[17th Miss Regt]

But we also learned that lithium-7 will do just about as well when it burned during the Castle Bravo test. Oops, that was supposed to be 6 megatons, not 15! Cleanup on aisle three!

[farming pharmer]

It was assumed that the lithium-7 would absorb one neutron, producing lithium-8, which decays (through beryllium-8) to a pair of alpha particles on a timescale of seconds, vastly longer than the timescale of nuclear detonation. However, when lithium-7 is bombarded with energetic neutrons, rather than simply absorbing a neutron, it captures the neutron and decays almost instantly into an alpha particle, a tritium nucleus, and another neutron. As a result, much more tritium was produced than expected, the extra tritium fusing with deuterium and producing an extra neutron. The extra neutron produced by fusion and the extra neutron released directly by lithium-7 decay produced a much larger neutron flux. The result was greatly increased fissioning of the uranium tamper and increased yield.

This resultant extra fuel (both lithium-6 and lithium-7) contributed greatly to the fusion reactions and neutron production and in this manner greatly increased the device's explosive output. The test used lithium with a high percentage of lithium-7 only because lithium-6 was then scarce and expensive; the later Castle Union test used almost pure lithium-6. Had sufficient lithium-6 been available, the usability of the common lithium-7 might not have been discovered.

Fascinating.