Lithium Neutron Cross Sections During the Manhattan Project and the Quest for the H-Bomb

Neutron cross sections of the stable lithium isotopes ⁶Li and ⁷Li were of interest in the 1940s and 1950s in part because of their reactions, which form tritium using moderated neutrons on ⁶Li and higher-energy neutrons on either isotope. Lithium remains of interest today for use as a blanket and shielding material in fusion reactors, where it can be used to breed tritium for a self-sustaining fuel cycle. During the Manhattan Project, the resonance in the ⁶Li(n,t) reaction was discovered and later became important for enhancing tritium production for nuclear technologies. The dominant natural isotope ⁷Li was and remains of interest because of the expense of enriching ⁶Li. It has been oft reported that the 1954 Castle Bravo nuclear test had a yield twice as large as expected because the nuclear explosive device designers had not properly accounted for the benefits from the ⁷Li isotope in the fuel; we note that this explanation is false.