THE experimental physicist Robert Fox Bacher was a key member of the Manhattan Project, the team of scientists who developed the atom bomb during the Second World War. He directed the experimental physics division at the Los Alamos Laboratory, New Mexico, a division known as the “G” division (G being for gadgets). When the operation reached the bomb production stage he became head of the bomb physics division.

Bacher was born in 1905 in Loudonville, Ohio. In 1926 he graduated with a bachelor’s degree from the University of Michigan. In 1930 he was awarded a PhD and given an appointment as a National Research Council Fellow at the California Institute of Technology (Caltech). In 1934 he went to Columbia University.

He moved on to the physics department of Cornell University in 1935, where he became Professor of Physics and Director of the Laboratory of Nuclear Studies. When the Second World War started he was affiliated with MIT, where he conducted research into radar. In 1943 he joined the hundreds of other young scientists working on the Manhattan Project.

He urged J. Robert Oppenheimer, the director of the Manhattan Project, not to place the enterprise under military control in order to increase secrecy and security. At the time, the Los Alamos Laboratory was officially classified as a military establishment, but Bacher strongly believed that, to be effective as scientists, the team needed to be able to think independently. The project remained under civilian control.

Bacher was a member of the team that assembled the weapon for the first nuclear explosion, on July 15, 1945 at Alamogordo in the New Mexico desert. The test, at a site called Trinity, was a test of a weapon of the design that destroyed Nagasaki on August 9 that year.

The nuclear weapon dropped on Hiroshima on August 6, 1945, used highly-enriched uranium instead of plutonium. The uranium design was so straightforward that the scientists were confident that it would work without testing. The plutonium bomb was much more complex, and so a test was scheduled.

By early July 1945, the Manhattan scientists had produced only enough plutonium for two weapons, and sufficient highly-enriched uranium for one. It was, therefore, possible to test a plutonium weapon and have just enough fissile material left over for the weapons dropped on Nagasaki and Hiroshima.

The core of the weapon used for the Trinity test consisted of a sphere of plutonium, surrounded by a mass of high explosives to compress the sphere to obtain the nuclear explosion. The plutonium weighed 6.2 kilograms (13 lb 9 oz). The core was assembled at the George McDonald ranch house, two miles from ground zero. It was then transported to the base of the 30 meter-high (100 ft) steel tower on which the bomb was to be exploded. The core was inserted into the weapon with some difficulty. On the first try it stuck, as the plutonium was hotter than the rest of the assembly. Plutonium is radioactive, and the heat produced during the radioactive decay processes heated the core. After a little while, the temperatures of the plutonium and the casing equalised and the core slid neatly into place, much to the relief of Bacher and the others in the assembly team.

The entire bomb was hoisted to the top of the tower. It worked perfectly, exploding with an explosive power equivalent to that of 20,000 tonnes of TNT.

When the extent of the destruction at Hiroshima and Nagasaki became clear, Bacher was concerned about the huge destructive power of such weapons. He believed that it would have been better if the threat of nuclear attack could somehow have been used to persuade the Japanese to end the war without the destruction of the two cities.

Bacher was a staunch friend and supporter of Oppenheimer, who had considered resigning from the Manhattan Project when difficulties arose over the production of plutonium. Bacher persuaded him to stay, knowing that, if he quit, the project would be seriously delayed.

Oppenheimer’s loyalty was questioned during the anti-Communist witch hunts, with a “trial” beginning on April 12, 1954, in an AEC Building in Washington. This bizarre episode began on November 7, 1953, when a former executive director of the Congressional Joint Atomic Energy Committee wrote to J. Edgar Hoover accusing Oppenheimer of being “an agent of the Soviet Union”. Given Oppenheimer’s role as “the father of the atomic bomb”, it is hardly surprising that the accusation was taken seriously.

Bacher testified in his colleague’s defence, saying that, in his opinion, Oppenheimer was not a security risk. This was a very brave thing to do during the hysterical days of the McCarthy inquisition. The AEC verdict went against Oppenheimer.

Bacher became one of the first members of the United States Atomic Energy Commission. As a commissioner, he testified before a joint congressional committee about the state of America’s nuclear weapons programme. After an investigation at Los Alamos, Bacher was surprised and shocked at how few nuclear weapons were in America’s arsenal.

In 1945, six weapons were produced and three were used. During 1946, five more were produced. American nuclear weapon production increased soon afterwards as a result of technical improvements in the production of plutonium and highly-enriched uranium. At the end of 1950, the arsenal contained more than 360 nuclear weapons.

In 1949, Bacher became Professor of Physics at Caltech, a small, independent university for research and teaching in science and engineering that has become one of the world’s leading institutions of scientific research and education. Together with the physicist Lee Alvin DuBridge, head of MIT’s wartime radar project who was appointed president of Caltech in 1946, Bacher considerably expanded the teaching and research activities.

Bacher headed Caltech’s division of physics, mathematics and astronomy. He rebuilt the physics department, starting with high-energy particle physics, a rapidly expanding field of study. He oversaw the construction and use of a new electron synchrotron (an electron accelerator) that enabled Caltech physicists to produce their own high-energy particles. He also established a flourishing theoretical physics group.

His first recruit was the theoretical physicist Richard Feynman. He also recruited Murray Gell-Mann, another brilliant physicist who received the Nobel Prize in Physics in 1969. Caltech operated a 200in telescope at its observatory on Palomar Mountain, then the world’s most powerful optical telescope. Bacher established radio astronomy, raising the money to build the Owen Valley radio observatory.

In 1962, Bacher was appointed provost of Caltech and vice-president and provost in 1969. He retired as professor emeritus in 1976. Bacher was president of the American Physical Society in 1946 and president of the International Union of Pure and Applied Physics between 1969 and 1972.

In 1946 he was awarded the President’s Medal for Merit. He was a member of the US President’s Science Advisory Committee during the Eisenhower Administration and a member of the US delegation to the nuclear test ban negotiations in 1958.

Bacher’s skill as a nuclear physicist was coupled with a strong, pragmatic personality that was invaluable in a crisis. A son and daughter survive him.

Robert Bacher, physicist, was born on August 31, 1905. He died on November 18, 2004, aged 99.