In July the first physics experiment completed at Los Alamos counted the number of secondary neutrons Pu239 emitted when it fissioned. “In this experiment,” says the Los Alamos technical history, “the neutron number was measured from an almost invisible speck of plutonium and found to be somewhat greater even than for U235.”1836 The experiment thus established what had not yet been confirmed despite the expensive rush of building: that plutonium emitted sufficient secondary neutrons to chain-react.
