Theodore Hall (October 20, 1925-November 1, 1999) was an American physicist who, during his work on Allied effort to develop the first atomic bombs during World War II (the Manhattan Project), gave a detailed description of the "Fat Man" plutonium bomb, and of processes for purifying plutonium, to the Soviet Union.

Unbeknownst to Hall, Klaus Fuchs, a Los Alamos colleague, and others still unidentified, were also spying for the USSR; none seems to have known of the others. Some of their information provided an independent and confirming source for the others. At only 19, Hall was the youngest scientist on the Manhattan Project. His wife said, after his death, that he had begun to develop strong feelings against the possibility of an emerging, militarized, United States, with a nuclear monopoly very early in his Los Alamos work.

Hall, with the help of his college friend Saville Sax , who had open Communist sympathies, together visited New York, where Hall, after some searching, arranged a meeting with a Russian diplomat. He presented a detailed sketch of the "Fat Man" nuclear device to the official, who transmitted the information to the NKVD from New York using a one-time pad cypher.

Until recently, nearly all of the severely damaging espionage regarding the Los Alamos nuclear weapons program was attributed to Klaus Fuchs. Hall was questioned by the FBI in 1951 but wasn't charged due to lack of evidence. Despite having been more damaging to U.S. security than celebrated Soviet collaborators Harry Gold , David Greenglass, and Julius and Ethel Rosenberg, Hall was never imprisoned. It is claimed that Hall was not a Communist sympathizer, but simply believed no nation should hold a nuclear monopoly.

Hall and his wife came under scrutiny in the early 1950's, but as with many Cold War spies, they went unpunished. This seems to have been primarily to keep secret the fact that the US has been partially successful in decrypting some messages carried in a Soviet one-time pad cypher (see VENONA). The US seems to have also been more interested in securing existing projects than in any way jeopardizing several years of counter-espionage efforts.

Hall left Los Alamos for the University of Chicago, where he switched to biology. There he pioneered important techniques in X-ray microanalysis. He went to work at Cambridge University, England in 1962.

In November of 1999, Theodore Hall died in Cambridge. He suffered from Parkinson's disease, though it was cancer of the kidney that killed him.

Further reading

• Joseph Albright and Marcia Kunstel. Bombshell: The Secret Story of America's Unknown American Spy Conspiracy (New York: Random House/Times Books, 1997). (An account of Soviet espionage against the US during WWII, including background on Hall and his activities, and on the VENONA project.)