 In 1943 development efforts were directed to a plutonium gun-type fission weapon called "Thin Man". Initial research on the properties of plutonium was done using cyclotron-generated plutonium-239, which was extremely pure but could only be created in tiny amounts. When Los Alamos received the first sample of plutonium from the X-10 Graphite Reactor in April 1944 a problem was discovered: reactor-bred plutonium had a higher concentration of plutonium-240, making it unsuitable for use in a gun-type weapon. In July 1944, Oppenheimer abandoned the gun design in favor of an implosion-type weapon. Using chemical explosive lenses, a sub-critical sphere of fissile material could be squeezed into a smaller and denser form. The metal needed to travel only very short distances, so the critical mass would be assembled in much less time. In August 1944 Oppenheimer implemented a sweeping reorganization of the Los Alamos laboratory to focus on implosion. He concentrated the development efforts on the gun-type device, a simpler design that only had to work with uranium-235, in a single group, and this device became Little Boy in February 1945. After a mammoth research effort, the more complex design of the implosion device, known as the "Christy gadget" after Robert Christy, another student of Oppenheimer's, was finalized in a meeting in Oppenheimer's office on February 28, 1945.
