might be making faster progress on an atomic weapon than they were. In a letter dated May 25, 1943, Oppenheimer responded to a proposal by Fermi to use radioactive materials to poison German food supplies. Oppenheimer asked Fermi whether he could produce enough strontium without letting too many in on the secret. Oppenheimer continued, "I think we should not attempt a plan unless we can poison food sufficient to kill a half a million men." 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 be created only 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; 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. In May 1945 an Interim Committee was created to advise and report on wartime and postwar policies regarding the use of nuclear energy. The Interim Committee in turn established a scientific panel consisting of Arthur Compton, Fermi, Lawrence and Oppenheimer to advise it on scientific issues. In its presentation to the Interim Committee, the scientific panel offered its opinion not just on the likely physical effects of an atomic bomb, but on its likely military and political impact. This included opinions on such sensitive issues as whether the Soviet Union should be advised of the weapon in advance of its use against Japan. Trinity The joint work of the scientists