X-10 supplied the Los Alamos Laboratory with the first significant samples of plutonium. Studies of these by Emilio G. Segrè and his P-5 Group at Los Alamos revealed that it contained impurities in the form of the isotope plutonium-240, which has a far higher spontaneous fission rate than plutonium-239. This meant that it would be highly likely that a plutonium gun-type nuclear weapon would predetonate and blow itself apart during the initial formation of a critical mass. The Los Alamos Laboratory was thus forced to turn its development efforts to creating an implosion-type nuclear weapon—a far more difficult feat.