During the Manhattan Project the need arose to find volatile compounds of uranium suitable for use in the diffusion separation of uranium isotopes. Uranium borohydride is, after uranium hexafluoride, the most volatile compound of uranium known. Uranium hexafluoride presented serious handling difficulties, causing the managers to seek alternatives. When the synthesis problems had been sufficiently defined it was decided to evaluate a boron compound. Enrico Fermi's purported comment when he observed the neutron cross section for boron—"My God! It's as big as the side of a barn!"—not only gave a name to the unit of cross section (barns), it also put an end to using uranium borohydride in the diffusion process.