Robert Christy

Robert Christy, who has died aged 95, was a theoretical physicist and astrophysicist who, as a member of the Manhattan Project team at Los Alamos, New Mexico, designed a vital gadget used in the first nuclear device ever detonated, and in “Fat Man”, the bomb that destroyed Nagasaki in 1945.

The Los Alamos Laboratory was organised in 1942 to design a nuclear weapon that would bring an end to the Second World War. A relatively simple “gun-type” fission weapon was developed (in which one lump of uranium-235 was fired at a second lump to create a supercritical mass — it was this type of bomb that devastated Hiroshima). In parallel, reactors were constructed in which uranium was irradiated and transmuted into plutonium. However the gun-type method proved impractical with plutonium, which tended to pre-detonate or “fizzle”. So in 1944 the Los Alamos scientists decided to develop the concept of “implosion” — the uniform compression of plutonium into a supercritical mass .

Christy, a member of the Los Alamos Theoretical Physics division, had been doing studies that led him to believe that a solid or near-solid sphere would implode more uniformly than a hollow sphere. The “Christy Gadget” involved the manufacture of two hemispheres to contain the plutonium, each coated with nickel to prevent it from oxidising, along with detonators, fuses and high-explosive lenses.

On Saturday July 14 1945 the assembled device was hoisted to the top of the 100ft tower at New Mexico’s Alamogordo Bombing and Gunnery Range, where it was due to be detonated two days later. No one knew exactly what to expect. Enrico Fermi was heard taking side-bets that the bomb would incinerate New Mexico and as a precaution a no-doubt startled state governor was warned that an evacuation of the state might be necessary.

At 5:29am on July 16 the device exploded with a force of 21,000 tons of TNT, evaporating the tower on which it stood. “The whole country was lighted by a searing light with the intensity many times that of the midday sun,” recalled Brigadier General Thomas Farrell. “It was golden, purple, violet, grey and blue. It lighted every peak, crevasse and ridge of the nearby mountain range ... Seconds after the explosion came first the air blast pressing hard against the people, to be followed almost immediately by the strong, sustained awesome roar that warned of doomsday.”

Immediately after the test Farrell turned to his superior, General Leslie Groves: “The war is over,” he observed. “Yes,” came the reply. “Just as soon as we drop one or two of these things on Japan.”

The son of an electrical engineer who had emigrated to Canada from England, Robert Frederick Christy was born on May 14 1916 in Vancouver, British Columbia. His parents died when he was young and he was brought up by relatives. It seemed unlikely that he would be able to afford to go to university. However, by coming top in the state in his school exams, he won a scholarship to the University of British Columbia, where he studied Physics. He was then accepted as a graduate student by Robert Oppenheimer, the leading American theoretical physicist, at the University of California, Berkeley.

After taking a PhD in 1941 Christy joined the Illinois Institute of Technology, but was soon recruited by Enrico Fermi to join a team at the University of Chicago, involved in the effort to design the first nuclear chain reactor. As he later recalled, there was no suggestion at the time that the objective was to develop a bomb; rather it was presented as a solution to submarine propulsion, avoiding the need for the vessels to carry tons of conventional fuel.

When Oppenheimer formed the Los Alamos Laboratory as part of the Manhattan Project, Christy was one of the first recruits to join the Theory Group under Hans Bethe. After the war, Christy joined the University of Chicago Physics department briefly before being recruited to join the Physics faculty at the California Institute of Technology (Caltech) in 1946.

He remained at Caltech for the rest of his academic career, working, first, on cosmic rays and later in astrophysics, investigating Cepheid variables, a class of pulsating stars which, due to the strong direct relationship between their luminosity and pulsation period, are used for establishing galactic distance scales. He won the Royal Astronomical Society’s Eddington Medal for this work in 1967 and the same year spent six months at Cambridge as a Churchill Fellow.

Christy served as chairman of Caltech’s Physics department and in 1970 became Caltech’s provost, a post he held for the 10 years. During an interregnum in 1977 he served as acting president of the institute.

Although he felt that the Bomb had probably saved lives, observing that the Japanese “probably would have lost millions if they had had to defend themselves against an invasion, and we would have lost hundreds of thousands”, Christy admitted that pictures of the devastation in Hiroshima and Nagasaki had been “very sobering” and after the war, like Robert Oppenheimer and others involved in the Manhattan Project, he opposed the further development of nuclear weapons.

This put him at odds with his former colleague Edward Teller (the “father” of the hydrogen bomb), with whom he and his wife had shared living quarters for a time after the war. When Teller testified against Oppenheimer during Atomic Energy Commission security-clearance hearings, in 1954, he was appalled: “I viewed Oppenheimer as a god ... and I was sure that he was not a treasonable person.” Less than a week later, Christy was visiting Los Alamos when he ran into Teller: “He approached me with his hand out to shake my hand. And I very deliberately refused to shake his hand,” Christy recalled. Later, Teller told an interviewer that the snub caused him to break down and weep: “I realised that my life as I had known it was over.”

Robert Christy was twice married. His first marriage was dissolved. He is survived by his second wife, Juliana, by their two daughters, and by the two sons of his first marriage.

Robert Christy, born May 14 1916, died October 3 2012