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. At the time plutonium-239 had been produced in minute quantities using cyclotrons, but it was not possible to produce large quantities that way. Compton discussed with Eugene Wigner from Princeton University how plutonium might be produced in a nuclear reactor, and with Robert Serber how the plutonium produced in a reactor might be separated from uranium.