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 (five times that of "cyclotron" plutonium), making it unsuitable for use in a gun-type weapon.