Operation Epsilon
en.wikipedia.org
en.wikipedia.org
It was then that the chemists asked what elements absorb neutrons, and after some discussion, they learned that Boron was a strong absorber... and that Boron was used in the production of Graphite. So a special batch was made using a different process, and the then-new method of spectrographic analysis was used to verify it as boron-free. And it worked as a moderator, something that nobody else knew. This is likely why the Germans thought it impossible to build a Uranium bomb.
[Edit/Update] Wikipedia tells it differently, but Leo Szilard was a key figure in the story, and his Wikipedia page[1] notes the role of Boron in graphite.
> Like the German researchers, Fermi and Szilard still believed that enormous quantities of uranium would be required for an atomic bomb, and therefore concentrated on producing a controlled chain reaction. [1]
The impetus for a bomb and the fundemental work came from outside of the USofA:
> At the University of Birmingham in March 1940, Otto Frisch and Rudolf Peierls examined the theoretical issues involved in developing, producing and using atomic bombs in a paper that became known as the Frisch–Peierls memorandum. [2]
> On 5 August 1941, Oliphant flew to the United States in a B-24 Liberator bomber, ostensibly to discuss the radar-development program, but was assigned to find out why the United States was ignoring the findings of the MAUD Committee. [2]
> Leo Szilard later wrote, "if Congress knew the true history of the atomic energy project, I have no doubt but that it would create a special medal to be given to meddling foreigners for distinguished services, and that Dr Oliphant would be the first to receive one." [2]
[2] https://en.wikipedia.org/wiki/Mark_Oliphant#Manhattan_Projec...
> Fermi and Szilard met with representatives of National Carbon Company, who manufactured graphite, and Szilard made another important discovery. He asked about impurities in graphite, and learned that it usually contained boron, a neutron absorber. He then had special boron-free graphite produced. Had he not done so, they might have concluded, as the German nuclear researchers did, that graphite was unsuitable for use as a neutron moderator.
A neat story, and although I haven't checked the citation I don't doubt it says so. But it probably didn't happen that way: many biography anecdotes are like that.
Here are some quotes from Voices of the Manhattan Project: Norman Hilberry's Interview (1965) Part 2. Primary source on the history of the Manhattan Project, so to speak. https://ahf.nuclearmuseum.org/voices/oral-histories/norman-h...
> This was just one of those things that Szilard had been needling the graphite companies for two years to try to get pure graphite. He had been working with the Speer Carbon Company... Boron was the one that we just had to keep out because it has such a terrifically high cross section for neutron absorption... But their facilities, a number of furnaces and so forth, were so few that I went down to National Carbon Company to see if we couldn't get them, now that we knew what had to be done, couldn't get them to start making stuff for us.
So it really can't be that Szilard made discovery about boron at meeting with National Carbon Company. Szilard already knew about boron, and developed boron-free graphite production process with Speer Carbon Company. But Speer was too small, so National Carbon Company was contacted to scale up the process.
America built the atom bomb because they had the resources- human, financial and industrial. And those were the result of deliberate policy not luck. We should give the US credit where it's due.
Don't we have the sound only? For all we know there could have been heavy winking, hand gestures or written notes involved.