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Scribbled in a notebook among the Canadian spy papers was the name Fuchs, but for a long time nobody thought to connect the name significantly with German-born Klaus Fuchs, an anti-Hitler refugee who was high in Anglo-American atom councils. Four years passed before Klaus Fuchs was arrested in England (and sentenced to 14 years). His confession led to the arrest of Courier Harry Gold in Philadelphia. The trail from Harry Gold led to the Rosenbergs, Greenglass and Soviet Spy Master Anatoli Yakovlev, who was ostensibly a Soviet vice consul in New York.
Worst Ever. Fuchs, said the joint committee, was by far the most damaging spy. "Fuchs alone has influenced the safety of more people and accomplished greater damage than any other spy not only in the history of the United States but in the history of nations." As a top member of the visiting British atomic-energy mission, he knew all the secrets of the Los Alamos weapons center. At Columbia University, he worked on the gaseous-diffusion method for separating U-235 —the process now used exclusively at Oak Ridge. He knew all the ideas for improving bombs, and the thinking on the hydrogen bomb. Fuchs fed his material to stubby Harry Gold, who took it to Yakovlev at furtive meetings in restaurants and bars, at the end of elevated lines, at a Childs restaurant.
David Greenglass, the only American among the top spies, was far less important to the Russians. He furnished Russia with mechanical details of the bomb, most importantly the high-explosive lenses used in the Nagasaki-type bomb, and a diagram of the bomb itself. But, the committee noted, he had nothing like Fuchs's fund of scientific principles and information.
The Gap. The Russians still had one major gap in their knowledge: they did not know how to make plutonium. That gap, the committee suggested, was filled by Bruno Pontecorvo, the Italian-born British physicist who quietly took his wife and three children on a trip to Finland last fall, then vanished behind the Iron Curtain. Pontecorvo was an expert on nuclear reactors, the devices which are needed to make plutonium.
'At Canada's Chalk River atomic center, Pontecorvo helped design the heavy-water pile, still the "reactor of most advanced design and performance." He knew the secrets of the plutonium-producing piles at Hanford. After the war, he was a senior officer at Harwell, the British atomic research center. Pontecorvo, whose brother and sister were lifelong Communists, might have been betraying reactor data from 1943 on, the committee guessed. He was rated by some colleagues as an even abler scientist than Fuchs. After Fuchs, said the committee, "Pontecorvo may be plausibly rated as the second deadliest betrayer . . . Certain it is that Russia today possesses nuclear reactors."