The Australian physicist Sir Marcus Laurence Elwin Oliphant, who died on 14 July 2000, was the last of the ‘Rutherford Boys’ — the brilliant team who, under the leadership of Lord Rutherford, made the Cavendish Laboratory in Cambridge the Mecca of nuclear research during the years between the two world wars. A figure larger than life, Oliphant exuded physical vitality and self-confidence. A genial extrovert, with a booming laugh, he spoke his mind bluntly, never shy of challenging authority. As a scientist he was imaginative and creative; as a public figure he had a strong commitment to freedom of speech and an abhorrence of secrecy in any walk of life.

Oliphant joined the Cavendish Laboratory in 1927, on an 1851 Exhibition Scholarship. He quickly endeared himself to Rutherford by his experimental skill. Making things with one's own hands was in the Cavendish tradition of ‘sealing-wax and string’, but Oliphant was also an enthusiast for new technologies, with a penchant for building particle accelerators.

Improving on the Cockcroft–Walton accelerator by designing ion sources of much greater intensity, Oliphant's main contributions were the study of the nuclear reactions that occurred when deuterium was bombarded with deuterons, which led to the discovery of tritium. Considering his later anti-nuclear campaigns, it is ironic that the main practical applications of his work should be in nuclear weapons — tritium is a booster for the fission bomb, and the fusion reactions are the basis of the hydrogen bomb.

After moving to Birmingham in 1937, Oliphant took on the task of converting a moribund physics department into a leading nuclear research centre. He saw to this by building a 60-inch cyclotron, the largest at the time in England. However, the completion of the machine was interrupted by the outbreak of the Second World War in 1939. Two projects more urgent and vital to the war effort — radar and the atom bomb — were to occupy his mind for the next six years.

To make a radar that would be of practical use in the war — for example, as a means of detecting approaching enemy aircraft — it was necessary to increase greatly the power in the radio beam and to make it work at much shorter wavelengths. Oliphant adapted the klystron (used as a radiofrequency source in the cyclotron) for this purpose. But John Randall and Harry Boot, working in Oliphant's department, soon developed a much more efficient device, the resonant cavity magnetron. Within a few months it became the tool that helped to win the Battle of Britain in 1940 and avert Hitler's invasion of England. It was largely Oliphant's indefatigability that made this possible.

Oliphant played a somewhat similar role in the development of the atom bomb. Research on the bomb began in England in 1939, but the main impetus came from the calculations by Otto Frisch and Rudolf Peierls, early in 1940, which showed that the critical mass for a divergent chain reaction, propagated by fast neutrons in uranium-235, was only a few kilograms. It was Oliphant who brought the Frisch–Peierls memorandum to the attention of government authorities. As a result, the MAUD Committee was set up, charged with the development of the atom bomb.

Most of the experimental research on the physics of the bomb was done in Liverpool, where Frisch joined James Chadwick's team. By 1941, the scientific feasibility of the bomb had been established, but the separation of the uranium-235 isotope was too difficult a task for Britain under wartime conditions.

While on a trip to the United States on business connected with radar, in the autumn of 1941, Oliphant discovered that no work on the atom bomb was going on t here, despite the fact that the report of the MAUD Committee had been sent to the relevant authorities. He immediately informed a few influential friends about the British findings. The outcome of Oliphant's indiscretion was the setting up of the Manhattan Project. He himself worked on the project, on the manufacture of uranium-235, in California and Tennessee.

Oliphant returned to Birmingham for only a few years before taking up, in 1950, permanent residence in his native country. During his time in Birmingham he designed the proton-synchrotron, in which particles could be accelerated to gigaelectronvolts (GeV). This work was based on the ‘phase stability’ principle, which he conceived before Vladimir Veksler and Edwin McMillan.

Oliphant was lured to Canberra by a promise from the Australian government to provide him with sufficient finances to set up a Research School of Physical Sciences at the new Australian National University. With his usual enthusiasm he threw himself into the project of creating a school of nuclear research equal to any in the world; this included a plan for an accelerator of novel design that would produce particles with a higher energy than achieved anywhere else. Although much of his planning came to fruition, he failed to build his ambitious 10-Gev ‘cyclosynchrotron’; it was simply beyond the capacity of a small country, and his dream machine remained a “white Oliphant” (as it was described at the time). However, this failure did not diminish his high standing in Australia — he had the unique distinction, for a scientist, of serving as a governor of South Australia, the state of his birth.

Oliphant was vehemently opposed to the use of the atom bomb on the Japanese cities. He never overcame his feelings of guilt about his part in the Manhattan Project, and he frequently expressed his views to the Australian media. He also took an active part in international campaigns against nuclear weapons, particularly in the Pugwash Conferences on Science and World Affairs. One of the 22 participants in the first conference in 1957, he fully shared the Pugwash precept that scientists have a moral duty to be concerned about the social impact of their work. Describing himself as a ‘belligerent pacifist’, he advocated his conviction that war itself is evil and must not be tolerated by humanity.