A bet, Little Boy, and spontaneous fission

Some of the information given in Owen Chamberlain’s obituary needs correction. Let me quote the problem sentences:

In the summer of 1943, [Emilio] Segrè and his group, including Owen, moved to Los Alamos, New Mexico, where they were assigned the task of determining the spontaneous fission probabilities of uranium and plutonium isotopes. Their initial results suggested an unexpectedly high rate of spontaneous fission, but later they determined that most of the “spontaneous” fission events were actually induced by cosmic rays. Nevertheless the rates were high enough to require a complete redesign of the bomb trigger.

That there would be high spontaneous fission rates was not unexpected. What was unexpected was that reactor plutonium would have as much plutonium-240 as it did. Plutonium-240, like uranium-238, has a high spontaneous fission rate. That has nothing to do with cosmic rays. As Niels Bohr and John Wheeler first argued, it is a barrier penetration matter. These wartime results are summarized in Segrè’s 1952 paper, ¹ which also has a brief summary of the theory. Also, the last sentence is misleading. What the results required was the use of implosion for the plutonium weapon. The uranium weapon—Little Boy—was able to use the original gun-assembly design, as Hiroshima sadly proved.