History
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 with a vapor pressure of 4 mmHg (530 Pa) at 60 °C. Uranium borohydride was discovered by Hermann Irving Schlesinger and Herbert C. Brown, who also discovered sodium borohydride.
Uranium hexafluoride is very corrosive, which presented serious handling difficulties, leading to serious consideration of the borohydride. However by the time the synthesis method was finalized, the uranium hexafluoride related problems were already solved. Borohydrides are nonideal ligands for isotope separations since boron occurs naturally with two abundant isotopes, 10B (20%) and 11B (80%).
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 (the barn); it also put an end to using uranium borohydride in the diffusion process.
Read more about this topic: Uranium Borohydride
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