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Robert F. Christy; physicist had key insight for Manhattan Project

NEW YORK — Robert F. Christy, who as a young Canadian-born physicist working on the Manhattan Project came up with a critical insight that led to the creation of the world’s first atomic bomb, died Wednesday at his home in Pasadena, Calif.

His death, at 96, came after a brief respiratory illness.

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Dr. Christy later turned his mind to the riddles of space and served as provost of Caltech. But he was more widely known as one of the last surviving leading scientists to have worked on the atomic bomb.

Dr. Christy was present at important junctures in the early atomic era, including the debut in 1942 of the world’s first nuclear reactor in Chicago. Most notable was his contribution to the design of the explosive core of the first atom bomb, which lit up the New Mexico desert during a nighttime test in 1945. The bomb dropped on Nagasaki also used his design.

The first bomb, developed in secrecy during World War II at Los Alamos, N.M., relied on implosion. The plan was to detonate a sphere of conventional explosives, the blast from which would compress a central ball of nuclear fuel into a very dense mass; that, in turn, would start a chain reaction that would end in a nuclear explosion.

But the team discovered that the interface between the detonating explosives and the hollow sphere could become unstable and ruin the crushing power of the blast wave.

Dr. Christy, while studying implosion tests, realized that a solid core could be compressed far more uniformly, and he worked to convince his colleagues of its superiority. He succeeded, and the hollow core was replaced with one made of solid plutonium metal.

A 1993 book, ‘‘Critical Assembly’’ ­— sponsored by the Department of Energy, which maintains the nation’s nuclear arsenal — said Dr. Christy’s insight reduced the risk that the core would lose its spherical form and thus fail to explode.

Robert S. Norris, an atomic historian and the author of ‘‘Racing for the Bomb,’’ called Dr. Christy’s breakthrough, known as the Christy pit, ‘‘a conservative solution to a problem they were having’’ that ‘‘increased the likelihood of a successful detonation.’’

Robert Frederick Christy was born in Vancouver and studied physics at the University of British ­Columbia. He was a graduate student at the University of California, Berkeley, under J. Robert ­Oppenheimer, who became known as the father of the atomic bomb.

Dr. Christy worked at the University of Chicago before being recruited by the Los Alamos team when Oppenheimer became its scientific director.

After the war, Dr. Christy joined Caltech in theoretical physics and stayed at the university for the rest of his academic career, serving as a faculty chairman, vice president, provost (from 1970 to 1980), and acting president (1977-78). He was elected to the National Academy of Sciences.

Dr. Christy came to question the wisdom of further developing nuclear arms, although he never ­became an activist.

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