Frisch–Peierls memorandum

The Frisch–Peierls memorandum was the first technical exposition of a practical atomic weapon. Written by Otto Frisch and Rudolf Peierls in March 1940 while they were both working at the University of Birmingham in England, the memorandum contained new calculations about the size of the critical mass needed for an atomic bomb, and helped accelerate British and U.S. efforts towards bomb development during World War II.

The memorandum was given to Marcus Oliphant, who passed it on to Henry Tizard, chairman of the Committee on the Scientific Survey of Air Defence who consequently requested the setting-up of what was to become the secret MAUD Committee. The memorandum (a copy of which is held in the Public Record Office at Kew) is dated March 1940.

The two men were the first to calculate that an atomic bomb would require about 1 kilogram (2.2 lb) of the isotope uranium-235. (The estimate of 1 kg turned out to be too low; see Critical mass.) Prior assumptions imagined that a fission-style bomb would require many tons of uranium, implying that it was theoretically possible, but militarily impractical. An earlier letter to President Franklin D. Roosevelt, signed by Albert Einstein (but written by Leó Szilárd), had suggested that if delivered by boat, one such bomb could destroy an entire port, but that it might prove "too heavy for transportation by air."

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