obituaries: Ernest Walton

Among the items on display in the nuclear physics section of the Science Museum, in London, is an apparatus consisting of an upturned tea- chest lined in lead and open on one side. From it rises a thick glass tube of a kind which once formed a part of petrol pumps, topped by an object resembling a tin top-hat.

Into that tea-chest, on the morning of 14 April 1932, squeezed Ernest Walton, and there he observed the tiny sparks or "scintillations" which showed that man had, for the first time, split the atom in controlled circumstances.

It took 19 years for the Nobel Prize committee to acknowledge the achievement, but its significance was instantly clear to physicists around the world. Niels Bohr, the great Danish theoretician, cabled his congratulations on the "wonderful new results", while in the United States the great experimentalist Ernest Lawrence interrupted his honeymoon to order colleagues in California to replicate the feat.

What Walton and his partner, John Cockcroft, had done was to create a lever to prise open the atomic nucleus. In doing so they had shown that a nuclear reaction released energy, and they had demonstrated the validity of Einstein's formula relating energy and mass.

Into the bargain, they achieved the ancient dream of the alchemist, of transmuting one element into another. Employing what was for the time a vast current of energy, a stream of hydrogen atoms (mass number one), was propelled down the glass tube at a target of lithium (mass number seven) in the wooden box. The scintillations observed through a microscope by Walton indicated that some of the projectiles were penetrating the lithium atoms, which were then splitting into two atoms of helium (mass number four).

The experiment, conducted in Lord Rutherford's Cavendish Laboratory in Cambridge, was one of the great landmarks in physics and helped generate a new branch of experimental science which today uses enormous atom-smashers occupying underground corridors miles in length and costing billions of pounds. The accelerators at Europe's Cern laboratories outside Geneva are descendants of the Cockcroft-Walton apparatus.

The son of an Irish Methodist minister, Walton had come to the Cavendish on a scholarship in 1927 from Trinity College Dublin. His early projects were unsuccessful, but his originality and application impressed the laboratory director, who thought him "original and able", and in 1929 Rutherford chose him to work with Cockcroft on a machine to accelerate particles for the purpose of splitting atoms.

Cockcroft was the senior scientist of the two. In 1929 he was 32, six years older than his Irish partner and an important figure in the Cavendish and in the university. His career was already turning towards scientific administration, a field in which he was later to excel - in wartime Cockcroft worked on the radar project before moving on to direct the construction of an early nuclear reactor in Canada, and afterwards he was the first Director of the Atomic Energy Research Establishment at Harwell.

It was Cockcroft's contacts with the Russian-born theoretician George Gamow which provided the basis for the 1932 experiment. His background in engineering, too, was vital in the design of the apparatus. He had, however, many other commitments and most of the labour of making the machine operate and of overcoming theoretical problems along the way fell on the younger man.

Walton was ingenious and dextrous - a colleague of the time recalled that he was "exceptionally clever with his hands and quite capable of making spare parts for watches", and even in his old age in Dublin he still took pleasure in working with his hands. This was famously the age of string-and-sealing-wax experimentation in physics, and Walton was a master of the art. Cockcroft, whom the Irishman later described as "a good scrounger", could conjure up equipment and materials, while no one could have been better than Walton at fitting them together and making them work. It was no accident that, on the morning when the entire apparatus was ready to perform its alchemy, it was Walton who was there to start it up.

Success made them famous. Even in 1932 "splitting the atom" was popularly recognised as significant. The more sensational reports linked it with the possibility of an atom bomb, not strictly a justifiable connection. Walton and Cockcroft continued to collaborate at the cutting edge of nuclear physics until 1934, when Walton returned to Trinity College in Dublin.

In those quieter waters he remained for the rest of his career, first as fellow and from 1946 as professor. Neither the college nor the Irish state had money for what was becoming a very expensive discipline and, although Walton continued to work on accelerators, he published little and contented himself with teaching and building his department.

This was a reflection of his temperament: he was a modest man and had been uncomfortable with the publicity that surrounded the events of 1932. His students were the beneficiaries, in particular because he had an exceptional gift for performing an elaborate experiment before an audience, explaining it as he went along, and actually producing the desired result at the end.

Once, he came close to returning to the mainstream of international science. Although he was educated in Belfast and never shook off his northern accent, he was a citizen of the southern state. (His father was from Limerick and was an ardent Home Ruler.) In consequence he was not called upon to help the war effort after 1939, as were many British scientists working in his field. But later in the war, probably in 1944, he was given a pressing invitation from a senior British scientist, Sir James Chadwick, to join an unidentified military project. Trinity College, which had already lost a number of young scientists to the British war effort, ruled that he could not be spared. Although neither the college nor Walton could have known it, the project in question was the Manhattan Project, making the atomic bomb. Walton, a religious man, later expressed relief that he had not had to make the decision.

In 1951, Walton and Cockcroft were finally awarded the Nobel Prize for Physics for the 1932 experiment. The citation declared that their work "may be said to have produced a totally new epoch in nuclear research".

The relative obscurity of Walton's later career and the high distinction of Sir John Cockcroft's have helped foster a view that the Irishman was in some way an undeserving partner in the prize. Cockcroft's biographers have concluded otherwise and Cockcroft himself remarked in the 1930s that "those of us who knew Walton are quite clear about his contribution to our joint work". Rutherford, too, is said to have declared that they should share the credit for their success. It seems fair to conclude that, had they been working alone, neither man could have concluded the experiment ahead of Ernest Lawrence's American team.

In 1934, on his return to Ireland from Cambridge, Walton married Winifred Wilson, a childhood sweetheart who had been a fellow-pupil at Methodist College, Belfast. They settled in the Dublin district of Dartry and had four children, all of whom chose careers in science.

Brian Cathcart

Ernest Thomas Sinton Walton, nuclear physicist: born Dungarvan, Co Waterford 6 October 1903; 1851 Overseas Research Scholar, Cambridge University 1927- 30, Senior Research Award of Department of Scientific and Industrial Research 1930-34, Clerk Maxwell Scholar 1932-34; Fellow, Trinity College Dublin 1934-74 (Emeritus), Erasmus Smith's Professor of Natural and Experimental Philosophy 1947-74; Nobel Prize for Physics 1951; married 1934 Winifred Wilson (two sons, two daughters); died Belfast 25 June 1995.