sufficiently large quantity that way. Compton discussed with Wigner how plutonium might be produced in a nuclear reactor, and with Robert Serber about how that plutonium might be separated from uranium. His report, submitted in November, stated that a bomb was feasible. The final draft of Compton's November 1941 report made no mention of plutonium, but after discussing the latest research with Ernest Lawrence, Compton became convinced that a plutonium bomb was also feasible. In December, Compton was placed in charge of the plutonium project. Its objectives were to produce reactors to convert uranium to plutonium, to find ways to chemically separate the plutonium from the uranium, and to design and build an atomic bomb. It fell to Compton to decide which of the different types of reactor designs the scientists should pursue, even though a successful reactor had not yet been built. He proposed a schedule to achieve a controlled nuclear chain reaction by January 1943, and to have an atomic bomb by January 1945. Development In a nuclear reactor, criticality is achieved when the rate of neutron production is equal to the rate of neutron losses, including both neutron absorption and neutron leakage. When a uranium-235 atom undergoes fission, it releases an average of 2.4 neutrons. In the simplest case of an unreflected, homogeneous, spherical reactor, the critical radius was calculated to be approximately: where M is the average distance that a neutron travels before it is absorbed, and k is the average neutron multiplication factor. The neutrons in succeeding reactions will be amplified by a factor k, the second generation of fission events will produce k2, the third k3 and so on. In order for a self-sustaining nuclear chain reaction to occur, k must be at least 3 or 4 percent greater than 1. In other words, k must be greater than 1 without crossing the prompt critical threshold that would result in a rapid, exponential increase in the number of fission events. Fermi christened his apparatus a "pile". Emilio Segrè later recalled that: Another grant, this time of $40,000, was obtained from the S-1 Uranium Committee to purchase more materials, and in August 1941 Fermi began to plan the building of a sub-critical assembly to test with a smaller structure whether a larger one would work. The so-called exponential pile he proposed to build was long, wide and high. This was too large to fit in