During the spring of 1944, Emilio Segrè's group in P-Division made the startling observation that the first samples of pile-produced 239Pu had an unusually high spontaneous fission rate, with a neutron emission approximately five times that of cyclotron-produced 239Pu. This finding confirmed the gnawing suspicions of Fermi, Segrè, Seaborg, and others that the neutron bath in the production piles at Clinton and Hanford might cause the formation of a significant quantity of 240Pu, an as-yet-unobserved spontaneously fissioning isotope of plutonium. However, the alarmingly high rate of the spontaneous fission was unexpected. This rate increased the neutron background enough to make it highly probable for a gun-assembled gadget to predetonate and thus undermined the plutonium gun program.

Determined not to lose the heavy investment made in plutonium production, Groves forced the laboratory to change course. The primary technical objective shifted from developing a gun weapon to developing a plutonium implosion assembly. Within days after Oppenheimer officially announced the spontaneous fission discovery, the laboratory reorganized its work force to focus on implosion. Two new divisions were established – X (Explosives) under Kistiakowsky, and G (Gadget) under Bacher. Most of the groups in these new divisions were moved out of the earlier Research and Ordnance Engineering divisions. Unfortunately, at this point experiments in the implosion diagnostic program were indicating that an implosion weapon would be extremely difficult, if not impossible, to achieve.