrough in implosion technology could be found.