In April 1941, the National Defense Research Committee (NDRC), asked Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program. Niels Bohr and John Wheeler theorized that heavy isotopes with odd atomic numbers, such as plutonium-239, were fissile. Emilio Segrè and Glenn Seaborg at the University of California produced 28 μg of plutonium in the 60-inch cyclotron there in May 1941, and found that it had 1.7 times the thermal neutron capture cross section of uranium-235. While minute quantities of plutonium-239 could be created in cyclotrons, it was not feasible to produce a large quantity that way. Compton conferred with Eugene Wigner from Princeton University about how plutonium might be produced in a nuclear reactor, and with Robert Serber from the University of Illinois about how the plutonium produced in a reactor might then be chemically separated from uranium it was bred from.