Robert Bacher, who has died aged 99, was a leading member of the team of scientists on the Manhattan Project who developed the first atomic bomb, exploded at the Trinity test site in the New Mexico desert on July 15 1945. He supervised the preparation of the plutonium core in the site's workshop, a converted ranch house, and drove it to the final assembly point at ground zero.
The core had to be mated with the TNT detonator designed to trigger the nuclear explosion. In a tense scene, panic rose among the ground zero assembly team when some of the components failed to interlock.
Calmly, Bacher deduced that some of the materials had expanded inside in the hot ranch house, while other well-insulated parts within the bomb casing that had come from the laboratory were still cool. Once the temperatures had equalised, the components fitted perfectly.
There had been earlier evidence that Bacher's brilliance as a nuclear physicist was coupled with a robust, pragmatic temperament of incalculable value in a crisis. A few months before, he had been instrumental in restoring the faltering confidence of the Manhattan Project director, Robert Oppenheimer, at a critical moment.
Oppenheimer, architect of the atomic bomb, began to fear that the team had hit a seemingly insoluble technical problem in obtaining high purity plutonium. Agonising over what he regarded as a personal misjudgment, he considered resigning. Bacher persuaded him that the project would be irreparably delayed if he quit, causing great potential damage, and Oppenheimer decided not to go.
When Bacher returned to academic life in 1949, he moved to Caltech, the California Institute of Technology, and was then mainly responsible for establishing its postwar international reputation in astronomy and high-energy physics.
Bacher was born in Loudonville, Ohio. He got a bachelor's degree from the University of Michigan in 1926 and a doctorate in 1930. He did short-term research projects first at Caltech, in 1930, with a one-year appointment as a national research council fellow, and then at the Massachusetts Institute of Technology (MIT) and the University of Michigan.
He moved to Cornell University in 1934 and a year later became professor of physics and director of the laboratory of nuclear studies. While at Cornell, he was affiliated to MIT's radiation laboratory and then joined the Manhattan Project from 1940 to 1945.
Bacher's tough-minded attitude was invaluable in solving problems Oppenheimer encountered when headhunting his Los Alamos team. The prospect of moving from prestigious universities to the wilds of New Mexico, and having, as civilians, to accept a military posting, aroused misgivings among many physicists. Others were unprepared to work on the ultimate weapon of mass destruction.
Bacher supported a rebellion by certain recruits who insisted on scientific autonomy and retaining civilian status as their price for working at Los Alamos. The terms were accepted by (later General) Leslie Groves, who was the army engineer in charge of the project. Although the laboratory was a military installation, Groves agreed to a civilian administration and staff until the large-scale weapons trials began.
Bacher was head of the experimental physics division at the laboratory and, after the bomb-production phase began, was made head of the bomb physics division.
After the war he became one of the first members of the US atomic energy commission, and served on the president's science advisory committee during the Eisenhower administration.
He joined Caltech in 1949 and rebuilt the physics department, starting with high-energy particle physics, then a new field. He was head of the physics, mathematics, and astronomy division from 1949 to 1962, provost from 1962 to 1969, and vice president and provost from 1969 to 1970.
In shaping Caltech's research programme in the burgeoning field of high-energy physics, he was responsible for bringing on board two of the most famous physicists, Richard Feynman and Murray Gell-Mann. He was the force behind the construction of a new electron accelerator so that the Caltech group could make its own high-energy particles. This ensured, given the brilliance of Feynman and Gell-Mann, that his team was at the forefront of theoretical physics as it entered a golden age.
He established radio astronomy and got the funds to build the Owens Valley radio observatory, which remains one of the leading facilities in the world.
Bacher was president of the American Physical Society in 1964, president of the International Union of Pure and Applied Physics from 1969 to 1972, and winner of the president's medal for merit in 1946. He was a member of the US delegation to the nuclear test ban negotiations in 1958.
A son and a daughter survive him.
· Robert Bacher, nuclear physicist, born August 31 1905; died November 18 2004